What Glass Is: Structure and Compositions

Sand, soda and fire

The first glass made by humans appeared more than four thousand years ago in Mesopotamia, between the Tigris and Euphrates rivers. Glassmakers mixed sand with the ash of salt-loving desert plants, which brought both soda (to lower the melting point) and lime (to make the glass stable).

They first heated the mixture into a half-fused “frit”, then ground it and remelted it in clay crucibles in wood-fired kilns at around 1000 °C. Minerals like copper and cobalt gave it colour. The earliest objects were small beads, made by winding hot glass around a metal rod, an ancestor of today’s lampworking. Glassmaking may have grown out of other crafts that worked with heat, such as metalworking or faience, a glazed ceramic. Mesopotamia also left us some of the oldest written glass recipes: cuneiform tablets from the library of the Assyrian king Ashurbanipal at Nineveh, from around 650 BCE, which describe the ingredients, the firing and even rituals for building the kiln.

The Roman writer Pliny the Elder told a different story: Phoenician merchants, cooking on a beach at the mouth of the river Belus, rested their pots on blocks of natron (soda) from their cargo, and found glass running from the fire the next morning. It is a legend rather than history, but it holds the right ingredients: sand, an alkali, and enough heat to join them.

The first vessels

By the fifteenth century BCE, glass was being produced on a large scale in Western Asia, Crete and Egypt. The first vessels were small bottles for scented oils and perfumes, made by core-forming: a rope of hot glass was wound around a core of sand and clay fixed to a metal rod, reheated so the coils fused, marvered smooth and decorated with trails of other colours. When the glass had cooled, the core was scraped out. In Egypt glass was still new and precious: some of the finest early vessels, such as a lotus-shaped glass and gold cup now in the Metropolitan Museum of Art, were buried with three foreign wives of the pharaoh Thutmose III, in the fifteenth century BCE. Phoenician traders carried these vessels across the Mediterranean.

Core-forming is close to what a lampworker still does: winding softened glass around a rod, turning it constantly, building up form and colour layer by layer.

Far to the east, glass reached China around the fifth century BCE, first as imported "eye beads", layered with concentric circles of colour, and soon in local versions. Chinese makers developed their own recipe, rich in lead and barium, unlike the soda-lime glass of Western Asia, and used opaque green and milky white glass to imitate jade: in the Chu kingdom of the Warring States period, glass bi discs took the place of jade ones in many tombs.

Hellenistic luxury

After the conquests of Alexander the Great in the late fourth century BCE, the courts of his successors wanted luxury on the table, and glass rose to meet them. Hellenistic workshops cast thick bowls in moulds and cut and polished them on a wheel like stone, and made mosaic glass, fusing slices of coloured canes into patterns. By the end of the first century BCE, workshops probably in Alexandria were making gold-band glass, with gold foil cased between bands of coloured and clear glass. These canes and slices are the ancestors of the millefiori and murrine that lampworkers still pull and cut today (see Techniques at the Torch).

Blowing glass

In the first century BCE, on the coast of what is now Syria, Lebanon and Palestine, glassworkers discovered that a gather of molten glass on the end of a hollow pipe could be inflated with breath. Glassblowing turned glass from a luxury into an everyday material. Under the Roman Empire it spread from the eastern Mediterranean across the empire, and by around 100 CE glassmakers in Alexandria were using manganese to make glass almost colourless (the chemistry of that trick is told in Colour in Glass).

Soon after came mould-blowing: glass inflated inside a carved mould of two or more parts, so that the same shape and relief decoration could be repeated many times. Some makers signed their moulds. Jugs and cups signed by Ennion, the finest maker of Roman mould-blown glass in the early first century CE, have been excavated at sites from Palestine to Spain, making him one of the first glassmakers in history known to us by name.

At the same time, on India's south-east coast near Pondicherry, the port of Arikamedu was making tiny drawn beads: long tubes of glass pulled by hand, cut into small pieces and reheated in ash to soften their edges. Between about 200 BCE and 400 CE these Indo-Pacific beads became the most widely traded beads in the world, found from Ghana to China, and the same method is still used at one bead-making site in India.

Byzantium and the eastern workshops

When Constantine made Constantinople the new capital of the empire in 330 CE, the centre of gravity moved east, and the workshops of the Levantine coast, in cities such as Tyre and Sidon, kept working. Byzantine glassmakers are remembered above all for light: gold glass tesserae, small cubes with gold leaf sealed under a thin layer of glass, became the shimmering background of Christian mosaics throughout the Byzantine period, and in the eleventh century Kyiv was still decorating its churches with tesserae made in Constantinople. Further east, Sasanian Persia was making thick bowls cut with hollow facets; one of them reached Japan and is kept in the Shōsōin, the imperial treasure house at Nara, built in the mid-eighth century.

Islamic glass: cut, lustre and enamel

After the Arab conquests of the seventh century, glassmakers in Syria, Egypt, Iraq and Iran built on Roman, Byzantine and Sasanian traditions and took them further. They cut glass in deep relief, carved honeycomb facets and Kufic inscriptions into its surface, and between the seventh and ninth centuries, in Egypt and Syria, they painted it with metallic lustre, staining the glass with a thin golden or brown film.

From the thirteenth century, workshops in Syria and Egypt mastered enamelling and gilding: coloured glass powders and gold painted onto finished vessels and fused in a second firing, with Cairo, Aleppo and Damascus as the great centres. Their masterpiece was the mosque lamp: tall enamelled lamps, often inscribed with the Verse of Light from the Qur'an, "God is the light of the heavens and the earth", hung in the mosques, madrasas and tombs of the Mamluk sultans. Fifty-nine glass lamps made for Sultan Barquq, who ruled from 1382 to 1399, still survive. A vessel of glass, holding a flame, glowing through words written in colour.

In the winter of 1400 to 1401, Timur's armies besieged and sacked Damascus, and great numbers of its artisans were taken to his capital, Samarkand. By the fifteenth century, fine glassmaking across the region was in steep decline.

Meanwhile, far to the south, glass was being made in West Africa. At Ile-Ife, in what is now Nigeria, excavations at the site of Igbo Olokun led by the archaeologist Abidemi Babalola showed that glass was produced locally between about 1000 and 1500 CE, with a recipe of its own. It was the first evidence of glassmaking in sub-Saharan Africa, and Ife's glass beads were treasured symbols of power.

Venice and Murano

In 1291 the Republic of Venice ordered its glassmakers to move their furnaces to the island of Murano, for fear that fire would spread through the wooden city. From the fourteenth century Murano became the centre of luxury glass in Europe, and its techniques, colours and secrets were guarded and copied for centuries. Murano learned much from the enamelled and gilded glass of the eastern Mediterranean, and as eastern production declined, the trade turned around: in 1569 the Ottoman grand vizier Sokullu Mehmed Pasha ordered six hundred plain glass lamps from Venice, possibly to be decorated elsewhere.

Beijing, Istanbul, Edo and Hebron

In 1696 the Kangxi Emperor set up a glass workshop attached to his palace in Beijing. It was run at first by Kilian Stumpf, a German Jesuit and skilled glassmaker, and over the following decades the imperial workshops joined European recipes to Chinese glassmaking and carving, producing vessels and the small, precious snuff bottles of the Qing court.

In Istanbul, a new chapter began with a Mevlevi dervish, Mehmed Dede, who is credited with founding the glassmaking of Beykoz, on the Asian shore of the Bosphorus. According to Istanbul's Directorate of Culture and Tourism, Sultan Selim III sent him to Italy in 1807 to learn the art of glass. Beykoz's best-known glass is çeşm-i bülbül, "nightingale's eye": clear glass with spiralling ribbons, most often blue, a close cousin of the latticino a lampworker twists at the torch.

In Japan, Kagaya Kyūbei, a glassware merchant in Edo (now Tokyo), first made Edo kiriko in 1834: clear glass cut with bold geometric patterns, among them nanako, a mesh of fine crossing lines that shimmers like fish roe. In 1846 the Satsuma domain, in the south, began making glassware, and its lord Shimazu Nariakira built it into a full-scale factory. Satsuma's researchers succeeded in making red glass, which for a time only Satsuma could produce in Japan, and Satsuma kiriko is known for bokashi, a soft gradation of colour where the cut passes through a coloured layer into clear glass.

In Hebron, in Palestine, glassblowing has been practised for centuries, with an old-city neighbourhood once named for its glassworkers. The Natsheh family were the only ones to keep the traditional methods alive; today they blow recycled glass at their workshop at the northern entrance to the city, where visitors can watch them work.

Other traditions live on in daily life rather than in museums. In Ghana, the Krobo people of the south-east make powder-glass beads from crushed bottles and old beads, packed into hand-made clay moulds around a cassava stalk that burns away to leave the hole. And in Firozabad, in Uttar Pradesh, known as India's city of glass, workers make almost half of all the glass bangles produced in India.

In the twentieth century, glass from beyond Europe also shaped studio glass. Kyohei Fujita (1921–2004) began his kazaribako, decorative boxes, in 1970, wrapping coloured glass and gold and platinum foil around molten glass and blowing it into a mould, inspired by the Rinpa school of painting. Yoichi Ohira (born 1946) moved from Japan to Venice in 1973 to study sculpture and went on, with the Muranese masters Livio Serena and Giacomo Barbini, to transform the murrina.

Borosilicate: a glass for the laboratory

The glass I work with is much younger. In 1884 the chemist Otto Schott founded a glass laboratory in Jena, Germany, together with Ernst Abbe and Carl Zeiss, to develop glass for science and optics. In 1893 they launched a borosilicate glass for laboratory thermometers: by replacing most of the soda in the recipe with boric oxide, Schott made a glass that expands very little with heat and resists thermal shock. In 1915 Corning, in the United States, launched the same idea for the kitchen as Pyrex. For most of the twentieth century borosilicate was the glass of laboratories and scientific glassblowers; only later did coloured borosilicate become widely available to artists.

A small, focused flame

Lampworking means working glass at a small flame rather than at a furnace. The earliest glass thought to have been made this way is a group of beads from around the fifth century BCE, although the identification is tentative. Small head pendants from the same period, possibly made in Carthage, have eyes of stacked dots and tiny applied noses and lips: details that suggest a narrow flame aimed at one small area at a time.

The lamp and the bellows

For most of its history, the heat came from an oil lamp. The worker blew air into the flame through a pipe, and later with foot-operated bellows, to make it hotter and more directed. The name lampworking comes from that lamp. The practice became widespread in Murano in the fourteenth century, and Venetian beads made a lume, "at the lamp", are documented from the seventeenth century. In the nineteenth century, when the rest of Murano's industry was in crisis, lamp-worked beads were the one sector that kept growing.

The figures of Nevers

In 1565, Louis Gonzaga became Duke of Nevers, in the Loire valley, and brought Italian glassworkers, lampworkers among them, to the town. From the mid-sixteenth to the late eighteenth century, Nevers was the centre of a remarkable sculptural tradition: small glass figures of religious scenes, beggars and even mechanical theatres, built at the lamp around skeletons of iron and copper wire. As early as the seventeenth century, travelling glassworkers also gave lampworking demonstrations to the public, showing people a material that seemed to move like liquid in their hands.

Women at the flame

The history of glass is often told through workshops of men, but at the lamp women were there early, and in public. The historian and archivist Rebecca Hopman, who researches itinerant glassworkers, identifies a Mrs. Johnston (sometimes written Johnson) as the earliest known woman itinerant glassworker. In December 1740 she worked at a tavern in Dublin, making figures, animals, swords and ships at a table over an oil lamp and spinning glass thread on a wheel. In 1743 she showed her work in Edinburgh, where the Caledonian Mercury printed a poem in her praise, and her reply.

Glass spinning was one of the great attractions of these shows: a thread of hot glass drawn from the flame onto a turning wheel, fine enough to be woven or looped into plumes, ribbons and even clothing.

In the nineteenth century women ran troupes of their own. From the 1870s to the 1890s, Nora Allen, billed as Madam Nora and as the sole proprietress of her troupe of glassblowers, workers and spinners, toured the north-east and Midwest of the United States with spun glass, glass weaving and a working glass steam engine, and other women followed her example. In the twentieth century the women of the Howell family carried the craft through fairs, shops and television. Grace Howell, who began at the lamp at about five, said she first made her "Bluebird of Happiness" at the 1939 New York World's Fair, and made scientific glass during the Second World War; Marie Howell demonstrated at the 1964 New York World's Fair and wore a dress of spun glass at Rockefeller Center in 1965.

Women also carried traditions that might otherwise have been lost. In Mauritania, the powder-glass beads now known as Kiffa beads, made from crushed glass mixed with saliva or gum arabic, decorated with a needle and fused over open fires, were first documented in 1949; the last traditional makers died in the 1970s, and since the early 1990s organised groups of women bead makers have brought the craft back. Their line continues in the women of today's studios, from Ginny Ruffner to Kristen Frantzen Orr (see Practice: Glass Tips).

Paperweights, instruments and glass flowers

In the mid-nineteenth century the technique was taken into the making of paperweights, mainly in France: tiny flowers and canes worked at the lamp and sealed inside clear glass. At the same time the flame became the tool of scientific glassblowers, who made thermometers, tubes and instruments for laboratories.

The most extraordinary example of what the flame can do may be the Glass Flowers at Harvard University: 4,300 models of 780 plant species, made by the father and son Leopold and Rudolf Blaschka in Germany between 1886 and 1936.

The torch, oxygen and borosilicate

Eventually the oil lamp gave way to the gas torch. Today it burns propane, natural gas or, in some countries, butane, mixed with air or with pure oxygen; quartz, which melts even higher, needs hydrogen and oxygen. Oxygen made it possible to work borosilicate at the torch, and borosilicate moved from the laboratory to the studio. In the 1970s and 1980s the American artist Bob Snodgrass developed and popularised fuming, vaporising silver and gold onto hot glass, and a generation of artists began to treat the torch as a sculptor's tool rather than a technician's.

Lampworking, also called flameworking or torchworking, is the art of shaping glass rods and tubes by melting them in the flame of a torch and working them with hand tools. Where a glassblower in a hot shop gathers glass from a furnace that keeps a whole pot of it molten, a lampworker melts only a small part of a rod at a time, exactly where it is needed. That is what makes the technique so precise: it is made for detail, for small scale, and for forms built gradually, one movement after another.

The temperature at which glass softens depends on its chemical composition, so each glass has its own working range and its own character at the flame (see What Glass Is).

The torch

The technique I practice consists of heating the glass with the flame of a torch that mixes propane (or butane) with oxygen. Adjusting the balance of the two changes not only the heat but the chemistry of the flame, and that chemistry also shapes the colour of the glass, as The Flame and Reading Heat explains. Contemporary lampworkers no longer use oil lamps, but the name has stayed.

The glass

Glass for the torch comes as rods, for solid work, and as tubes, for hollow forms that are expanded with breath. Soft glass melts at lower temperatures and is traditional for beads; borosilicate needs more heat but is far more resistant once finished, which is why it is the glass of all Rain and Rivers jewelry.

Daughter of the sun. Boro gives glass the quality of permanence. It softens her, makes her tender, and holds her in her vulnerability. It offers her a refuge, somewhere to rest.

The tools

Most lampworking tools are simple, and many are made of graphite, which withstands high heat, has low friction and does not stick to molten glass. A marver is a flat surface for rolling and smoothing hot glass; a paddle is a marver on a handle. Tweezers and pliers pull and pinch details. Beads are wound around a mandrel, a steel rod coated with a release agent (clay-based or boron nitride) so the finished bead slides off.

The kiln

The work is not finished when it leaves the flame. Glass that cools unevenly keeps stress inside it and can crack days later, so finished pieces are annealed: held in a kiln at a set temperature and cooled slowly, for borosilicate at around 560–565 °C. The kiln and its protocol are part of The Studio.

What it makes

Beads, figures, marbles, paperweights, scientific instruments, sculpture and jewelry all come from the same gestures: heat, turn, gather, pull, shape. In my own practice, the torch is how Rain and Rivers is made: wearable sculptures in borosilicate glass that hold the fluidity of water and the rawness of stone.

The next entry moves into the studio: how to set up a workspace for the torch, safely, and what equipment it needs.

Before you begin, set up a well-equipped and safe workspace. Here is what you will need.

The bench

Work on a fire-resistant surface, such as a metal table or a bench covered with a non-combustible board, with nothing flammable on or under it. Sit at a height where your forearms can rest on the bench, so your hands stay steady during long sessions. A heat-resistant dish or a bed of vermiculite or fibre blanket gives you a safe place to set down hot tools and pieces.

The torch

The torch is the heart of the bench, and the glass decides which torch you need. Torches fall into two families:

• Air-fuel torches: hand torches and small bench burners that mix a fuel such as propane, butane or MAPP-type gas with the surrounding air. They are simple and inexpensive, and suit soft glass (soda-lime glass such as Italian rods), but they do not run hot enough to work borosilicate beyond the very smallest pieces.
• Oxygen-fuel torches: often called dual-gas torches, they mix a fuel (usually propane) with pure or concentrated oxygen. Oxygen is not a fuel itself; it makes the fuel burn much hotter. The hotter, adjustable flame is necessary for borosilicate.

Most lampworking bench torches are surface-mix: oxygen and fuel leave the torch head through separate ports and only mix in the flame, which makes them quieter and less prone to flashback. Pre-mix torches combine the gases inside the torch and give a small, sharp flame, but are louder and carry more flashback risk (although I was recently recommended this option in order to consume less oxygen). If you are starting with borosilicate, a small surface-mix oxygen-propane bench torch is the usual first step. The flame itself is the subject of the next entry, The Flame and Reading Heat.

Essential tools

• Mandrels: stainless steel rods, dipped in bead release and dried, around which beads are wound. The release coating lets the finished bead slide off.
• Tweezers and pliers: for handling, pulling and shaping hot glass.
• Graphite paddle: to flatten and shape glass. Graphite does not stick to hot glass and conducts heat away more gently than steel.
• Marvers: flat surfaces, usually steel, brass or graphite, used to shape and smooth glass.
• Kiln: used to anneal glass, relieving internal stress and preventing cracks (see the annealing section below).

Glass rods and tubes

Rods and tubes come in many colours and diameters, offering endless possibilities. Start with clear or single-colour rods while you learn the basic techniques. Tubes are used for hollow work; rods for solid forms. Borosilicate rods and colours are made to a coefficient of expansion of about 33, and must not be mixed with soft glass in the same piece: the two expand and shrink by different amounts and will crack apart as they cool. Entry 01, What Glass Is, explains why. Specific torch and kiln models are listed at the end of this entry.

An oxygen-propane torch connects to two separate supplies. Rules for gas installations vary by country, so follow your gas supplier's instructions and your local regulations, and have the installation checked if you are unsure.

Oxygen: concentrator or cylinder

Oxygen concentrators draw in room air and separate out the nitrogen, delivering gas that is roughly 90% oxygen or more, at low pressure. They need no refills or high-pressure regulator, but their output is limited: one or two suit small and medium torches, while larger torches need cylinder oxygen. Check the oxygen flow and pressure your torch needs before choosing.

Oxygen cylinders store oxygen at very high pressure and can supply large torches. They must be fitted with a proper oxygen regulator, kept upright and chained or strapped to a wall or stand so they cannot fall, and moved with the valve protection cap on.

Oxygen makes everything burn more fiercely. Keep oil and grease away from oxygen fittings (they can ignite in pressurised oxygen), never use oxygen to blow dust off yourself, and remember that clothing soaked with oxygen from a leak catches fire very easily.

Propane

Propane is heavier than air. A leak does not rise and disperse: it sinks and collects in low places such as basements, pits and floor drains, where a spark from a light switch can ignite it. For this reason propane cylinders are normally kept outside the workroom, upright, in a ventilated place away from heat and ignition sources, with the gas piped in. Never store or use propane cylinders below ground level. The exact rules on where and how much gas you may keep are set by local regulations and your supplier.

Regulators, hoses and flashback arrestors

• Regulators reduce cylinder pressure to the working pressure the torch needs. Use the regulator made for each gas.
• Hoses should be rated for the gas they carry and kept in good condition. In Europe, oxygen hose is blue and propane (LPG) hose is orange.
• Check valves stop gas flowing backwards into the wrong line, but they do not stop a flame.
• Flashback arrestors stop a flame travelling back up the hose towards the regulator and cylinder. Most also contain a check valve. Fit them on both the oxygen and fuel lines with cylinder supplies, as your torch and regulator makers recommend, and test or replace them as they advise. Some concentrators are sold for use without an arrestor on the oxygen side, because arrestors can restrict their low-pressure flow; follow the concentrator and torch makers' guidance.

Leak testing and daily routine

Test every new or disturbed connection for leaks with a leak-detection solution or soapy water: brush it on with the gas on and watch for bubbles. Never look for a leak with a flame. If a connection bubbles, shut off the gas and fix it before using the torch. If you smell gas (propane has a strong added odour), turn off the supply at the cylinder, put out any flame, ventilate, and do not operate switches until it clears.

For a typical surface-mix torch, light the propane first with a striker, then add oxygen. To shut down, turn off the oxygen first, then the propane. At the end of a session, close the cylinder valves and bleed the gas from the lines. Always check your own torch's instructions: the order matters.

Safety in lampworking cannot be overstated. These are the main areas, followed by the checklist I follow before every session.

Eye protection

Hot glass gives off two things your eyes need protecting from. The first is sodium flare, the bright yellow-orange glow produced when sodium in the glass enters the flame, which hides what the glass is doing. The second is invisible: infrared radiation from the hot glass and flame, and some ultraviolet. Long-term exposure to infrared is linked to cataracts.

Didymium lenses filter sodium flare very well, which is why they are standard for soft glass. But on their own they give very little infrared protection, so they are not enough for borosilicate, which is worked much hotter. For borosilicate, use lenses made for boro work, which filter infrared and ultraviolet as well as sodium flare, in a shade suited to the size of your flame.

Lens colours and what they are for

Lampworking lenses come in several tints, and the colour is a first clue to what each one filters. Names and ranges change from maker to maker, so always check the protection a lens is rated for, not only its colour.

• Rose, pink or lavender: didymium. Lenses containing the rare earths praseodymium and neodymium, which give them their rose-purple tint and absorb the yellow sodium flare, so you can see the glass itself. For soft glass and small work. Because they block little infrared, safety guides recommend wearing them with at least a shade 2, preferably shade 3, filter.
• Blue-purple: polycarbonate sodium-flare lenses. They do the same job as didymium in a lighter, more impact-resistant plastic. Some are combined with welding filters to reach shade 3 or 5; others give little infrared protection, so check the rating.
• Amethyst: ACE (Amethyst Contrast Enhancer). Filters sodium flare, ultraviolet up to 390 nm and near infrared, and makes reds, blues and greens easier to read. Made for soft glass.
• Green with sodium-flare filtering: boro lenses. Lenses such as Phillips' Green ACE add extra infrared and ultraviolet protection for borosilicate, in shades: 2, a little darker than ACE and still suited to soft glass; 3, for most small to medium borosilicate work; and 5, for very large or very hot pieces. Polycarbonate boro lenses such as BoroView and BoroTruView follow the same logic: 3 for smaller work, 5 for larger.
• Light green: infrared and UV filters without sodium-flare filtering. For kiln work (fusing, slumping and casting) and for looking into a furnace, where heat and brightness matter more than sodium flare.

The shade number tells you how much infrared a lens blocks: to be rated shade 2, a lens must block at least 85% of infrared between 780 and 2,000 nm, and shade 3 at least 91%. Soft glass at a small torch needs at least shade 2; borosilicate, with its hotter oxygen-fed flame, needs at least shade 3, and larger, hotter work a darker shade still. Buy lenses from a glass supplier that publishes their infrared and UV protection.

Anyone sitting close and watching the flame needs the same protection. Clear safety glasses protect only against flying fragments, so they are suitable for people passing through the room, not for watching the torch.

Ventilation and fumes

The torch flame produces combustion gases, and glass releases fumes as it is heated. Some colours contain metals: copper, for example, and certain reds, oranges and yellows are coloured with cadmium and selenium compounds. Copper oxide fumes are toxic; iron oxide fumes are irritating. Fuming, where silver or gold is vaporised in the flame to colour the glass, deliberately produces metal vapour, and so does burning off metal leaf or foil.

That is why active, forced extraction should run throughout the whole session, not only when you notice smoke. A good system captures fumes close to the torch, draws them away from your breathing zone (never past your face) and vents them outdoors. A canopy hood high above the bench catches mainly heat and gives limited protection on its own. An extraction fan also needs makeup air: an opening or supply that lets fresh air into the room to replace what is extracted. Without it the fan moves far less air and can pull fumes back from other appliances. Size the system with your supplier or a ventilation specialist, and follow local regulations. Read the safety data sheet for any colour or powder you use.

Clothing

Wear natural fibres such as cotton, wool or linen. Synthetics like polyester and nylon can melt onto skin when they catch a spark or hot fragment. Avoid loose sleeves and dangling scarves, tie back long hair, and wear closed shoes. Remove rings and bracelets that could trap heat against your skin.

Fire and burns

Keep a fire extinguisher nearby that is suitable for the fires a studio could have. A multipurpose dry powder extinguisher rated for solids, flammable liquids and gas fires (ABC in the European and many other systems) is a common choice; check the classes on the label against your local fire safety advice. With a gas fire, the first action is always to shut off the gas supply: a gas fire put out while the gas keeps flowing can re-ignite explosively. A fire blanket is also useful.

Glass stays hot long after it stops glowing. For a burn, cool it under cool running water for 20 minutes, remove nearby jewelry and clothing unless it is stuck to the skin, and cover it loosely with cling film. Do not use ice, butter or creams. Seek medical help for large or deep burns, or burns to the face.

My checklist before every session

• Ventilation: active forced extraction throughout the whole process.
• Eye protection: boro-rated lenses for whoever is at the torch and anyone watching closely.
• A fire-resistant surface for all work, such as a metal table, clear of anything flammable.
• Gas: hoses and connections checked, flashback arrestors in place, no smell of gas.
• An annealing protocol decided before you start: a kiln preheated to the annealing temperature for borosilicate (around 565 °C / 1050 °F), or, for very small pieces only, a planned flame-annealing and slow-cooling method.
• Water and a burns kit on the table, and a fire extinguisher nearby.
• Steel wool is only handled with metal tongs near the flame: fine steel wool catches fire very easily.
• Mineral powders: avoid inhaling them; wear a suitable dust mask or respirator at every stage where powders are handled, and keep their containers closed.

When glass cools, the outside cools and stiffens before the inside. That difference locks stress into the piece, and stressed glass can crack hours, days or even weeks later. Annealing removes this stress: the piece is held at a temperature where the glass can relax internally without losing its shape, then cooled slowly and evenly.

The annealing point of laboratory borosilicate such as Duran is 560 °C, and lampworkers usually anneal borosilicate at around 565 °C (1050 °F). This is noticeably hotter than soft glass, which anneals at roughly 500 to 520 °C depending on the brand, so a schedule written for soft glass will not properly anneal borosilicate. Always check the annealing temperature your glass maker recommends.

A typical approach:

1. Preheat the kiln to the annealing temperature before you start working, and keep it there.
2. Garage the work: place finished pieces into the hot kiln straight from the flame, before they cool, so they never pass through a sharp temperature drop.
3. Soak at the annealing temperature. The hold time depends on the thickness of the thickest part: thicker pieces need much longer.
4. Cool slowly through the range just below annealing, down past the strain point, then let the kiln cool to room temperature with the door closed.

Flame annealing (evenly heating the finished piece in a soft, cooler flame) followed by slow cooling in fibre blanket or vermiculite can work for very small, thin pieces, but it is less reliable than a kiln. When choosing a kiln for borosilicate, check that its maximum temperature comfortably exceeds the annealing temperature, and if you anneal beads, look for a mandrel port or a door that lets you load pieces quickly.

A summary of torches and kilns often recommended for borosilicate. Models, names and prices change, and some are discontinued, so check current specifications, gas requirements and maximum temperatures with the maker or a glass supplier before buying.

Torches

Kilns

With the bench set up and the gas safely connected, the next step is learning to read the flame itself, in The Flame and Reading Heat.

The temperature at which glass softens depends on its chemical composition, and borosilicate needs far more heat than soft glass (see What Glass Is). That is why borosilicate is worked with a fuel burned in oxygen rather than in air. Propane burned in air reaches a theoretical maximum of about 2,000 °C; propane burned in pure oxygen can reach roughly 2,800 °C under ideal conditions. Real flames run below these figures, and the glass never approaches flame temperature: it only needs roughly 1,000–1,250 °C to be fully workable.

Anatomy of the flame

• The inner cone: the small, sharply outlined blue cone at the face of the torch. This is where the gases first react. The gas inside the cone itself is not yet fully burnt.
• The hot spot: the hottest part of an oxy-fuel flame sits just beyond the tip of the inner cone. Glass held a little in front of the cone heats fastest.
• The outer envelope (or feather): the larger, paler flame around and beyond the cone, where combustion finishes using oxygen from the torch and from the surrounding air. It is cooler, and it gets cooler the further you move from the torch.

Distance is therefore one of your controls: much of torch work is simply choosing where in the flame the glass sits.

Oxygen and fuel

Complete combustion of propane needs five volumes of oxygen for every volume of propane (C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O). In practice a torch flame also draws oxygen from the room, so lampworkers often quote torch ratios of roughly 2 to 4 parts oxygen to 1 part propane, depending on the torch and the flame size. Treat these as background: you set the flame by eye and ear.

Types of torch

• Surface-mix torches keep oxygen and fuel in separate channels until they leave the torch face, where they mix and burn. Most torches sold for glass work are surface-mix. Because the gases never mix inside the body, they are less prone to flashback.
• Premix torches mix the gases inside the torch before they exit. They give a tight, sharp flame but need more care, as mixed gas inside the torch can burn back.
• Air-propane torches, which draw air into the flame, are suitable for soft glass but generally struggle to bring borosilicate to working heat.

Whatever the torch, follow the manufacturer's instructions for lighting and shutting down, fit flashback arrestors and check valves where your supplier and local regulations require them, and keep the room well ventilated. Most oxy-propane torches are lit with propane first, then oxygen added, and shut down oxygen first, but your torch's manual is the authority. Gas set-up and ventilation are covered in The Studio.

Flame control is one of the most important skills in borosilicate work. Chemistry matters because many glass colours contain metals, and metals change colour depending on how much oxygen surrounds them. In chemical terms, oxidation means losing electrons and reduction means gaining them. An oxygen-rich flame favours higher oxidation states; a fuel-rich flame, full of carbon monoxide and unburnt carbon, takes oxygen away from metal oxides and pushes them towards lower oxidation states or even the bare metal.

Neutral flame

A balanced ratio of oxygen and propane.

A sharp, well-defined blue inner cone with a thin, translucent outer envelope. Little or no yellow or orange at the edges. A steady, clean hiss.

Used for: general melting of borosilicate, fuming with silver, clean joins, tube work.

Oxidising flame

An excess of oxygen.

The inner cone narrows and shortens; the flame becomes compact and sounds more aggressive, with an intense hiss. The cone is very sharp and can look blue-violet. Because the heat is concentrated in a smaller space, it feels hotter and more focused, although a strongly oxidising flame does not reach a higher peak temperature than a neutral one.

Used for: clean, bright joins in clear borosilicate (many workers run slightly oxidising for clear), and keeping copper in its oxidised Cu²⁺ state, which colours glass blue to turquoise and green. Iron behaves similarly: oxidised Fe³⁺ leans yellow-brown, reduced Fe²⁺ leans blue-green. Too much oxygen can make some colours boil, so "oxidising" in practice usually means slightly oxidising.

Reducing flame

An excess of fuel. The fuel takes oxygen away.

The inner cone grows, softens and loses definition. The outer envelope expands and becomes slightly luminous and feathery; a faint yellow-orange touch at the edges is normal. The sound is softer and more muffled than a neutral flame. The effective temperature is lower, and the flame is rich in unburnt carbon and carbon monoxide, which is one more reason good ventilation is essential.

Used for: striking copper towards warm tones, drawing out metallic lustre, encasing in a reducing envelope, softening an oxidised fume before striking. Here is what actually happens with copper: Cu²⁺ (blue-green) is reduced to Cu⁺, which on its own is nearly colourless in glass, and then to metallic copper, Cu⁰. Tiny particles of metallic copper, or of cuprous oxide (Cu₂O), give the reds, ambers and browns; metallic copper at the surface gives a coppery sheen. Silver works in a comparable way: silver ions reduced to metallic silver particles produce yellows and ambers, and on many silver-bearing colours a reducing flame brings up a metallic, iridescent surface. Surface reduction effects can be burnt away by a later hot oxidising flame. The colour side of this is explored in Colour in Glass.

Bushy and pinpoint flames

A bushy flame, broad and soft, heats larger sections evenly. It is ideal for sculpting and for large gathers that need consistent heat to avoid cracks and thermal stress.

A pinpoint flame concentrates intense heat on a small area: ideal for fine lines, attaching small components and detail on pendants and hollow forms.

Thermal shock and preheating

Glass cracks when one part expands or contracts much more than another. Borosilicate expands only about 3.3 millionths of its length per degree (written COE 33), roughly a third as much as typical soft glass, which is why it tolerates the flame so well. It is not immune. A thick rod pushed straight into a hot flame can crack or pop, and fragments can fly, which is one more reason to wear proper eye protection at all times.

• Introduce rods and tubes in the cool tail of the flame and move them slowly forwards, rotating all the time.
• The thicker the glass, the more slowly it should be warmed. Scratches and chips on the surface make cracking more likely.
• Thin parts lose heat in seconds. Pass the whole piece through the flame regularly so that thin sections, joins and fine details never fall far behind the rest.
• Never quench hot glass.

Working in and out of the flame

You do not shape glass only in the flame. A common rhythm is to heat a zone in the flame, then lift it out to shape it, where the glass holds its position better and you can see clearly. While you turn, soft glass stays centred; when you stop, it sags. Many workers also keep a heat base: the whole piece stays evenly warm in the outer flame while only the working area is brought to full softness, so cold areas do not crack next to hot ones.

Glass has no single melting point. It softens gradually, and manufacturers describe that softening with a set of standard reference points defined by viscosity (how stiff the glass is). For borosilicate 3.3 (the family that includes Duran, Simax and Pyrex 7740), the data sheets agree closely:

So borosilicate's working range runs from roughly 820 °C, where it first starts to move, up to around 1,250 °C, with most gathering, shaping and pulling happening in the upper part of that range. The annealing range lies between the strain point and the annealing point, roughly 510–565 °C: a piece is held near the annealing point so stress can relax, then cooled slowly through the strain point. Annealing is covered further in Finishing, Combining and Troubleshooting.

Most solids start to glow a very dull red at about 525 °C, shifting through red, orange and yellow towards white as they get hotter. The table below pairs the traditional glow colours with what borosilicate is doing at those temperatures. Read it as a rough guide, not a thermometer.

Why glass is harder to read than metal

Glow-colour charts were made for iron and steel, and even they disagree by 100 °C or more. Glass is harder still:

• Clear glass glows weakly. A transparent material gives off much less visible light than a dark, opaque metal at the same temperature. Clear borosilicate can be well above its softening point while showing only a faint glow.
• Much of the orange is not glow at all. The bright yellow-orange you see around hot glass is mostly sodium flare: sodium in the glass, and in dust or fingerprints on its surface, emits intense yellow light at about 589 nanometres in the flame. Borosilicate contains far less sodium than soft glass (a few per cent of sodium oxide rather than around 15 per cent), so it flares less, but the flare still masks the true glow.
• Your eyewear changes the picture. Didymium lenses filter exactly that sodium yellow, which protects your eyes and lets you see the glass, but it also removes much of the colour you would otherwise use to judge heat. Lenses made for borosilicate usually add stronger infrared protection, darkening the view further. Never take your glasses off to read colour; choose eye protection rated for borosilicate work by its supplier.
• Light and thickness matter. A glow obvious in a dim room vanishes in daylight, and a thick gather glows more than a thin stringer at the same temperature.

Because colour is so unreliable, experienced lampworkers mostly read heat by how the glass behaves. What matters is viscosity, and viscosity shows itself in movement:

• Edges and corners: when sharp edges start to round over and the surface turns glossy, the outer layer has passed its softening point. This is how fire polishing works.
• Response to gravity: stop turning for a moment and watch. Glass that holds its shape is stiff; glass that sags slowly is at shaping heat; glass that drops off-axis within a second or two is very hot.
• Gathering: a rod end that pulls itself into a round ball by surface tension is at working heat. If it only mushrooms unevenly, it needs more heat or more time.
• Resistance to tools: stiff glass pushes back against a paddle or tweezers and can crack; soft glass gives smoothly; overheated glass sticks and smears.

Heat also has depth. The surface softens first, while the core lags behind, so a piece can look ready on the outside while still being stiff inside. Slower heating in a softer flame gives a gather that is soft all the way through, which is what you want before shaping anything substantial.

With the flame understood and the heat readable, the next step is to put both to work: Techniques at the Torch moves from the first gathers to advanced shaping.

Introducing glass to the flame

Borosilicate expands very little when heated (its coefficient of expansion is about 3.3 × 10^-6 per °C, roughly a third of soft soda-lime glass), which is why it tolerates heat so well. It can still crack or "pop" if a cold rod goes straight into a hot flame, especially thicker rods, tube ends and many colour rods. So start every rod by flashing it: pass the tip in and out of the cooler outer flame for several seconds, then move it gradually into the heat, always working from the tip back.

Once the tip is warm, rotate the rod constantly so it heats evenly on all sides. Keep out of the hottest part of the flame until the glass is warm through.

Gathering

Gathering is collecting molten glass into a workable mass. As the end of the rod softens, keep turning it and let surface tension pull the glass back into a ball, or "gather". Feed more rod into the flame to make it bigger. Turning steadily keeps the gather centred: stop, and gravity pulls it off-axis.

Points and punties

A punty is a handle: a scrap rod (or a pulled point) fused to the glass so you can hold and turn the piece while you work on the other end. To attach one, heat the end of the punty and a small spot on the piece until both are soft, touch them together, and let them fuse before you rely on the joint. A good punty sits on the axis of the piece, so it turns true, and is joined just firmly enough to hold yet come off cleanly afterwards.

On tube, you make a "point" by heating a band of the tube all the way round until it is soft, then pulling the two ends apart outside the flame so the tube narrows to a thin neck. Points serve as handles and air passages, and let you close a tube neatly.

Shaping and forming

Once gathered, the glass can be shaped with heat, gravity and tools. Graphite paddles flatten and smooth, tweezers pull and push, and gravity does a surprising amount of the work. Shape in short passes, returning to the flame whenever the glass stiffens. Forcing stiff glass leaves chill marks and builds stress.

Mandrels and bead release

Beads are wound on a mandrel: a thin stainless steel rod dipped in bead release, a clay-based slurry that stops the glass bonding to the steel and leaves a clean hole. For borosilicate, use a release rated for its higher working temperatures. Let it dry completely, warm the coated mandrel in the flame, then wind molten glass onto it while turning, keeping both ends of the bead seated on the release. After annealing, clean the release out of the hole under water to keep the dust down.

Adding colour

Glass can be coloured with pre-coloured rods, with frit (crushed coloured glass) or with powders. Roll the hot gather in frit, then reheat gently and melt it in. Use only colours made for borosilicate: soft glass colours are not compatible and will crack. Colour is the subject of Colour in Glass.

Stringers, ribbons and twisties

Stringers are thin rods of glass, often 1–2 mm across, used for drawing lines, dots and fine details. To make one, heat a small gather evenly, take it out of the flame, wait a moment, then pull it apart steadily, either with a second rod or tweezers. Ribbons are flattened canes, pressed with a paddle or pulled from a flattened gather, for broader bands of colour. Twisties are canes in which two or more colours spiral around each other: build a striped gather between two punties, heat it evenly, then twist and pull at the same time.

Keeping a heat base

One of the habits that separates a piece that survives from one that cracks is keeping the whole piece warm while you work on one part of it. Small areas cool very quickly, and a hot spot next to a cold one creates stress. Wave the whole piece through a soft flame regularly so it never gets cold before it is finished.

Flame annealing and kiln annealing

When you finish, flame anneal: warm the whole piece evenly in a soft, bushy flame so it is at one temperature, then put it straight into a hot kiln. Kiln annealing relieves the stress that working leaves in the glass. Schott gives the annealing point of Duran borosilicate as 560 °C, and kiln schedules for borosilicate jewelry typically hold at around 560–565 °C (about 1,040–1,050 °F), then cool slowly. Times depend on thickness: follow your manufacturer's guidance. More on this in Finishing, Combining and Troubleshooting.

Sculpting in glass

Sculpting means shaping molten glass into more intricate forms, such as animals, flowers or abstract shapes. It requires precise control of temperature and movement: heat only the part you are shaping, with a smaller, sharper flame, while keeping the rest of the piece warm.

Encasing

Encasing means layering clear glass over a coloured base to create depth and dimension, and to protect delicate surface work. It is especially popular for marbles, pendants and the cores of beads. Apply the clear in overlapping wraps, slowly so you don't trap air, then melt it smooth.

Implosion

Implosion is one of the hallmark techniques of borosilicate, and the pattern you see in many boro pendants and marbles. Lines or dots of colour are laid along a clear form, then the face of the piece is heated so the glass melts inward and draws the colour towards the centre and back. Repeating this builds a three-dimensional, often flower-like pattern seen through a lens of clear glass. Because it means long, hot working of thick clear glass, borosilicate's resistance to thermal stress suits it especially well.

Fuming, often taught at this stage, is covered in the next entry: Colour in Glass.

Borosilicate comes in a huge range of tubing as well as rod, and much of its character comes from tube work, which grew out of scientific glassblowing.

Hollow work and blow tubes

Hollow forms, such as hollow beads, vessels or ornaments, are made from glass tubes rather than rods. Close one end of the tube, heat a band of glass until it is soft, take it out of the flame and blow gently into the open end while rotating it, so the glass expands evenly: like traditional glassblowing, on a smaller scale. A blow tube, a length of flexible hose with a mouthpiece attached to the open end of the glass, lets you blow while both hands turn the work, and keeps your face away from the flame.

• Even wall thickness: uneven walls are weak spots. Heat evenly all round, then blow in short puffs out of the flame; glass that is too hot balloons where it is thinnest.
• Let it breathe: air expands as it heats. Any closed hollow piece needs a way for the air to escape while you work it, or it can bulge or burst.

Seals: butt seals and ring seals

In tube work a "seal" is a fully fused joint. The butt seal joins two tubes of the same diameter end to end: heat both ends until they are soft, bring them together squarely, then heat the joint all the way round while blowing gently so the wall evens out and the seam disappears. A ring seal fixes one tube inside another, the inner tube passing through the end of the outer one, so the two are joined in a ring of glass. It is used for double-walled and internal structures, and is harder because the inner tube is heated through the outer one.

A good seal has the same wall thickness as the tube on either side and no visible line or bubble at the join. A thin or lumpy seal is where a piece cracks later.

Tacking before sealing

For large or complex pieces, sections can first be tacked together with small spot joins, so the placement can still be checked and adjusted. Once the placement is right, the joints are heated and fully fused into proper seals. A tack is never a finished joint: the small contact area concentrates stress, so every tack has to be properly sealed, and the whole piece kept warm while you do it. (This step is sometimes loosely called a "cold seal", but that is not a standard lampworking term: the usual words are tack, then seal.)

The lathe: perfecting symmetry

For large, symmetrical tube work, such as goblets, vases or large hollow sculptures, a glass lathe holds the tube in chucks at both ends and rotates it evenly while you apply heat. It is more common in large and scientific work than in jewelry.

• Centre the glass in the chucks so it runs true without wobbling.
• Heat evenly with a soft, bushy flame before you start shaping; uneven heat causes distortion.
• Choose the speed: fast enough that soft glass doesn't sag, slow enough that it doesn't throw outwards. Larger diameters generally need slower speeds.

Whatever you make, the order of work matters as much as the individual techniques. A simple encased pendant shows the sequence:

1. Prepare: lay out rods and tools, check the kiln is at temperature, and put on eye protection before lighting the torch.
2. Flash and gather: warm a clear rod gradually, then build a gather at its tip.
3. Add colour: lay on stringers, dots or frit, melting each in before the next.
4. Encase and shape: wrap the work in clear, melt it smooth and shape it with heat, gravity and a paddle.
5. Form the bail: add the loop or hole the pendant will hang from while the body is still warm.
6. Flame anneal the whole piece evenly and put it straight into the kiln.

Throughout, keep the heat base: no part of the piece should cool much before it reaches the kiln.

These patterning techniques are most closely associated with the glassmakers of Murano, in Venice, who perfected them in the furnace, with teams pulling canes many metres long. The same principles work at the torch on a smaller scale. They demand precision and practice, and reward it.

Filigrana: the family name

Filigrana (filigree glass) is the generic name for blown glass decorated with canes laid lengthwise: canes with threads of white, and sometimes coloured, glass inside clear glass. The style originated on Murano in the 16th century and was revived there in the 20th. Its main types are vetro a fili (straight threads), vetro a retorti (twisted threads, also called zanfirico) and vetro a reticello (a net pattern).

Latticino: twisted cane

Latticino comes from latte, milk, after the opaque white glass, lattimo, traditionally used for the threads. The name is older than filigrana but still widely used for twisted cane.

• Prepare the canes: at the torch, build a clear gather with stripes of white or colour on it (or a core with threads around it), then heat it evenly between two punties.
• Control the twist: twist and pull at the same rate for a consistent spiral; a lathe helps with longer canes.
• Use it: as decoration in marbles, beads or sculpture, encased in clear glass for depth.

Reticello

Reticello is the classic net pattern: a fine mesh of white threads in clear glass, with a tiny air bubble trapped in each diamond of the net. In the furnace it is made from two layers of twisted cane spiralling in opposite directions: one blown as a cup, the other pushed inside it, with air caught between their ribs forming the bubbles. The term is also used loosely for any criss-cross pattern.

• At the torch, lay twisted canes in parallel around a form, then lay a second set twisted in the opposite direction over or inside them.
• Use a soft, even flame, rotate constantly so the pattern doesn't distort, and encase it in clear glass, working slowly to avoid unwanted bubbles.

Millefiori and murrine

A murrina (plural murrine) is a slice cut from a cane with a pattern or image running through it, so the design shows in cross-section. Millefiori, Italian for "a thousand flowers", is the name for many flower- or star-shaped murrine used together. Patterned cane is ancient, and Venetian glassmakers revived murrine on Murano in the early 16th century. The word "millefiori" itself only entered English in the 19th century.

• Build a cane with a floral or geometric design, by layering colour over a core, adding stripes and dots, or bundling smaller canes together, then heat the whole mass evenly.
• Pull slowly: stretching the cane with even heat and tension keeps the cross-section in proportion as it shrinks; uneven heat distorts the pattern.
• Slice and embed: cut thin slices, preheat them so they don't crack, place them on the surface of the piece and encase them in clear glass.

Once the techniques are in your hands, the next step is the colour that runs through them: Colour in Glass.

When we add metals to glass, the metal atoms have electrons that can absorb energy from light. White light contains every colour: red, orange, yellow, green, blue, violet. Each colour corresponds to a different energy. When metals absorb some colours, they remove them from the light passing through the glass. What remains is the colour we see. Cobalt glass, for example, absorbs mainly yellow, orange and red light, so what reaches your eye is blue.

Metals are the most common way to colour glass, but not the only one. There are four main mechanisms.

1. Dissolved metal ions

Transition metals such as cobalt, copper, iron, chromium, manganese and nickel have electrons that absorb particular energies of visible light. For example: Co²⁺ gives blue, Cr³⁺ green, Mn³⁺ purple. These colours are usually transparent because the ions are fully dissolved in the glass network. The exact shade depends on the ion's oxidation state (how many electrons it has lost) and its surroundings in the glass, which is why one metal can give different colours in different recipes and flames.

2. Metallic nanoparticles

In some glasses, the metal doesn't stay dissolved as an ion but forms tiny metallic particles, a few to a few tens of nanometres across. These interact with light through surface plasmon resonance: the free electrons on the particle's surface oscillate together when light hits them, and the frequency of that oscillation decides which wavelengths are absorbed and scattered. The particles form when metal ions are reduced to neutral atoms, which group together into nuclei that grow.

The colour depends on the metal, the particle size and the concentration. Gold is the classic case: small particles absorb green light, so the glass looks ruby red; as they grow, the absorption shifts to longer wavelengths and the colour moves towards purple and blue, and very large particles scatter light and make the glass cloudy or brownish. Silver particles absorb blue-violet instead, giving yellows and ambers. So there is no universal rule that smaller particles mean cooler tones: for gold it is the opposite.

3. Crystals in the glass

Here microscopic crystals do the work, in two different ways. Cadmium sulphide (CdS) and cadmium sulphoselenide (CdSSe) absorb all light above a certain energy: CdS cuts out blue and looks yellow, and adding selenium moves the cut-off through orange to red. When these crystals are very small the glass can be transparent (the red of old traffic-light lenses); larger crystals, or an added opacifier, make it opaque. Other crystals, such as tin oxide, absorb nothing but scatter all light, making the glass white and opaque.

4. Colour centres (structural defects)

A less familiar mechanism. Sometimes colour comes not from a pigment but from defects in the glass structure that trap electrons. High-energy radiation such as X-rays or gamma rays creates these colour centres and can turn clear glass brown, which is why cerium is added to glass meant for radiation environments.

A metal-by-metal guide

"Oxidised" and "reduced" describe the chemistry; at the torch, an oxygen-rich flame pushes towards the first and a fuel-rich flame towards the second.

Glass is transparent because its structure lets light through: it scatters very little of it and barely absorbs it.

Glass is made mostly of silicon and oxygen. The electrons in these atoms absorb ultraviolet and infrared, but not visible light. And because glass has no grains or crystal boundaries inside it, there is nothing for light to bounce off. So visible light simply passes through.

Glass can be made opaque by adding special compounds to the molten glass.

Tin oxide (SnO₂) is the classic example; fluorides and calcium phosphate are others. They form microscopic crystals as the glass cools, and the crystals scatter light in every direction, so the glass looks milky or opaque.

Transparent = everything is mixed at the atomic level.
Opaque = there are tiny "solid things" inside that scatter the light.

The manganese story

Since at least Roman times, manganese has been used (alongside antimony) to decolourise glass, neutralising the greenish tint of the iron in sand. It works in two ways. Chemically, it turns strongly coloured blue-green Fe²⁺ into much paler Fe³⁺. Visually, a little purple Mn³⁺ sits opposite green on the colour wheel, so the remaining tints cancel out. The result: glass that looks colourless.

There is a side effect. Years of ultraviolet light from the sun turn the nearly colourless Mn²⁺ back into purple Mn³⁺, and clear glass slowly goes lavender. This is solarisation, and collectors call the result "sun-purpled" or "sun-coloured amethyst" glass.

In American bottle making, manganese was the standard decolouriser from around the 1870s–1880s until about 1920. The First World War disrupted supply: Germany is often named as the source, though research shows much of it came from India and Brazil. The bigger reason was technological: manganese worked well in the pots of hand production but poorly in the large tanks feeding the new automatic bottle machines, and selenium was cheaper. That is why purple-tinted bottles and windows usually date from before the 1920s.

Selenium and cerium

Selenium decolourises the same visual way: a trace of pink, often balanced with a little cobalt, masks the green. Cerium oxide works chemically, oxidising Fe²⁺ to Fe³⁺, and also resists radiation browning. Today, purer, low-iron raw materials do much of the work.

Striking

Some glasses look transparent or pale while hot, but colour appears when they are gently reheated. The colour depends on particles that haven't formed yet. When the glass is very hot, the metals are dissolved as ions. As it cools, tiny nuclei form; reheated to a moderate temperature, these grow into particles large enough to colour the glass. Striking glasses usually contain gold, silver, copper, selenium or cadmium.

Step by step: let the piece cool until the glow has gone. Reheat it gently (in the kiln, or in a cooler part of the flame) without bringing it back to a glow. Nanoparticles form. The colour appears. Stop when you like it: overstriking can make some colours dark, muddy or opaque, and reheating to a full glow can dissolve the particles so you have to start again.

Reduction

When the flame has little oxygen, some metals are reduced. This can produce metallic surfaces, iridescent colours and darker tones. It is very common with silver-rich glasses, and with gold and copper glasses. Reduction effects sit on the surface, so an oxidising flame can burn them off again.

Fuming

Fuming means vaporising fine silver or pure (22–24 carat) gold in the flame so that its vapour settles as a microscopically thin film on hot glass. A small piece of metal, usually stuck to the end of a glass rod, is held in a hot, oxidising flame, and the piece is held downstream so the vapour washes over it. Encased in clear, the film gives soft colours.

The thickness of the film decides the result. Light silver fuming looks pale yellow to amber when you look through it, and over a dark background heavier silver often reads as blue. Gold gives soft pinks that deepen to rose and magenta, and heavy gold looks metallic or mirror-like. Both keep developing in the kiln, so judge the colour after annealing.

The variables that control the result:

• Glass temperature: warm enough for the vapour to stick, not so hot that it softens; many lampworkers fume just below a visible glow.
• Distance from the metal along the flame: a few centimetres is typical; closer generally deposits more.
• Angle to the vapour stream: square-on gives an even layer, oblique a gradient.
• Speed and number of passes: slower passes deposit more, and each extra pass adds depth.
• Flame: hot and oxidising to vaporise the metal (gold needs more heat than silver), then neutral to encase. Strong reheating of unencased fume can burn it off.

Encased fuming: fume an inner layer, encase it in clear glass, then add more layers. The result is dimensional, almost holographic. You can also layer silver over gold for unusual hues.

Reactive glasses

Some glasses react with each other where they touch. The classic examples involve sulphur: a sulphur-bearing glass meeting one that contains copper, silver or lead forms dark metal sulphide at the boundary, seen as a dark line or halo. A well-known soft-glass pairing is ivory with copper turquoise. Used on purpose, these reactions draw fine outlines; by accident, they muddy a design.

Dichroic glass

Dichroic glass carries a coating of many very thin layers of metal oxides and silica, evaporated by an electron beam in a vacuum chamber: often 30 to 50 layers, less than a thousandth of a millimetre in total. Light reflecting from the layer boundaries interferes, so one band of colours is reflected while the complementary colours pass through, and both shift with the viewing angle. It is optical interference, the same physics as a soap bubble. Modern coatings grew out of NASA-related research into optical filters. For lampworking, use dichroic made for your glass type, encase it, and avoid overheating, which can dull or break up the coating.

Working with colour at the torch

• Overlay transparent colours: a base colour with a different transparent hue on top creates colour that seems to shift with the light.
• Gradients: vary the heat across a piece to create colour gradients, especially in sculptural work and with striking colours.
• Preheat colour: keep colour rods warm in a cooler part of the flame before adding them, for even colour and less thermal shock.
• Mind the thickness: transparents look far lighter in a thin stringer than in a thick gather, and cobalt can look almost black in a heavy piece.

Why the borosilicate palette is different

Borosilicate began as laboratory glass, so coloured boro rod for studio artists is recent compared with the long tradition of coloured soft glass. It is also harder to colour: boro is high in silica and boron, low in alkali and worked much hotter, so colourants dissolve and behave differently; volatile ingredients such as sulphur, selenium and cadmium are harder to hold; and every colour must match the low expansion of clear boro (COE around 33). For a long time boro offered fewer and paler colours, with bright reds and yellows especially difficult. The gap has narrowed: silver- and gold-rich striking colours became a boro strength, and brighter cadmium-based opaques followed. The range is still smaller than soft glass, and more dependent on heat and flame. More on the glass itself in What Glass Is: Structure, Types and Borosilicate.

Testing colours

• Make a labelled test piece for every new colour, with maker, name and batch number: batches can vary.
• Test it as you will use it: thin and thick, encased and on the surface, struck and unstruck, neutral and reducing flame. Note what you did.
• Anneal before judging, since many colours shift in the kiln.
• Look in different light: daylight and indoors, against white and dark, with light passing through and reflecting off.
• Test combinations for reactions and compatibility before a finished piece.

Safety with colour

These add to the studio basics in The Studio: Workspace, Safety and Equipment.

• Cadmium and selenium colours: cadmium compounds are toxic and carcinogenic, and selenium compounds are toxic too. In finished glass they are bound in, but overheating these colours can release fumes. Work with good ventilation, don't boil or burn them, read the supplier's safety data sheet, and keep such glass away from food use.
• Fuming: metal fumes are hazardous to breathe. Fume only with strong local extraction that carries vapour out of the studio. Use only fine silver and pure gold: never sterling, plated metal, solder or unknown metal, which can contain copper, zinc, lead or cadmium.
• Dust and hygiene: grind coloured glass wet, wipe dust with a damp cloth, wash your hands after handling rods and frit, and don't eat or drink at the bench.

Always follow your suppliers' safety data and local regulations.

With colour understood, the next step is the finished piece: Finishing, Combining and Troubleshooting.

As glass cools, the outside stiffens before the inside. When the core finally contracts, it pulls against a surface that has already set, and that tension stays locked in the glass as stress. Stressed glass can look perfect and still crack days or months later. Annealing removes it by holding the piece where the glass can relax without losing its shape, then cooling it slowly and evenly. See What Glass Is.

The temperatures that matter

These are manufacturers' figures for laboratory borosilicate 3.3 glasses such as Duran and Pyrex 7740. Coloured borosilicate rods are close but not identical, so check your glass maker's recommendation. Many lampworkers set their kilns to around 565 °C (1,050 °F) for boro.

Kiln-carrying and batch annealing

The safest practice is to kiln-carry (also called garaging): keep the kiln at annealing temperature while you work, and put each piece in as soon as it leaves the flame, before it has had a chance to cool unevenly. When the session ends, the kiln runs its soak and cooling cycle on everything inside.

Batch annealing means letting finished pieces cool in a fibre blanket or vermiculite and annealing them together later. Small, solid, evenly thick boro pieces often survive this, but complex pieces, thick-to-thin joins and inclusions can crack before they reach the kiln. If you batch anneal, bring the kiln up slowly from room temperature so the cold pieces are not shocked.

An example schedule

Schedules depend on thickness, the size of the piece and your kiln, so treat this as an illustration, not a rule:

1. Garage at about 565 °C while you work.
2. Soak once the last piece is in: commonly 30 to 60 minutes for small jewelry-scale pieces, longer for thick ones.
3. Cool slowly through the strain point, for example at a few tens of degrees per hour, to around 480 °C (900 °F). A fast cool here puts stress back in.
4. Let the kiln cool naturally to room temperature with the door closed.

Don't push the soak temperature much higher to save time: closer to the softening range, fine details begin to slump.

Re-annealing, not "thermal cycling"

A correctly annealed piece does not get stronger by being heated and cooled again. Re-annealing is a repair, for pieces that cooled without a proper anneal, show stress, or have been reworked. Never put a cold, annealed piece straight back into the flame: warm it gradually in the kiln, work it, then anneal it again.

Checking stress with polarised light

Stress in glass is invisible to the eye but visible in polarised light. A polariscope is two polarising filters set at right angles over a light, so the field looks dark. Unstressed glass between them changes nothing. Stressed glass becomes doubly refracting and lets light through, so stress shows as bright or coloured bands, typically around joins, thick areas and inclusions. A well-annealed piece looks even and quiet. Two sheets of polarising film over a light make a simple version. It is not a perfect test, but it shows what your annealing is actually doing.

Cold working covers every technique used to finish a piece after it has been annealed. Always anneal first: cold working a stressed piece is a reliable way to crack it.

Safety first: dust

Grinding, drilling, polishing and blasting all produce fine dust. Glass itself is amorphous, not the crystalline silica that causes silicosis, but fine glass and abrasive dust is still harmful to breathe. The core rules:

• Work wet. Run grinders, drills and rotary tools with a constant flow of water. It keeps dust out of the air and the glass cool.
• Wear a respirator rated for fine particles (FFP3 or P100) for any dry work, sandblasting included, plus safety glasses or a face shield.
• Clean wet. Wipe and mop rather than sweep, so settled dust doesn't go back into the air.
• Keep electrics safe around water: use equipment made for wet use on a protected circuit.

Sandblasting

Despite the name, glass should never be blasted with sand. Silica sand breaks down into respirable crystalline silica, which causes severe and permanent lung disease, and its use for blasting is banned or tightly restricted in many countries. Use a dedicated abrasive in an enclosed blasting cabinet with a dust collector:

• Masking: vinyl, resist tape or photoresist film protects the areas you don't want to etch, for precise, clean designs.
• Textures: adjust pressure, distance, grit size and time for anything from a subtle frost to a deep carve.
• Contrast: sandblast some areas and leave others polished, or use it to bring out details in sculpted work.

Grinding, polishing and engraving

• Diamond tools: diamond wheels, discs and bits for precise grinding, removing excess material and refining edges.
• Polishing: work through progressively finer abrasives, removing the scratches of each grit before moving to the next, then finish with a polishing compound such as cerium oxide on a felt or cork wheel to restore clarity and shine.
• Engraving: for fine details or a signature, use small diamond burrs in a rotary tool, kept wet; experiment with different tips.
• Fire polishing: a quick pass through the flame restores shine but softens detail, and the piece must be annealed again.

Graphite for smoother finishes

Finishing also begins at the torch. Graphite's smooth surface and high heat tolerance make it ideal for marvering hot glass, and custom-shaped graphite tools can create specific textures or refine shapes with great precision, which means less cold work later. A slightly cooled graphite or tungsten punty also releases more cleanly, leaving fewer marks during punty transfers.

The real artistry in borosilicate often lies in combining techniques and materials. The rule behind all of it is compatibility: everything fused together has to expand and contract together.

Borosilicate and soft glass

Borosilicate and soft glass cannot be fused together. Borosilicate 3.3 has a coefficient of expansion (COE) of about 33 on the scale lampworkers use, while soft glasses are typically around 90 to 104: roughly three times as much. As a joined piece cools, the soft glass tries to shrink far more than the boro, and the tension cracks it, often right away and sometimes later. They also work at very different temperatures: in a flame hot enough for boro, soft glass boils, burns and loses its colour. Scientific glassblowers join them only through a graded seal, a series of intermediate glasses that step the expansion gradually. At the bench: keep rods separate, label them, and never mix scrap.

Compatibility also matters within borosilicate. Coloured boro is formulated to sit close to clear, but some colours expand a little more or less than others. Test a new pairing in a small piece, and check it in the polariscope, before using it in a finished design.

Metals in borosilicate

It is tempting to think a metal works as long as it is compatible with the glass's COE. In reality almost no metal matches borosilicate, which expands less than nearly any metal. What decides whether a metal survives is how thin it is, whether it melts or oxidises at torch temperatures, and how far its expansion differs from the glass.

Two safety points. Fuming produces metal vapour, so only fume under strong local extraction that pulls fumes away from your face. And never encase plated, coated or unknown metals: plating can flake, and coatings and alloys can release toxic fumes.

Organic and found materials

• Organic inclusions: leaves, fibres and other organic matter char and give off gas when encased in hot glass, leaving dark marks and bubbles that can weaken the piece. What can be encased is the mineral ash left once organic matter is fully burned, or a material that has been prepared and tested.
• Found materials and residue: in my own practice I combine glass with residue and found materials, which give it a different kind of permanence.
• Multiple layers: combine encasing, fuming and patterning in a single piece; each layer adds a dimension.

Encapsulating movement

• Imply motion: sculpt flowing forms such as spirals, waves or flames by stretching, twisting and rotating the hot glass.
• Layer for depth: build up clear and coloured layers to create the illusion of depth and movement.
• Optical effects: lenses, clear glass and trapped bubbles make the movement shift as you look from different angles.

Most problems come back to three things: heat, cleanliness and compatibility.

Cracking

When a piece cracks tells you why. During work it is usually thermal shock: glass introduced too fast, or a thin section cooling while you heat another. After finishing it usually means stress: no anneal, too short a soak, too fast a cool, or mismatched materials. Heat evenly, always anneal in a kiln, and use the polariscope to see where stress gathers.

Scorching and sooting

Scorch marks appear when the glass sits in a reducing flame, one with too much fuel and too little oxygen, or when it is held inside the inner cone. Soot settles on the surface, and metal-bearing colours, especially silver and copper ones, are reduced and turn grey, brown or metallic. Adjust the flame towards neutral or slightly oxidising (see The Flame and Reading Heat) and work just beyond the tip of the inner cone, where the flame is hottest and cleanest. Surface soot usually burns off in an oxidising flame; soot trapped under encasing stays.

Muddy colour

If colours look muddy or dull, the glass may be overheated, the flame chemistry may be wrong, or the colours may be reacting with each other. Experiment with temperature, flame and colour combinations, and see Colour in Glass for why metals behave as they do.

Boiling colours

Many coloured rods tolerate less heat than clear. Pushed too hot, they boil: the surface fizzes, tiny bubbles form and the colour can grey. Preheat colour gently and melt it further out in the flame.

Bubbles

Bubbles come from air trapped while handling the glass: when wrapping a gather, encasing, folding, or laying frit, which holds air between its grains. Dirty or damp glass adds more. Work slowly, wrap from one end so air is pushed out, and keep your glass clean. An unwanted bubble can often be heated, lanced with a pointed tool and smoothed over.

Devitrification

A cloudy, frosted or crusty surface means the glass has started to crystallise. It happens when glass spends too long in the range where crystals grow, usually through overworking, and dirt (fingerprints, dust, marker residue) gives crystals somewhere to start. Some colours, often opaques, are more prone. Clean rods with alcohol and avoid unnecessary reheating. See What Glass Is for why glass crystallises at all.

Thermal gradients

Uneven heat causes many of these problems, but used deliberately it is a tool. For sharp detail without deforming the whole piece, keep the base slightly cooler while concentrating intense heat on the area you are working. Return the whole piece to the flame regularly so the cooler part doesn't crack.

All of this knowledge only becomes useful at the bench, through repetition. The last entry, Practice: Tips, Masters and the Journey, is about how that practice grows.

• Keep rotating. Always keep the rod or tube turning in the flame so it heats evenly and doesn't sag. Turn at a steady speed: an uneven rhythm gives an uneven form.
• Control the heat. Practise adjusting flame size, chemistry and position. A reducing flame (more fuel, less oxygen) is softer and cooler, and some colours need it, for example to draw out metallic lustre or warm copper tones; a neutral flame is the everyday working flame for borosilicate. The Flame and Reading Heat explains each one.
• Work in and out of the flame. Where you hold the glass matters as much as the flame itself. The hottest zone is just beyond the tip of the inner cone; the gentler outer flame warms glass slowly, keeps a piece at temperature and reintroduces cooled glass without shocking it.
• Use gravity. Tilting the rod or mandrel helps stretch and shape the piece naturally.
• Be precise with tools. A graphite paddle smooths surfaces; tweezers create fine details. Touch hot glass briefly and lightly. Over time you develop a feel for moving the glass subtly.
• Experiment. New techniques, colours, forms, and unconventional materials. Change one thing at a time, so that you know which change made the difference.
• Keep a journal. Record designs, techniques, temperatures and results, as this journal does. It is the best reference you will have.
• Keep learning. Workshops, the glass community, books and articles.

Time at the torch only becomes skill when it is spent on purpose. Many lampworkers divide their practice into three kinds of work: drills that train the hands, tests that teach you your materials, and records that let you learn from both.

Drills

A drill is a small exercise repeated until it becomes automatic. Good first drills are:

• Gathers: melting the end of a rod into a round, centred ball, again and again, until it stays on axis without effort.
• Pulling points and stringers: heating a section of rod evenly and pulling it out of the flame into a thin, even thread. Aim for the same diameter every time.
• Sets of identical pieces: ten round beads, ten drops or ten small spheres of the same size. Matching is harder than making one good piece, and trains your eye for heat, time and quantity of glass.
• Joins: attaching one rod or tube to another cleanly and evenly, which is the base of all techniques at the torch that build a form from parts.

Short, focused sessions are usually more productive than long, tired ones. Fatigue makes rotation uneven, and it is also when accidents happen.

Sample boards and colour tests

A colour on the rod is often not the colour it becomes. Borosilicate colours can shift with the flame chemistry, with heat, with time, with encasing in clear glass and with striking in the kiln, as Colour in Glass explains. A rod sold under the same name can also behave a little differently from one maker, or one batch, to another.

The answer is the sample: a small, standard test piece made from each new colour. A useful test shows the colour in more than one state, for example worked in a neutral flame and in a reducing one, bare and encased, thin and thick. Label each sample with the maker, colour name and code, and the conditions, and keep them together on a board or in a box. After a year, that board is a map of your materials no catalogue can give you.

The glass notebook

A notebook turns results into knowledge. For each session or piece it is worth recording:

• the date and what you set out to do;
• the glass used: maker, colour, code and rod or tube size;
• the flame: torch, flame chemistry, and anything you changed;
• the sequence of steps, with a quick sketch;
• how the piece was annealed;
• the result, good or bad, and what you would try next.

Write down failures as carefully as successes. A crack, a cloudy colour or a bubble is information, and Finishing, Combining and Troubleshooting helps you read it. This journal itself grew out of notes like these.

Study and emulate

• Break down techniques: take complex pieces apart into their component techniques and try to replicate them.
• Incorporate ideas from work you love, blended with your own language.
• Ask for feedback from peers and mentors.

Copying a master's piece is a good exercise for learning; selling it as your own work is not. Use what you learn to find your own voice.

Flameworkers to know

These artists work in soft glass and borosilicate, at the scale of a bead and of a sculpture. Each has something to teach a learner.

Leopold and Rudolf Blaschka. The father and son behind the Glass Flowers at Harvard, introduced in A History of Glass and Lampworking. Their lesson is patient observation: every model began with the study of a real plant.

Paul Stankard (born 1943, United States). Stankard trained in scientific glassblowing at what is now Salem Community College in New Jersey, graduating in 1963, and spent his first working years making apparatus for chemical laboratories. In 1969 he began experimenting with paperweights, and he went on to make botanically accurate flowers, roots and small figures he called "root people", encased in clear glass. He has also written about growing up with dyslexia, identified only in adulthood, and the self-directed learning he built around it. His lessons: laboratory discipline can become the foundation of art, and there is more than one way to learn.

Ginny Ruffner (1952–2025, United States). Ruffner learned lampworking through years of work in commercial flameworking studios in Georgia. In 1984, as artist in residence at Pilchuck Glass School, she taught its first lampworking class and introduced borosilicate there; the harder glass let her make much larger pieces than were usual at the torch. She combined flameworked glass with painted surfaces and is credited with helping establish lampworking as a fine art medium. After a car accident in 1991 left her unable to speak or walk, she relearned her skills and returned to work, directing a team of assistants. Her lessons: borosilicate opened the torch to sculpture, and a practice can survive almost anything.

Lucio Bubacco (born 1957, Murano). The son of the glass artist Severino Bubacco, he worked from the age of fourteen in a large glassworks, making thousands of small figures, and at seventeen opened his own workshop dedicated to lampworking. He works in Murano soft glass, building large, free-standing figurative sculptures full of moving bodies and themes from classical mythology. His lesson: thousands of small repetitions are what make fluent, ambitious work possible.

Emilio Santini (Murano, now in the United States). Born into a Murano glassmaking family, Santini worked in glass factories as a boy during summer holidays and learned lampworking from his father at seventeen. In 1988 he moved to Virginia and opened a lampworking studio. He has taught for many years at the Studio of the Corning Museum of Glass and at other major schools. He has described the Studio as a place "where technique reigns first, in the service of art": a good sentence to keep above any bench.

Shane Fero (born 1953, United States). A flameworker for more than four decades, whose work includes a long series of glass birds, Fero has a studio near Penland School of Craft in North Carolina and has taught there many times. He is a past president of the Glass Art Society's board, which gave him its Lifetime Membership Award in 2014. His work is in collections including the Corning Museum of Glass. His lesson: a long career is built with others, through teaching and through service to the community.

Kristen Frantzen Orr (born 1951, United States). A bead maker whose detailed floral beads are in the collection of the Corning Museum of Glass, given by the International Society of Glass Beadmakers. She came to glass from illustration and calligraphy, and describes building layers of transparent colour in a way inspired by watercolour, with multi-coloured canes for detail; a single bead can take her hours. Her lesson: what you already know from another art can come with you to the torch.

Bob Snodgrass (United States). Working at the torch since 1974, he is credited with developing fuming, silver and gold vaporised onto borosilicate, which began, by his account, as an accident (see A History of Glass and Lampworking and Colour in Glass). His lesson: pay attention to accidents, because some of them are discoveries.

Workshops and collaboration

Hands-on workshops teach what books and videos can't: the sound of a well-set flame, the moment a teacher takes a piece out of the heat. Collaborating with other artists leads to new methods and fresh perspectives.

Stay inspired

• Nature: the forms and structures of plants, water, stone, landscapes.
• Other arts: painting, sculpture, architecture, translated into glass.
• Science: glass is deeply connected to optics and chemistry, as the colour entry of this journal shows.

Many glass schools offer flameworking alongside furnace glass and cold working. Programmes, dates and course content change every year, so check each school's current offer before planning a trip. A few of the best known:

• The Studio of the Corning Museum of Glass (Corning, New York, USA). Opened in 1996, it runs classes for beginners and advanced students, with a hot shop, a flameworking studio and a coldworking studio, next to one of the world's great glass collections.
• Pilchuck Glass School (near Stanwood, Washington, USA). Founded in 1971 by Dale Chihuly, Ruth Tamura, and Anne Gould Hauberg and John H. Hauberg, on a former tree farm north of Seattle. Its small summer classes run for one to three weeks, and flameworking is among the areas taught.
• Penland School of Craft (Penland, North Carolina, USA). Founded in 1929 by Lucy Morgan, it teaches many crafts in short workshops and longer, eight-week "concentrations". Glass was added under its second director, Bill Brown.
• Pittsburgh Glass Center (Pittsburgh, Pennsylvania, USA). Founded by the glass artists Kathleen Mulcahy and Ron Desmett and opened to the public in 2001, it has a dedicated flame shop where students work seated at tabletop torches, with classes open to complete beginners.
• Abate Zanetti glass school (Murano, Italy). It descends from the Scuola di Disegno per Vetrai, the drawing school for glassworkers founded in 1862 by the abbot Vincenzo Zanetti as a companion to the Murano glass museum. Besides its school programme, it offers glass courses, including a basic lampworking (lume) course.
• Salem Community College (Carneys Point, New Jersey, USA). Its two-year Scientific Glass Technology degree (Associate in Applied Science), which the college describes as the only programme of its kind in the United States, trains scientific glassblowers to build laboratory apparatus to technical drawings. It is the path Paul Stankard took, and a reminder that the same torch serves both science and art.
• Barcelona Glass Studio (Barcelona, Spain). My recommendation closer to home. Founded in 2015 by the glass artists Agustina Ros and Ferran Collado, it is a flameworking studio dedicated to borosilicate glass that has grown from a small workshop into an open studio and community. It offers intensive and thematic courses, private classes, masterclasses and co-working at the torch, and works to spread glass culture in Barcelona and across Europe. A good place to start, and to keep going.

Local studios and individual artists also teach. One good teacher near you, followed by regular practice, can be worth more than a famous course followed by months away from the torch.

Communities of glass

• International Society of Glass Beadmakers (ISGB). Founded in 1993 by a small group of bead makers in Prescott, Arizona, as the Society of Glass Beadmakers, it took its current name in 2003 to reflect its international membership. It promotes and teaches glass beadmaking as wearable, sculptural and functional art, and holds an annual conference called the Gathering.
• Glass Art Society (GAS). Founded in 1971 by Fritz Dreisbach, Mark Peiser and William Bernstein, it held its first conference at Penland that year. Its annual conference gathers artists of every glass technique and has met in many countries, Murano among them.

Local guilds and online groups do the same on a smaller scale: sharing problems, suppliers and each other's progress.

A culture of safety

Good studios share a culture in which safety is part of the craft, not an extra. The checklist in The Studio is the starting point; these habits sit around it:

• Learn gas handling from a qualified source. Have your torch, regulators, hoses and flashback arrestors installed and checked as the supplier and local regulations require, and test connections for leaks with a suitable leak-detection solution, never with a flame.
• Protect your eyes properly. Use eyewear rated for the glass you work and for the intensity of your flame. For borosilicate, eyewear makers offer lenses with stronger infrared and UV protection than standard didymium; ask your supplier what suits your work.
• Ventilate every session. Extraction should run throughout, especially when fuming with metals, burning off release agents or working with powders.
• Stop when tired. Concentration is a safety tool; a tired lampworker is more likely to burn themselves or drop hot glass.

The journey in lampworking, alike glass, rarely moves in a straight line. There are weeks when everything cracks and weeks when a technique suddenly makes sense. Time at the torch, attention to what the glass tells and the habit of going back the next day.

To oxidate

At one of the residencies I am creating at, El Convent de Sant Agustí, I am beginning to think about what it means to oxidate.

Oxidation is often described as deterioration, but it is also a form of conversation, a slow exchange between a body and its atmosphere.

Metals breathe with the air, surfaces register their encounters with humidity, salt, and touch. The object darkens, blooms, stains. In this sense, oxidation is less about loss and more about exposure: a material allowing the world to write itself into its skin.

To oxidate is to accept permeability. It is to understand that no surface can remain sealed forever, that transformation begins the moment something enters into relation with its surroundings.

I find something reassuring in this process, the way a structure slowly records time, weather, and contact. A lamp that tarnishes, a mechanism that gathers dust, a piece of glass that clouds slightly with use: these are not malfunctions of the object, but traces of its participation in the world.

The gentleness of failure,
the subtle intelligence
carried by things
in the process
of undoing themselves.

07.03.26

Archivos Porosos

Here I have begun to gather a range of performative instructions on, How to Practice Porosity. 

25.11.25, 16:16, five days after the new moon.

What happens when years of artistic research gets flattened into a trend?

My practice didn't begin with jewellery. It began with interactive installations, driven by questions about nature, technology, and more-than-human relationships.

Rain and Rivers is the result of years of research, experimentation, failures, residencies, programming, and more-than-human collaborations. It was shaped through DIY practices, creative coding, and a desire to rethink our relationship with nature in the context of the climate emergency.

Recently, I've watched elements of this visual and conceptual language fall in the work of major brands. Seeing ideas that emerged from an anti-capitalist, research-driven practice become absorbed into commercial aesthetics has made me realize how easily the histories behind artistic work can disappear.

By sharing where these ideas came from, how they evolved, and the questions that shaped them is a way for me to preserve these languages before its context gets flattened into a trend, or becomes detached from the values that gave rise to it in the first place.

16.04.26, 18:43, waxing crescent.

The patience of small motors

Lately my attention has been resting on the slow companionship between movement and erosion: how objects continue to act even as they wear down, how a gesture can persist through small mechanical repetitions. Lamps that hesitate before glowing, motors that stutter and continue, artifacts that seem to breathe through their own fragility.

I have been thinking about the strange duality of living with decay: how deterioration is never simply disappearance, but a form of ongoing negotiation with time.

I inaugurated a small constellation of these lamps and motorised artifacts within a performance at Mostra Festival at Casa Montjuïc, titled Acción y Destino, a reinterpretation of the works by Dania and Rupert Clervoux. It is an attempt to stage this coexistence with entropy, to let light, motion, and fatigue share the same choreography.

The same week I was awarded a residency at Hangar, here in Barcelona, where I will continue to follow these threads over the coming months. I am curious to see how these machines and materials might keep teaching me about endurance, about the tenderness of malfunction, about the quiet intelligence of things that are slowly coming apart.

07.03.26, 13:15, five days after the full moon.

Shedding: Skin Scent I–II

Skin Scent I–II is a wearable sculpture holding two perfume compartments with distinct yet layering skin scents. It is a collaboration between Rain and Rivers and Ludmila Furman, made for Apoc Store's 100 One of Ones collection.

This collaboration was initiated as a reflection on shedding, developed in connection to the year 2025: the yin wood snake. Activated by body heat and the oil base of the structure, the scents unfold gradually as an extension of the skin. The piece was designed around release and intimacy.

Scent I carries earthy tones of vetiver, moss, and mineral notes, deepened by labdanum, myrrh, and incense, whilst Scent II introduces a lighter, floral layer built around frangipani, jasmine, ylang-ylang, and green tea, supported by musky and woody notes.

The piece is a 1/1 edition, forged by hand in Barcelona using handblown borosilicate glass and stainless steel. Each compartment is a refillable bottle.

25.11.25

The Softest Hard: porous methodologies across borders

The Softest Hard is a series of sculptures that accumulate gestures of failure and fragility, made during my residency at Hangar in Barcelona and shown in July 2026 at Casa R.A.R.O, within the Art Nou circuit. The pieces are built from blown glass and reconstructed found objects, with multispecies collaborations and low-tech languages. They emerged from investigating porosity as a methodology: a way of inhabiting decomposition and sustaining sensitivity in contexts of accelerated change and deep erosion.

For something to decompose, it must first be porous.

For three months I commuted the same route every day, observing the places where waste accumulates and documenting what circulates and what remains. Unwanted worlds bound together in a bag and left for who dares. My fifteen-minute walk to the tram tended to stretch to thirty, as I peeked into each world and filled my bag with plastic tubes, aluminium structures and bits of broken glass, to work with what leaks, what resists, what remains. I like to think of myself as a magpie when I collect.

What began as a poetic gesture slowly revealed broader logics of extraction, excess and disposal, and recycling systems sustained by marginalised and invisible bodies and forms of labour. At €0.20 per kilogram of iron and €1 per kilogram of aluminium, the prices expose circuits of exploitation and displaced ecological violence.

During this time I shared a staircase with a space occupied by recyclers, who dismantle found objects every day under precarious conditions. The proximity between their gestures and my own created a productive tension around the material conditions and privileges from which my work operates. Over time that tension became exchange, with tools, materials and care circulating between us. Some of the recyclers began setting fragments aside for me, and I gathered leftovers from my walks and from other Hangar residents for their recycling. These relationships became part of my methodology of porosity, and the sculptures that emerged embody tangled, often invisible anatomies, assembled from what those exchanges accumulated.

The research unfolded as the European Parliament approved the "Return Regulation", a migration framework that undermines fundamental human rights. The Softest Hard turns towards those who sustain the metabolism of a system that excludes them, and questions political communities that reinforce borders rather than expanding rights.

This is where porosity stops being only a material quality. A pore is a minimal opening through which to pass, sweat, breathe and disintegrate; a porous methodology lets proximity, rather than the line, decide what is altered and who is allowed through. Materials, tools and care crossed a staircase that, on paper, separated two very different ways of working.

01.07.26, 12:00, three days after the full moon.

Not All Flowers Follow the Sun

On 28 August 2026, supported by the Casa RARO production grant, I opened Not All Flowers Follow the Sun at Casa RARO in Barcelona: an installation and investigation in process, a series of motorised, decomposing sunflower heads out of step with the sun. In my meadow the flowers follow neither the sun nor the programmed code; motorised and in decay, they move out of phase.

The piece began with the wish to learn from the ways in which flowers decompose. Since then I have been sharing time with sunflowers, learning about their porous threshold: how they are entered, altered and shared.

El girasol que gira en busca del sol,
gira, busca, encuentra y vuelve.

A flower of a thousand heads

What looks like a petal is an entire flower. Sunflowers are plural: the head hides whole flowers between its petals, pollinated by the thousand. Una flor de mil cabezas, peluda, con escamas, with roots that tangle into mycorrhizal networks. Its reproductive body is collective. Each small flower carries both pollen and stigma, and can pass through a pollen-shedding phase before it becomes receptive.

Planted densely, sunflowers lean away from one another, keeping each body out of its neighbour's shade. Tal vez su mirada nunca se trató de seguir una estrella, sino de atender a quienes la rodeaban.

Waste lands produce waste bodies

The sunflower has had its multiplicity tamed into a single heavy head. A plant evolved to scatter, clasificada como maleza, classified as a weed, is now a domesticated body. It is also introduced into phytoremediation: planted into landscapes transformed by extraction, industry, warfare and waste, for its capacity to absorb contaminants from soil and water.

The plant temporarily embodies materials displaced by human activity. Its roots pull heavy metals, radioactive elements and organic pollutants from the ground and store them in its tissues, mostly in roots and leaves, sometimes as far as the seeds. As contaminants accumulate they alter growth, damage tissue and weaken seeds. Repair, carried by vulnerable bodies.

Sunflowers grown for phytoremediation are considered contaminated before they grow, and are managed as contaminated waste. And the contamination keeps changing hosts: bees and butterflies meet the metals in nectar and pollen, insects absorb more as they feed on leaf and stem, predators inherit the encounter through their prey. Toxicity only changes host.

Waste lands produce waste bodies.

Phytoremediation exposes the unequal distribution of ecological repair: which beings are made responsible for damage they did not cause, and which species become sacrificial subjects when human labour and ecological responsibility are outsourced. It is the same question The Softest Hard asks of the bodies that sustain a city's recycling.

The shield flower

Es una flor que ha crecido entre relatos de sacrificio. In Nahua worlds the sunflower already sat within systems of warfare, offering, solar power and death. Chimalxóchitl, the shield flower, was offered to Huitzilopochtli. Ceremonial language turned war and death into floral speech: xochiyaoyotl, guerras floridas; xochimiquiztli, muertes floridas. Warriors fallen in battle and the sacrificial dead became xochimicque, entering the solar cycle and returning as birds that move among flowers and drink their nectar.

In both stories the sunflower enters an economy of transfer: something passes through it that is not its own. An intermediary body. A flower with a thousand flowers in its head carries soil, metal, warfare, poison, insects, microbes, predators, humans, histories.

Seres de sacrificio para ser llamados maleza.
Its body is never only its own.

Before dismounting the exhibition in September, I set up one last arrangement of the decaying heads, thinking about how delicious that rotting plant must be to the butterfly.

28.08.26, 15:00, one day after the full moon.

The writings above are about looking and thinking. The next entry turns to doing: prompts, rituals and exercises to try.

Here is one of the first prompts I wrote down:

Take a photo that captures movement or stillness: flowing fabric, wind in your hair, light on your skin. I like to think of this as being weathered.

I have begun to collect a series of images that portray this. It is part of a growing study in tracing the climate through everyday forms.

24.07.25, 10:00, new moon.

A book I read and recommended in September 2025 is Quantum Listening by Pauline Oliveros. Oliveros (1932–2016) was a composer who, from the 1960s onwards, brought meditation, political activism and experimental music together, eventually creating Deep Listening: a practice she said was for humanitarian purposes, specifically healing.

The book is short, almost a manifesto for listening as activism. It begins with questions I keep returning to:

What is the difference between hearing and listening?

Quantum listening, as she describes it, is listening to listening: attuning to our bodies, to the earth and to one another in a world that keeps getting louder. Through simple exercises she proposes Deep Listening as the ground for a transformed way of living together, where compassion guides how we make and how we act.

The piece that resonated with me most was Sonic Meditations, her text scores from the early 1970s: instructions rather than notation, open to anyone, trained or not. They accompanied me while reworking A Skin of Soil with Akyute and Alice Sparkly Kat for Mira Festival 2025, and when, with Akyute, we placed contact microphones on the motorised sculptures of A Practice of Porosity for Turbina, presented by Mira Festival at Las Tres Chimeneas in Badalona, to build a soundscape from deep listening.

06.09.25, 18:45, a day before the full moon.

For that installation I was also reading other sound artists. Elena Biserna's book Going Out: Walking, Listening, Soundmaking gathers scores for walking, and one of them begins with the sounds closest to you: those of your own body as it moves, the first dialogue between you and the place around you. If you can hear even the quietest of them, you are moving through a place still scaled to the human body; your voice and footsteps speak to it, and it answers by giving them their particular echo. Then the score asks you to try to move without making any sound.

Which is the quietest sound of your body?

17.09.25, 13:24, four days before the new moon.

At Casa Raro in Barcelona I took part in a six-month collective mentorship, with weekly meetings to re-evaluate our artistic practices. One of our encounters ended with a collective exercise to receive critique. It can be done with any group of friends or fellow makers.

1. Each artist brings one photograph chosen to represent their work, along with its technical description and a one-paragraph artist statement.
2. Looking at each one, everyone answers out loud: What does it have? What is it missing? What is our question?
3. The artist listens, and cannot reply.

After the exercise you are left not only with the thoughts others have shared about your work, but also with a range of writings of your own about theirs.

In my case, my image, a close-up of one of my motorised flowers, was perceived as contrasting, cold, industrial and lacking human presence (as negatives), and sensitive and poetic (as positives). I found this very interesting, because what I was constantly looking for and critiquing in the work of others was that it was often lacking intimacy.

So my question was: as someone whose work is perceived as cold, industrial and lacking human presence, how can I integrate the intimacy and warmth I am looking for?

Friendship, a constant mirror.

07.03.26, 13:15, five days after the full moon.

Here is an egg cleanse shared with me by a friend.

An egg cleanse is a folk ritual used to absorb and release negative energy.

1. Pass a room-temperature egg slowly over your body, head to feet, while holding the intention to draw out what no longer serves you.
2. Crack it into a clear glass of water and wait five to ten minutes. Strings or bubbles may reveal envy, stress, or emotional heaviness.
3. Add salt before flushing, to seal and banish what has been released.

This ritual is rooted in Mesoamerican traditions. It is believed that the egg holds memory and echoes the body's silent cries beneath its shell.

24.07.25

I gathered eucalyptus leaves, a thistle, the hair on my brush accumulated from this month, clove, rosemary, ginger and rose petals, and nested a glass jewel within.

My hair fumed the glass with an amber tone, from the heavy metals it holds, opening a new line of investigation on phytoextraction.

This was something I had dreamt about a few months earlier, and had initiated by planting mustard seeds and experimenting with phytoextraction processes. So it was a nice surprise to see it had worked spontaneously with glass. 

06.10.25, 23:46, full moon.

In November I came across Metro Listening, a call and a score by the Bureau for Listening, and it connected with my own understanding of porosity as a practice and a methodology.

It proposes that to listen underground is to give up the search for easy answers and stay with the questions that rise from the noise: the disruptions and the silences, the disappearances and the arrivals, the histories held in the concrete and stone beneath us. The metro does not only carry bodies from one place to another; it carries their dislocations too, and is at once shelter and escape, freedom and confinement. Listening there becomes a way of being with the unspoken, the unnoticed and the unseen.

The score is for one or more people, on board an underground train. In short:

1. Without further instructions or context, improvise an act of "metro listening".
2. From those first impressions, form your own understanding of what metro listening is.
3. Keep developing and refining the practice.
4. After ten to thirty minutes, reflect: what has changed in your perception, awareness or engagement with the metro? How does your body feel, and what does it seem to ask for? What intentions are emerging for your practice?
5. Write a brief, open invitation for others on the train to explore their own version.

What will you hear beneath the surface?

16.11.25, 23:46, four days before the new moon.

Going through my notes for Archivos Porosos, I started to draft a series of points as a kind of manifesto. Here are ten of my favourites.

1. Permitirse pudrir. Confiar que la descomposición también es una forma de cuidado. Remember that decay is a form of generosity.
2. Considerar la humedad como una metodología.
3. Let our bodies be wetlands: zones where categories dissolve.
4. Revise our research not by organising it, but by watering it. Regar el archivo.
5. To shed, like bark, skin and scales, untangle the stories that no longer root us. Desenredarse.
6. Acoger las contradicciones, and contradict ourselves often. Contradiction is evidence of decomposition.
7. Accept that some of the things we inherit are moist, heavy, and not entirely ours. Ask ourselves what parts of our practice are inherited, and what parts need burial.
8. Remember to carry our migrations in our joints with tenderness, and that art is sometimes a way of softening those hinges. Entenderla como fuerza de arraigo y diálogo. Approach our knees, those hinges of stability, as containers of generational migration, departure, and endurance. Ask: where do the stories of our ancestors settle in the body, and how do they weather over time?
9. Trust any process that leaks a little.
10. Ask the soil for advice. Remember to consult the microorganisms.

25.11.25, 16:16, five days after the new moon.

If you managed to read this far, know I appreciate you. The next entry gathers the words of this practice into a porous glossary.

Accumulate (Acumular): To gather fragments.

Scavenge (Carroñar): To digest discarded matter in order to nourish what has yet to germinate.

Compost (Compostar): To accumulate, ferment, and decompose in order to emerge.

Contaminate (Contaminar): To cultivate relationships.

Contradict (Contradecir): Evidence of decomposition. Truths occupying the same space.

Care (Cuidar): To create around a fragility.

Decomposition (Descomposición): A process of generosity, in which one form ceases to sustain itself in order to nourish another.

Untangle (Desenredar): To release the stories that can no longer find a place to take root.

Scale (Escala): A human fiction.

Stagnation (Estancamiento): Movement too slow to be recognised.

Ferment (Fermentar): To allow relationships to produce knowledge.

Humidify (Humedecer): A methodology for mutation.

Intimacy (Intimidad): A relationship built through sustained proximity.

Language (Lenguaje): Organic matter capable of decomposing.

Oxidation (Oxidación): A slow conversation through the movement of electrons. See What Glass Is for the electrons themselves.

Porosity (Porosidad): To be permeated by proximity. See How to Practice Porosity.

Proximity (Proximidad): The minimum distance required for something to be altered.

Residue (Residuo): Matter awaiting new alliances.

Revisit (Revisar): To water an idea.

Sediment (Sedimento): Where your past versions gather.

Suppurate (Supurar): An intelligence of deterioration.

Glass (Vidrio): An amorphous solid. See What Glass Is.

01.07.26, 12:00, three days after the full moon.

The next entry steps back from the torch, to the furnace, the kiln and the many other worlds of glass.

Furnace glassblowing has existed since the Roman world adopted the blowpipe in the first century BC (see A History of Glass and Lampworking), and its basic tools have changed surprisingly little.

Three heats

A hot shop is organised around three kinds of heat. The furnace holds a crucible of molten glass, kept at around 1,100 to 1,200 °C for the soft soda-lime glasses used in most studios, day and night, for weeks at a time. The glory hole is a second, open-mouthed furnace used to reheat the piece again and again while it is being worked. The annealer (called a lehr in factories) is the oven where the finished piece cools slowly so it does not crack, the same principle as the kiln annealing described in Finishing, Combining and Troubleshooting.

The tools

• Blowpipe: a hollow steel tube, usually a little over a metre long, used to gather glass from the furnace and blow into it.
• Punty (pontil): a solid steel rod used to hold the piece from the bottom once it is transferred off the blowpipe, so the opening can be worked. The scar it leaves on the base is the pontil mark, a useful clue when dating old glass.
• Marver: a flat steel (originally marble, hence the name) table on which the hot gather is rolled to shape it and cool its skin.
• Blocks and paddles: wooden tools, often of fruit wood such as cherry, soaked in water. The water turns to steam on contact and the glass rides on a cushion of vapour, so the wood shapes it without sticking.
• Jacks: large tongs with blunt blades used to form necks and openings. Flameworkers will recognise the same idea in smaller form.
• Shears, tweezers and newspaper: a pad of folded wet newspaper held in the hand is still one of the best tools for shaping a hot gather.

Teamwork

Large pieces need several people. The lead glassblower, called the gaffer, sits at the bench and shapes the piece, while assistants gather glass, blow when asked, hold heat shields and bring bits of hot glass for handles and decoration. Murano workshops were organised as a piazza led by the maestro. The choreography is close to music: everyone moves on the gaffer's count, because hot glass will not wait.

What a flameworker can take from it

The physics is identical to the torch: glass moves when it is hot, sets as it cools, and must be kept turning so gravity does not pull it off centre. The difference is scale and source of heat. At the torch the flame comes to the glass and the work is small and precise. At the furnace the glass goes to the heat and a single gather can weigh several kilograms. Many artists move between both, and techniques such as cane pulling, incalmo and the Venetian patterns described in Techniques at the Torch began in the hot shop.

Kilnforming, sometimes called warm glass, shapes glass by heating it inside a closed kiln rather than in an open flame. The maker arranges cold glass, programmes a firing schedule, closes the lid and lets controlled heat do the forming. It rewards planning more than speed of hand.

Fusing

Fusing joins pieces of flat glass, frit (crushed glass), powder or stringer into one piece. The result depends on how hot the kiln goes. The figures below are typical for the soft fusing glasses most studios use; every glass maker publishes its own schedules, and these should always come first.

The same lesson as in What Glass Is applies here, only more strictly: all glass in a fused piece must be compatible. Fusing glass is sold in families tested to work together, commonly identified by their expansion (COE 90 and COE 96 are the two best known). Glass from different families, or ordinary window glass mixed with art glass, will usually crack on cooling, sometimes days later. Borosilicate can be fused too, but only with other borosilicate and at higher temperatures.

Slumping and draping

Slumping lets a fused panel sag into a mould to become a bowl or plate; draping lets it fall over a form. Moulds are made of ceramic, stainless steel or fibre board, coated with a separator called kiln wash (or lined with fibre paper) so the glass releases cleanly. Because glass at slumping temperature still has a skin, slow even heating and a long hold matter more than heat.

Kiln casting and lost wax

Kiln casting fills a mould with glass and melts it in place. In the lost-wax method a sculpture is first made in wax, then surrounded by a refractory mould of plaster and silica. The wax is steamed or melted out, leaving a cavity the exact shape of the original. The mould goes into the kiln with chunks of glass in a reservoir above it, and as the glass softens it flows down to fill the space. Thick castings hold enormous internal stress while they cool, so they are annealed for a very long time: days for a palm-sized piece, weeks or even months for monumental work. Afterwards the mould is broken away and the glass is ground and polished.

Pâte de verre

Pâte de verre (French for "glass paste") packs finely crushed glass mixed with a binder into a mould, then fires it just enough for the grains to fuse. The result is grainy, translucent and soft-looking, more like sugar or alabaster than ordinary glass. Ancient makers in Egypt and Mesopotamia used related methods, and the technique was revived in France in the late nineteenth century by Henry Cros, followed by makers such as Gabriel Argy-Rousseau and Almaric Walter. It allows colour to be placed grain by grain, almost like painting with glass.

How windows were made

For most of history a perfectly flat, clear sheet of glass was a luxury. Three older methods each left a signature you can still spot in old buildings:

• Crown glass: a gather was blown into a bubble, opened and spun on a punty until centrifugal force flung it out into a wide, thin disc. The disc was cut into panes; the thick centre where the punty had been, the "bullseye", was the cheapest piece and often used in modest windows.
• Cylinder (broad sheet) glass: a long cylinder was blown, its ends cut off, the cylinder split along its length and flattened in a kiln. It made larger panes, with a slight wave and seed bubbles.
• Plate glass: molten glass was cast on an iron table, rolled flat, then ground and polished on both faces. The process was developed in France at the end of the seventeenth century for the royal glassworks that later became Saint-Gobain, and made great mirrors such as those at Versailles possible. It was clear and flat but slow and expensive.

The ripples in old windows are a record of these methods, not of glass flowing over centuries, as What Glass Is explains.

Float glass

Almost every window made today is float glass, a process developed in Britain by Alastair Pilkington and Kenneth Bickerstaff and announced in 1959. A continuous ribbon of molten glass is poured onto a bath of molten tin. The glass is lighter than the tin and does not mix with it, so it floats and spreads into a sheet whose faces are flat and parallel without any grinding. Left to itself the ribbon settles at about 6 mm thick; rollers at its edges stretch or hold it for thinner or thicker sheets. One face of every float sheet touched the tin, and under ultraviolet light it glows faintly, which is how glass workers tell the two sides apart.

Stained glass

A stained glass window is a mosaic of coloured glass held in strips of lead called came, shaped like an H in section so a piece of glass sits in each side. The colour mostly comes from the glass itself, coloured in the melt with metal oxides (see Colour in Glass). Details such as faces and folds of drapery are painted on with dark vitreous paint, a mixture of ground glass and metal oxide, and fired in a kiln so they fuse permanently to the surface. From the early fourteenth century, glass painters also used silver stain, a silver compound that turns glass yellow to amber when fired, allowing gold and blue in a single piece. The great windows of Chartres, mostly made in the late twelfth and early thirteenth centuries, still hold much of their original glass.

In the late nineteenth century the Tiffany studios in New York popularised the copper foil method: each piece is wrapped in thin copper tape and the seams are soldered, allowing far smaller and more curving pieces than lead came, which made the famous lamps possible.

Decorating cold glass

Glass can also be decorated once it is cold and annealed:

• Cutting: rotating stone or diamond wheels grind facets and grooves. Heavy lead crystal became the classic material because its high refractive index makes cut facets sparkle.
• Engraving: fine lines are cut with small copper wheels fed with abrasive, or scratched with a diamond point. In stipple engraving, used by Dutch artists in the eighteenth century, the image is built from thousands of tiny dots tapped into the surface.
• Cameo glass: a vessel is made of two or more layers of different colours, and the outer layer is carved away to leave a relief, usually white over dark. The Roman Portland Vase is the most famous example; reproducing it took the English glassmaker John Northwood years of work, finished in 1876.
• Etching and sandblasting: a resist protects parts of the surface while the rest is frosted with abrasive. Sandblasting is covered in Finishing, Combining and Troubleshooting. Acid etching uses hydrofluoric acid, one of the most dangerous chemicals in any craft, and belongs only in properly equipped industrial settings.

The borosilicate in a lampworker's rod began as a laboratory and engineering glass (see A History of Glass and Lampworking), and it has many relatives doing quiet, essential work.

Optical glass

Lenses depend on glass with exactly known optical properties: how much it bends light (its refractive index) and how much it spreads colours apart (its dispersion). In the 1880s in Jena, the chemist Otto Schott worked with the physicist Ernst Abbe and the instrument maker Carl Zeiss to develop new glass types systematically, element by element. Their work transformed microscopes and cameras and is the same tradition that gave lampworkers borosilicate.

Optical fibre

An optical fibre is a strand of extremely pure silica glass, about as thin as a human hair, with a core and a surrounding layer of slightly different refractive index. Light sent down the core reflects off that boundary and stays trapped inside, travelling for kilometres. In 1970 researchers at Corning (Robert Maurer, Donald Keck and Peter Schultz) made the first fibre clear enough to be practical for communication. Today undersea fibre-optic cables carry the vast majority of data between continents: when you load a web page from another country, the message has very likely travelled through glass.

Strengthened glass

Glass almost always breaks from tiny flaws on its surface, which grow into cracks when the surface is pulled into tension. Strengthened glass works by putting the surface into compression so that cracks cannot open. Tempered (toughened) glass does this with heat: the sheet is heated and its surfaces are cooled suddenly with air jets. When it does break, it shatters into small blunt cubes, which is why it is used in car side windows and shower doors. Chemically strengthened glass, used on most phone screens, does it by ion exchange: the glass is bathed in molten potassium salt, and larger potassium ions take the place of smaller sodium ions near the surface, crowding it into compression. It is the same lesson about stress that annealing teaches, turned to advantage.

Glass in the everyday

Glass also insulates houses as glass wool, reinforces boats and wind turbine blades as fibreglass, holds medicines in vials (often borosilicate, for the same chemical resistance prized in the laboratory), and covers solar panels. The material in a pendant and the material that carries a video call are cousins.

What can be recycled

Container glass, the soda-lime glass of bottles and jars, is one of the most recyclable materials there is: it can be melted and remade many times without losing quality. Crushed recycled glass, called cullet, melts at a lower temperature than raw sand, soda and limestone, so every tonne used saves energy and raw materials, and avoids the carbon dioxide released when limestone and soda ash break down in the furnace. As a rough guide, glassmakers often quote energy savings of around 2 to 3 per cent for every 10 per cent of cullet in the batch.

Not all glass belongs in the bottle bank, though. Borosilicate (ovenware, laboratory glass and lampworked pieces), drinking glasses, window glass, mirrors, ceramics and glass-ceramic hobs melt differently from container glass. A small piece of borosilicate in a batch of bottle glass can stay unmelted and leave stones or weak spots in new containers, which may then be rejected. So borosilicate should be kept out of the glass recycling bin unless your local service says otherwise.

Waste at the torch

Lampworking produces little waste compared with many crafts, and most of it can be used again. Short ends of clear rod become punties; offcuts of colour can be melted together into new rods, crushed into frit or used for test samples (see Practice). Failed pieces teach: breaking a cracked piece cleanly and looking at the break often reveals whether the problem was stress, compatibility or a cold join. What cannot be reused should be wrapped and disposed of safely so it does not cut anyone handling the rubbish.

Caring for glass jewelry

Borosilicate is one of the toughest glasses for jewelry: it resists heat, water, soap and most chemicals, and does not tarnish or fade. It is still glass, and the main risk is a hard knock against stone, tile or metal. A few habits help a piece last for decades:

• Take it off for sport, heavy work, swimming in the sea and sleep, when knocks are most likely.
• Clean it with warm water, a drop of mild soap and a soft cloth or brush, then dry it. Avoid abrasive powders, which can dull a polished surface.
• Store pieces separately, in a soft pouch or a lined box, so they do not rub against each other or against harder stones.
• Check cords, chains and metal fittings from time to time; they wear long before the glass does.
• If a piece chips or cracks, stop wearing it, as a cracked edge can be sharp.

Looked after this way, glass outlives almost everything around it: museums are full of glass beads several thousand years old. The next entry follows glass into science and technology, through stories that run from a king who believed he was made of glass to the chips of artificial intelligence.

The king who was made of glass

Charles VI of France came to the throne in 1380 as a boy of eleven. In his twenties he began to suffer episodes of severe mental illness. In 1392, riding through a forest near Le Mans, he suddenly turned on his own companions with his sword and killed several of them before he could be restrained. A few months later, in January 1393, came the disaster remembered as the Bal des Ardents, the Ball of the Burning Men. The king and five courtiers danced at a masquerade disguised as "wild men", in linen costumes coated with resin and covered in flax. A torch carried by the king's brother came too close, and the costumes caught fire. Four of the dancers died. The king survived because the young Duchess of Berry threw her skirts over him; one other dancer saved himself by leaping into a tub of wine.

In the years that followed, Charles became convinced that his body was made of glass and might shatter at a touch. Contemporary accounts say he refused to let people near him and had iron rods sewn into his clothes to protect himself. He is often described as the first recorded case of what historians now call the glass delusion.

A fear of the age

Charles was not alone. Between the fifteenth and seventeenth centuries, physicians across Europe recorded patients who believed they were made of glass: some would not sit down for fear of breaking, others wrapped themselves in straw. Miguel de Cervantes wrote a short novel about it, El licenciado Vidriera ("The Glass Graduate", 1613), in which a brilliant young lawyer believes he has turned to glass and becomes, in his fragility, a sharp-tongued truth teller. Descartes used the delusion in 1641 as his example of a mind out of step with the world. As late as the nineteenth century, Princess Alexandra of Bavaria, daughter of King Ludwig I, was said to believe that as a child she had swallowed a glass piano.

Historians link the rise of the delusion to the material itself. In those centuries clear glass was new, precious and almost magical: transparent yet solid, beautiful yet fragile. The historian Edward Shorter has suggested that the mind tends to attach its fears to the new materials of each age. Glass, at that moment, was the newest material of all. It is a reminder, the podcast suggests, that every powerful new technology also enters our imagination, and is easy to project our hopes and fears onto. The delusion faded as glass became ordinary. Today a clear window is so common we barely see it.

An island of furnaces

In 1291 the Republic of Venice ordered its glass furnaces moved out of the city, built largely of wood, to the island of Murano in the lagoon, where a fire could not spread to the rest of Venice. The move also concentrated the craft in one place, and Venice guarded it closely. Murano's glassmakers were honoured citizens, and their daughters could marry into noble families, but they were forbidden to leave the Republic or teach their skills abroad. Those who fled risked severe punishment. The knowledge was too valuable to share.

Cristallo

Around the middle of the fifteenth century, the Murano glassmaker Angelo Barovier is credited with perfecting cristallo: a soda glass so clear and colourless that it looked like rock crystal. It was made with carefully purified plant ash and, very likely, manganese to cancel the green tint of iron (the chemistry is explained in Colour in Glass). For buyers used to greenish, bubbly glass, it was astonishing: a material that seemed to be almost nothing at all. Venice grew rich on it, and on mirrors, beads and filigrana glass (see A History of Glass and Lampworking).

Lenses, telescopes and a new sky

Clear glass made good lenses possible. Spectacles were invented in northern Italy in the late thirteenth century, and Venetian guild rules of around 1300 already mention "glass for the eyes". For the first time, people whose sight failed with age could keep reading and working for decades longer.

In 1608 spectacle makers in the Netherlands put two lenses in a tube and made the first telescopes. A year later Galileo Galilei, teaching in Padua, just inland from Venice, heard of the invention and built his own, grinding his own lenses. In January 1610 he saw four moons circling Jupiter, proof that not everything in the heavens revolves around the Earth, and published his observations that spring. His support for the idea that the Earth moves around the Sun led to his trial by the Church in 1633 and to house arrest for the rest of his life. A few decades later, microscopes opened the opposite world: Robert Hooke's Micrographia (1665) showed fleas, cork cells and the eye of a fly, and Antonie van Leeuwenhoek, using tiny glass lenses of his own making, became the first person to see bacteria.

The podcast's point is that this shook people deeply. Glass did not just improve sight. It changed the size and shape of the universe people lived in.

The same grain

Glass and computer chips begin with the same substance: silica, silicon dioxide, the mineral of quartz and of most sand (see What Glass Is). To make glass, silica is melted with fluxes and allowed to cool without crystallising. To make chips, the opposite happens: the oxygen is stripped away. Quartz is heated with carbon in an electric arc furnace to make raw silicon, which is then purified until only about one atom in a billion is anything other than silicon. That ultra-pure silicon is melted and a seed crystal is slowly drawn out of it, growing a single perfect crystal ingot. The ingot is sliced into wafers, and on those wafers billions of transistors are built.

Glass is woven through the whole process. The silicon is melted in crucibles of fused quartz glass, made from some of the purest quartz on Earth, much of it mined around the small town of Spruce Pine in North Carolina. When a hurricane flooded the area in 2024, the chip industry held its breath until the mines reopened.

Printing with light

The patterns on a chip are printed with light, in a process called photolithography. Light shines through a mask, a plate of extremely pure, ultra-flat quartz glass carrying the circuit design, and is focused down by lenses onto the wafer, shrinking the pattern many times. The most advanced machines, made by the Dutch company ASML, use extreme ultraviolet light, which glass absorbs, so they replace lenses with mirrors: some of the smoothest surfaces ever made, built by Zeiss, the same company whose collaboration with Otto Schott in Jena in the 1880s created modern optical glass and, in time, the borosilicate lampworkers use (see Beyond the Torch).

A new Murano

The podcast draws a striking parallel. Most of the world's most advanced chips, the ones that power AI, are made by a single company in Taiwan, TSMC, built on decades of accumulated skill, suppliers and know-how that are very hard to copy. Like Murano's glass secrets, that knowledge has become a source of wealth and a matter of state, and the island sits at the centre of competition between great powers. Five hundred years apart, two small islands mastered the art of turning sand into something that seems like magic.

Artificial intelligence runs on silicon, but it also depends on glass at almost every step.

Light through fibre

The data centres that train and run AI models move enormous quantities of information between thousands of chips, and between buildings and continents. Much of it travels as pulses of light through optical fibre, hair-thin strands of ultra-pure silica glass (see Beyond the Torch). As AI systems grow, engineers are bringing fibre closer and closer to the chips themselves, replacing copper wires with light.

Chips built on glass

Chips are mounted on a base called a substrate, usually made of layered resin. The largest AI processors are now so big that these resin substrates warp in the heat of manufacture. Glass is flatter, stiffer and can be made to expand at almost the same rate as silicon, so chipmakers including Intel and Samsung, and the company Absolics, are developing glass-core substrates for future AI chips, with the first aiming for production around 2026. The quality that makes borosilicate trustworthy at the torch, its stability under heat, is the same quality that makes glass attractive to chip designers.

Memory written in glass

Hard drives and magnetic tape last a few decades at most. Microsoft's Project Silica stores data inside glass instead: an ultra-fast laser writes tiny three-dimensional marks, called voxels, deep inside a glass platter, and a microscope with a camera and machine learning reads them back. Glass does not need power to keep data, does not rust, and resists heat, water and magnetism. In 2026 the team reported that the method works not only in costly fused silica but in ordinary borosilicate glass, the same family of glass as kitchen ovenware and lampwork, storing several terabytes on a platter about the size of a DVD, with an expected life of up to ten thousand years. The glass that holds a Rain and Rivers pendant may one day hold the memory of the internet.

Clear, but not thinking

The podcast returns to a thought that links the glass delusion to today. Cristallo seemed magical because it was so clear that it seemed to disappear. A chatbot can seem to understand us because its language is so fluent. But a large language model predicts likely words from patterns in its training; it does not see, feel or know the world the way a person does. Like a clear pane, it is easy to look through it and imagine there is someone on the other side. Knowing how a technology is made, whether glass or AI, is one of the best ways to use it well, and not to be fooled by it.

The Lycurgus Cup: Roman nanotechnology

A Roman glass cup made in the fourth century AD, now in the British Museum, looks jade green when lit from the front and glows ruby red when lit from behind. In 1990 scientists found why: the glass contains tiny particles of gold and silver, only tens of nanometres across, that scatter and absorb light differently depending on its direction. The Romans had made nanotechnology without knowing it. The same physics colours gold ruby glass at the torch (see Colour in Glass).

Prince Rupert's drops

Drip molten glass into cold water and it freezes into a tadpole-shaped drop. The head is so strong it can survive a hammer blow, but snap the thin tail and the whole drop explodes into powder. The outer skin cools first and locks the inside in tension and the surface in compression. The drops amazed the court of Charles II and the early Royal Society in the 1660s, named after Prince Rupert of the Rhine, who brought them to England. They are the same principle as the toughened glass of car windows and phone screens.

The mirror cast in Pyrex

For the 200-inch (5-metre) mirror of the Hale Telescope at Palomar Observatory in California, Corning cast a disc of Pyrex borosilicate weighing about 20 tons, ribbed like honeycomb on the back to save weight. The first pour, in March 1934, failed when parts of the mould broke loose and floated up through the molten glass; that disc is now on display at the Corning Museum of Glass. The second, poured in December 1934, was cooled by less than one degree a day for nearly a year to anneal it. From 1949 the telescope it became part of was the largest in the world for almost three decades. Borosilicate was chosen for exactly the reason it suits jewelry: it barely moves with changes in temperature.

Glass from the earth and the sky

Nature makes glass too. Lightning striking sand fuses it into hollow tubes called fulgurites. Volcanoes make obsidian (see Lichen and Volcanic Glass). In the Libyan Desert lies a pale yellow natural glass, formed about 29 million years ago, probably by the heat of a meteorite impact or airburst; a carved piece of it sits at the centre of a scarab on a pectoral found in the tomb of Tutankhamun. And on 16 July 1945, the first nuclear test in New Mexico turned the desert sand into a green glass now called trinitite: the moment the atomic age began left behind a layer of glass.

The screen in your hand

The cover glass on most smartphones descends from a 1960s Corning research programme that strengthened glass by ion exchange (see Beyond the Torch). It was shelved in the early 1970s for lack of a market, then revived in 2007 for the first iPhone: a glass technique that waited almost forty years for its moment. The ultra-flat glass is made by the fusion process, also invented at Corning in the 1960s, in which molten glass overflows both sides of a trough and the two streams meet below it, so neither face ever touches a tool.

From a frightened king to the chips that run artificial intelligence, glass keeps turning up wherever people push at the edge of what they can see and do. The next entry leaves technology for the volcano, where the earth makes its own glass and lichens slowly turn it back into soil.

How volcanoes make glass

Volcanic glass forms for the same reason all glass does (see What Glass Is): molten rock cools too quickly for its atoms to arrange themselves into crystals, and the disorder is frozen in place. Lava rich in silica is thick and sticky, and when it meets air or water it can solidify before crystals have time to grow.

• Obsidian is dense volcanic glass, usually black, sometimes brown, green or banded, or spotted with small white crystals in "snowflake" obsidian. Thin edges are often translucent. It fractures into edges sharper than steel, which is why people shaped it into blades for many thousands of years.
• Pumice is volcanic glass so full of gas bubbles that it can float.
• Volcanic ash is not ash in the sense of something burnt: it is mostly tiny shards of glass and rock, made when gas-rich magma shatters in an explosive eruption.
• Pele's hair and Pele's tears, named after the Hawaiian volcano deity, are threads and droplets of glass drawn out by the wind from fountaining lava.

Glass that does not last forever

Natural glass is not permanent. Over very long times it slowly takes in water and, given enough time and warmth, begins to crystallise: it devitrifies, as an overheated rod does at the torch, only over millions of years. This is why most obsidian is geologically young, and very old obsidian is rare. The slow intake of water forms a thin hydration rind on every fresh surface, and archaeologists measure its thickness to estimate the age of obsidian tools, a method called obsidian hydration dating.

A lichen is not one organism but a partnership. A fungus builds the body, and inside it lives a photosynthetic partner, a green alga or a cyanobacterium, that makes sugars from light. The fungus offers shelter, moisture and minerals; the partner offers food. Research in recent years has found that many lichens host further companions, such as yeasts and bacteria, so a lichen is closer to a small community than to an individual.

Lichens can live where almost nothing else can: on bare rock and fresh lava, on roofs, gravestones and old glass. They dry out and wait, and come back to life when wet. Many grow only millimetres a year, which makes them timekeepers of a kind: lichenometry estimates how long a surface has been exposed from the size of the lichens on it.

Lichens settle on the surface of volcanic glass and begin, grain by grain, to dissolve it. They work in two ways at once:

• Physically: the fungal threads, called hyphae, grow into the micro-fractures and bubbles of the glass. As the lichen swells when wet and shrinks when dry, it prises small fragments loose.
• Chemically: the lichen releases acids, oxalic acid above all, along with compounds found only in lichens. These draw metal ions such as calcium, magnesium and iron out of the glass, and often lock them into new crystals of oxalate at the boundary between lichen and stone.

In the language of the first entry: the acids carry away the modifier ions that sit in the gaps of the glass network, and the silica network, left open, weakens and crumbles. Because volcanic glass is disordered and unstable, it weathers faster than crystalline rock of a similar composition, and studies of young lava flows have found that rock covered by lichens weathers faster than bare rock beside it.

For something to decompose, it must first be porous.

Porosity is the precondition of this alliance. Obsidian and volcanic glass, with their micro-fractures and vesicles, are open enough to be inhabited, and it is that openness that lets the lichen take hold.

What the lichen begins, others continue. Ecologists call the colonisation of a new, bare surface primary succession, and lichens and mosses are often its pioneers. As the glass breaks down and lichens die and decay, the first thin soil gathers in the cracks. Moss follows, then ferns, then flowering plants, insects and birds.

The metals travel too. Obsidian and volcanic ash are dense with iron and other elements; as the lichen acids free them, they pass into the soil and into the tissues of what grows next. A metal released from a shard of glass can become part of a leaf, a wing, a flower. Colour becomes a language shared between the geological and the living, a way the volcano speaks itself through everything it becomes.

Capelinhos

Between 1957 and 1958, the Capelinhos volcano erupted off the western tip of Faial. It added new land to the island and buried houses, fields and a lighthouse under glassy ash, and the emigration that followed reshaped the community. Decades later, the recovery is carried by the beings able to reach it: lichens and mosses preparing the ground, the Azorean heather Erica azorica and the faya tree Morella faya taking root, seeds arriving in the digestion of migrating birds and on the backs of waves, terns returning each season to nest on the bare cones.

The rhythm is more-than-human.
An other velocity,
to undo the urgency
with which we usually relate to nature.

The usual vocabulary reads Capelinhos as disaster: loss, burial, departure. I am interested in what the eruption looks like from the position of the lichen, the tern, the mineral, the soil still forming, without erasing the people of Faial. Seen from there, the eruption is also an invitation, and human resilience becomes one thread in a wider symbiosis in which glass, stone, fungus, alga, plant, bird and human compose one another.

Glass also comes back from the sea. At the crossroads of transatlantic routes, the Azorean shore has received centuries of human residue since the fifteenth century: wine bottles, medicine jars, tableware and tiles, thrown into the water or lost in shipwrecks. The Atlantic grinds them against volcanic sand until they become sea glass, softened and frosted, and returns them to the beaches of São Miguel and Faial. The frost is not only scratching: in seawater, glass slowly loses some of its sodium and calcium, leaving a pitted, hydrated skin.

Lichens live on human glass too. On old church windows they settle with fungi in dust and moisture, and their acids pit and cloud the surface, one of the reasons medieval glass in damp places is so carefully conserved. It is the same digestion as on obsidian, only faster: medieval potash glass is far less durable than borosilicate or modern window glass.

This is where my investigation is heading. In my proposal for Faial I describe following this long metabolism as one continuous body: collecting, within local regulation, small amounts of ash, soil, volcanic glass and sea glass while walking again and again from the bare cone to the recovering slopes; carrying those minerals into my flamework in Barcelona, so that the territory itself colours the glass; and making glass structures to be offered to the lichens and slowly digested by them.

Alongside it, a lexicon: the community's words for the land and the eruption, gathered with the signs that structure the place, the seasons of the birds, the spread of the lichens, the returns of the sea. Listening across registers, as Pauline Oliveros proposed in Quantum Listening, as a continuation of How to Practice Porosity.

Decay as an intelligence, a multispecies labour of repair.

From the volcano back to words: the final entry gathers the language of glass into a glossary to keep at hand while reading and making.

Alkali: a compound of sodium or potassium (soda or potash) added to silica as a flux, lowering its melting point so it can be melted in a furnace. Most historical glass is soda glass or potash glass.

Amorphous solid: a solid whose atoms have no repeating long-range order. Glass is the best known example. See What Glass Is.

Annealing: holding glass at a temperature where internal stress can relax, then cooling it slowly and evenly so it does not crack later. See Finishing, Combining and Troubleshooting.

Annealing point: the temperature at which stress in a glass relaxes within minutes. For common lampworking borosilicate it is around 560 °C.

Annealer (lehr): the oven in which glass is annealed. "Lehr" usually means a long tunnel annealer in a factory.

Applied decoration: hot glass added to the surface of a piece as threads, dots, handles or prunts.

Aventurine: glass containing tiny flat crystals, often of copper, that glitter inside it. Developed in Venice; the name comes from a ventura, "by chance".

Batch: the mixture of raw materials (sand, soda, lime, colourants and often cullet) loaded into a furnace to be melted.

Bead release: a clay-based coating on a mandrel that stops the glass from sticking, so the finished bead slides off. See Techniques at the Torch.

Blowpipe: the hollow steel tube used at the furnace to gather and blow glass.

Blow tube: a lampworker's tube or hose for blowing into hollow work at the torch.

Borosilicate: glass in which boron oxide replaces much of the soda and lime, giving it low thermal expansion and strong resistance to heat and chemicals. Known by brand names such as Duran and Pyrex (in its original form). The glass of Rain and Rivers. See What Glass Is.

Boro: the workshop nickname for borosilicate.

Bullseye: the thick centre of a disc of crown glass, where the punty was attached.

Butt seal: a join between two tubes of the same diameter, end to end. See Techniques at the Torch.

Came: the H-shaped strip of lead (or zinc, brass or copper) that holds the pieces of a stained glass window.

Cameo glass: layered glass carved so that one colour stands in relief against another.

Cane: a solid rod of glass, plain or patterned, often pulled from a larger gather. Patterned cane is the raw material of filigrana and millefiori.

Casing: covering one layer of glass with another of a different colour. At the torch this is usually called encasing.

Cast glass: glass shaped by melting it into a mould, in a furnace or a kiln.

Chemically strengthened glass: glass toughened by exchanging small sodium ions at the surface for larger potassium ions, putting the surface into compression. Used on most phone screens.

COE (coefficient of expansion): how much a glass expands for each degree it is heated. Glasses with different COE cannot usually be combined. Common borosilicate has a COE of about 33 (3.3 × 10^-6 per °C). See What Glass Is.

Cold working: shaping or finishing glass when it is cold: cutting, grinding, polishing, engraving, sandblasting.

Colour centre: a defect in the glass structure that traps an electron and absorbs light, causing colour. Sunlight turns some old manganese-cleared glass purple this way.

Compatibility: whether two glasses can be fused without cracking as they cool. It depends mostly on matching expansion and viscosity.

Crizzling: a network of fine cracks in old glass caused by an unstable recipe with too much alkali and too little lime. The glass slowly takes in moisture and decays.

Crown glass: early window glass made by spinning a blown bubble into a flat disc.

Crystal (lead crystal): glass containing lead oxide, which makes it heavy, bright and easy to cut. In everyday speech, any fine clear glass, though glass is not a crystal in the scientific sense.

Cullet: broken or recycled glass added to the batch. It melts more easily than raw materials and saves energy.

Devitrification: the growth of crystals in glass that has been held too long at certain temperatures, leaving a frosted, scummy or rough surface. See What Glass Is.

Diamond point engraving: scratching a design into glass with a diamond or hard metal point.

Dichroic glass: glass that shows one colour by transmitted light and another by reflected light. Modern dichroic glass is coated with many microscopic layers of metal oxides. See Colour in Glass.

Dispersion: how much a glass spreads white light into its colours. High dispersion gives cut crystal its fire.

Edo kiriko: Japanese cut glass from Tokyo (formerly Edo), with bold geometric patterns, first made by the glass merchant Kagaya Kyūbei in 1834. See A History of Glass and Lampworking.

Encasing: covering a core, often coloured, with a layer of clear glass at the torch. See Techniques at the Torch.

Enamel: a coloured glass powder mixed with a medium, painted onto glass and fired so it fuses to the surface.

Enamelled glass: glass decorated with enamel and often gold, fused on in a second firing. Brought to great heights in thirteenth- and fourteenth-century Syria and Egypt, then in Venice. See A History of Glass and Lampworking.

Engraving: cutting a design into the surface of glass with a wheel, a point or a burr.

Etching: frosting or cutting the surface of glass with acid or abrasive, usually through a resist that protects the rest.

Faience: a glazed, quartz-based ceramic from ancient Egypt and Mesopotamia, a close ancestor of glass.

Filigrana: the family name for Venetian techniques using cane with threads of white or coloured glass inside clear glass. See Techniques at the Torch.

Fire polishing: briefly reheating glass so its surface melts slightly and turns glossy, removing tool marks or cold-worked texture.

Flameworking: another name for lampworking, shaping glass from rods and tubes in a torch flame. Some use "flameworking" for borosilicate and "lampworking" for soft glass, but the two words mean the same craft. See A History of Glass and Lampworking.

Flashback arrestor: a safety valve fitted to gas lines that stops a flame travelling back into the hose or cylinder. See The Studio.

Flashing: (1) passing a cold rod in and out of the flame to warm it gently before working. (2) In glassmaking, coating clear glass with a thin layer of coloured glass.

Float glass: flat glass made by floating molten glass on a bath of molten tin. Almost all modern window glass. See Beyond the Torch.

Flux: an ingredient that lowers the melting temperature of silica, such as soda, potash or boron oxide.

Frit: glass crushed into grains, from fine to coarse, used to add colour and texture.

Fuming: vaporising a precious metal, usually silver or gold, in the flame so a thin film settles on the glass and colours it. See Colour in Glass.

Fusing: joining pieces of glass by heating them together in a kiln until they melt into one.

Gaffer: the lead glassblower in a hot shop team.

Gather: a mass of molten glass collected on the end of a rod, punty or blowpipe. To gather is to collect it. See Techniques at the Torch.

Glass spinning: drawing a thread of hot glass from the flame onto a turning wheel, so fine it can be woven or looped into plumes, hair or fabric. A favourite of travelling lampworkers from the eighteenth century. See A History of Glass and Lampworking.

Glass transition: the temperature range in which a cooling liquid stiffens into a glass instead of crystallising. See What Glass Is.

Glory hole: the open furnace used to reheat work during glassblowing.

Gold ruby: red glass coloured by tiny particles of gold, which only develops its colour after reheating (striking). See Colour in Glass.

Graphite: a slippery, heat-resistant form of carbon used for paddles, marvers and reamers, because hot glass does not stick to it.

Grisaille: grey or brown vitreous paint used for details on stained glass, or a window painted mostly in it.

Heat base: a core of heat kept in the body of a piece while you work on one part, so it does not cool too far and crack. See Techniques at the Torch.

Hot shop: a studio built around a furnace, glory hole and annealer for glassblowing.

Implosion: a borosilicate technique in which a design is built on the inside of a sphere or form and seen through clear glass, often from the back. See Techniques at the Torch.

Incalmo: joining two separately blown bubbles of different colours at their open rims to make one vessel with bands of colour.

Iridescence: a rainbow shimmer on glass. On ancient glass it comes from weathering of the surface; on modern glass, from a metallic coating or fuming.

Jacks: large tweezer-like tools with blunt blades used to shape necks and openings.

Kiln: an insulated oven with precise temperature control, used for annealing, fusing, slumping and casting. See The Studio.

Kilnforming: all the ways of shaping glass inside a kiln, such as fusing, slumping and casting. Also called warm glass.

Lampworking: shaping glass from rods and tubes in a small, controlled flame. The name comes from the oil lamps, blown by bellows, that early makers used. See A History of Glass and Lampworking.

Latticino: twisted cane with spiralling white or coloured threads, or glass made with it. See Techniques at the Torch.

Lathe: a machine that holds and turns tube evenly in the flame, for perfectly symmetrical work.

Lichen: a partnership between a fungus and an alga or cyanobacterium, often with other microbes. Lichens are among the first beings to settle on volcanic glass, and their acids slowly dissolve it into soil. See Lichen and Volcanic Glass.

Lost-wax casting: making a wax model, encasing it in a refractory mould, melting the wax out and filling the space with glass.

Mandrel: a steel rod coated in bead release, on which beads are wound so they have a hole.

Marver: a flat, smooth surface of steel or graphite on which hot glass is rolled to shape it. Also the verb: to marver.

Millefiori: "a thousand flowers": slices of patterned cane arranged side by side and fused, so the patterns appear as a field of tiny flowers. See Techniques at the Torch.

Mosaic glass: glass made by fusing slices of coloured cane side by side, developed in the Hellenistic world. The ancestor of millefiori. See A History of Glass and Lampworking.

Mosque lamp: a tall enamelled and gilded glass lamp hung in mosques and tombs, the masterpiece of Mamluk Egypt and Syria. See A History of Glass and Lampworking.

Mould-blowing: inflating hot glass inside a carved mould so the same shape and relief can be repeated. Developed in the first century CE; the glassmaker Ennion signed his. See A History of Glass and Lampworking.

Murrina (plural murrine): a slice of patterned cane, or glass made from such slices.

Network former: an oxide, above all silica, that builds the linked framework of glass. Boron oxide is another.

Network modifier: an oxide, such as soda or lime, that breaks links in the framework, making the glass easier to melt and changing its properties.

Neutral flame: a flame with a balanced ratio of fuel and oxygen, neither oxidising nor reducing. See The Flame and Reading Heat.

Obsidian: natural volcanic glass, used for tools and ornaments for hundreds of thousands of years before glass was made by people. See Lichen and Volcanic Glass.

Opal (opalescent) glass: glass made milky by tiny particles or crystals that scatter light.

Oxidising flame: a flame with more oxygen than is needed to burn the fuel. It keeps colours clean and bright. See The Flame and Reading Heat.

Pâte de verre: "glass paste": crushed glass packed into a mould and fired until the grains fuse, giving a grainy, translucent texture.

Point: a narrow neck pulled on a tube or rod, used as a handle or to close a tube.

Polariscope: a viewer with two polarising filters that shows stress in glass as bands of colour or light. See Finishing, Combining and Troubleshooting.

Pontil mark: the rough scar left on the base of a blown piece where the punty was broken off.

Potash glass: glass using potassium from plant or wood ash as its flux, typical of medieval northern Europe (forest glass, Waldglas).

Powder-glass bead: a bead made from crushed glass, either fused in clay moulds, as in the Krobo beads of Ghana, or mixed with a binder, shaped by hand and fused over a fire, as in the Kiffa beads of Mauritania. See A History of Glass and Lampworking.

Prunt: a blob of glass applied to a vessel and often stamped with a pattern, both as decoration and to give a grip.

Pumice: volcanic glass so full of gas bubbles that it can float on water. See Lichen and Volcanic Glass.

Punty (pontil): a rod attached to a piece to hold it while another part is worked. At the torch, often a scrap rod.

Quartz (fused silica): glass made from pure silica. It can stand extraordinary heat but needs very high temperatures and a hydrogen flame to work.

Reamer: a pointed tool, often graphite or brass, used to open and widen holes and tube ends.

Reducing flame: a flame with more fuel than oxygen. It pulls oxygen from metal oxides in some colours, giving metallic or lustrous surfaces. See The Flame and Reading Heat.

Refractive index: a measure of how much a glass bends light. Lead crystal has a high refractive index, which makes it sparkle.

Reticello: glass with a fine net pattern made from two layers of latticino twisted in opposite directions, often with tiny air bubbles trapped in each mesh. See Techniques at the Torch.

Ring seal: a join where a smaller tube passes through and is sealed inside a larger one, as in many scientific instruments.

Sandblasting: frosting or carving glass with a jet of fine abrasive. See Finishing, Combining and Troubleshooting.

Scientific glassblowing: making and repairing laboratory apparatus from borosilicate and quartz tube, the craft from which much borosilicate art grew.

Seed: very small bubbles in glass, common in old window and hand-blown glass.

Silica: silicon dioxide, the main ingredient of almost all glass, found as quartz sand.

Silver stain: a silver compound painted onto glass and fired, turning it yellow to amber. Used in stained glass since the early fourteenth century.

Slumping: heating flat glass in a kiln until it sags into or over a mould.

Soda-lime glass: the everyday glass of bottles, jars and windows, made mostly of silica, soda and lime.

Soft glass: lampworkers' name for glasses with higher expansion than borosilicate, such as Italian soda-lime rods (COE about 104). They melt at lower temperatures and must be heated more gently.

Softening point: the temperature at which glass begins to sag under its own weight. Around 820 °C for common lampworking borosilicate.

Strain point: the temperature below which glass is rigid enough that stress can no longer relax. Cooling must be slow down to this point.

Stringer: a thin thread of glass pulled from a rod, used to draw lines and details.

Striking: reheating certain colours so that particles or crystals grow inside them and the colour appears or deepens. See Colour in Glass.

Tack fuse: fusing just enough for pieces to stick together while keeping their shape.

Tempered (toughened) glass: glass heated and cooled rapidly at the surface so it is held in compression, making it much stronger. It breaks into small blunt pieces.

Thermal shock: cracking caused by uneven heating or cooling, when one part of a piece expands or contracts faster than another. See The Flame and Reading Heat.

Twistie: a lampworker's twisted cane, made by twisting two or more colours together while hot.

Venetian glass: glass made in Venice and on the island of Murano, famous since the fifteenth century for cristallo, filigrana, millefiori and fine blown work.

Vitreous: glassy; made of or like glass. Vitreous paints and enamels are fused onto glass by firing.

Viscosity: a liquid's resistance to flow. Glass becomes steadily less viscous as it heats, which is what lets us shape it. See The Flame and Reading Heat.

Weathering: slow chemical change on the surface of glass buried or exposed for long periods, often leaving iridescent or flaking layers on ancient glass. Lichens and fungi can speed it up; see Lichen and Volcanic Glass.

Working point: the temperature at which glass is soft enough to be shaped easily. Around 1,250 °C for common lampworking borosilicate.

Zwischengoldglas: German for "gold between glass": a design in gold leaf sealed between two closely fitting layers of glass, most famously in Bohemian beakers of the early eighteenth century.

Words are a good place to end and to begin: every one of them points back to a gesture at the torch or an object you can go and see. Return to any entry above to read more, or begin again at What Glass Is.