Written in Rain III: A Porous Glossary

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.