A Glossary of Glass

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.