Part of The Grand Glossary: A Collector's Reference. This section covers the specialist vocabulary of art glass and decorative glass, from the fundamental processes of the furnace to the identification of specific glass types, techniques, and makers. Terms are arranged alphabetically within thematic groupings.
Processes and Materials
Annealing
The controlled cooling of glass in a specialised oven called a lehr, designed to relieve the internal stress introduced during the blowing or forming process. Glass that is cooled too rapidly develops tension between its outer and inner layers, rendering it brittle and vulnerable to cracking. The annealing cycle may last several hours, with the temperature reduced at a carefully managed rate. The duration and profile of the anneal affect the structural integrity and longevity of the finished piece.
Batch
The raw mixture of silica sand, soda ash, limestone, and additional minerals and colourants that is melted together in a furnace to produce molten glass. The precise composition of the batch determines the working characteristics and finished properties of the glass, including its colour, clarity, refractive index, thermal behaviour, and susceptibility to deterioration. Glass chemists adjusted batch recipes carefully; small variations in alkali content could produce glass prone to crizzling.
Cullet
Waste or broken glass added to a new batch of raw materials before melting. Cullet lowers the effective melting point of the batch, reduces fuel consumption, and improves the homogeneity of the melt. It also allows glass houses to recycle off-cuts, rejected pieces, and broken stock. In some historical traditions cullet was carefully selected for quality and colour consistency.
Frit
A partially fused or pre-melted mixture of glass-forming materials that has been quenched in water to shatter it into small fragments. Frit is added to the batch as a homogenising agent, or used in powdered form in decorative techniques such as marvering coloured powders into a gather. Frits may be prepared in a range of colours and opacities for specific decorative applications.
Gather
A mass of molten glass collected on the end of a blowpipe or punty rod by dipping it into the furnace pot. The glassblower works the gather by blowing air through the pipe, rotating the rod, and manipulating the glass with tools before it cools and stiffens. Multiple gathers may be applied to build up layers of colour, thickness, or decoration. The size and quality of the gather are critical to the success of the piece.
Glory Hole
A secondary opening or chamber in a glass furnace, separate from the main melting pot, used to reheat a piece during working. Glass cools rapidly and loses workability within seconds; the glory hole allows the glassblower to restore plasticity without returning the piece to the main furnace. It is an essential element of the hot glass studio and is used repeatedly during the making of a single piece.
Lehr
A temperature-controlled tunnel kiln through which freshly made glass passes, either continuously on a conveyor belt or in batches, to be slowly and uniformly cooled during annealing. Modern lehrs operate at precisely controlled temperatures across a graduated series of zones. The lehr is as fundamental to glass production as the furnace itself.
Marvering
The process of rolling a gather of molten glass on a smooth, flat surface — traditionally of iron marble (from which the term derives), later of steel — to shape, consolidate, and chill the outer skin of the glass. During marvering, coloured frits, powders, or other inclusions can be pressed into the surface of the gather before being encased in a subsequent layer of clear glass. The technique is central to many colour and inclusion techniques in studio and traditional glass.
Glass Types and Compositions
Flint Glass
A highly refined, lead-bearing glass developed in England in the late 17th century, credited to the experiments of George Ravenscroft under the patronage of the Glass Sellers’ Company. The addition of lead oxide in significant proportion produces a glass of exceptional clarity, density, and refractive quality, with a characteristic brilliance and a deep, resonant ring when struck. It is well suited to cutting and engraving. The term derives from the original use of calcined flint as the silica source, though this was later replaced by sand. English lead crystal set the standard for fine tableglass across Europe.
Lead Crystal
Glass containing a significant proportion of lead oxide, typically 24% or more by weight (full lead crystal contains 30% or more). Lead crystal has a higher refractive index than standard soda-lime glass, giving it exceptional brilliance and a prismatic quality when cut. It is denser and heavier than soda glass, cuts cleanly to a sharp edge, and produces a clear, sustained tone when struck. In the United Kingdom, the term lead crystal is legally defined and regulated: glass must meet specified lead content and refractive index thresholds to be marketed as such. The finest English lead crystal was produced at houses including Stuart and Sons, Webb Corbett, Royal Brierley, and Thomas Webb and Sons.
Opalescent Glass
Glass with a milky, translucent body that transmits and scatters light to produce an internal glow reminiscent of an opal. The effect is produced by adding fluorides, phosphates, or other opacifiers to the batch, which form submicroscopic particles that scatter short wavelengths of light. Opalescent glass is distinct from opaque glass in that light passes through the body of the piece. It was widely used by René Lalique, by the Northwood, Fenton, and Imperial factories in America, and by various English manufacturers, and appears frequently in Art Nouveau and Art Deco decorative glass.
Soda Glass
Glass made with soda ash (sodium carbonate) as the principal flux, producing a lighter, less refractive material than lead crystal. Soda glass has a lower working temperature and remains plastic for longer, making it well suited to lampworking and the production of fine, complex forms. Venetian and Murano glass is typically a refined soda glass, using barilla or levantine soda of exceptional purity. Soda glass does not ring when struck and has a lower density than lead crystal.
Uranium Glass
Glass to which uranium oxide has been added as a colourant, producing shades from pale straw yellow through to vivid apple green. Uranium glass fluoresces a strong, characteristic green under ultraviolet light, which provides a reliable identification test. It was produced widely in Bohemia, England, and America from the mid-19th century until uranium became a controlled material in the mid-20th century. It is also known as Vaseline glass when it exhibits the characteristic yellow-green colour of petroleum jelly. Uranium glass is mildly radioactive but is generally considered safe for display and handling under normal conditions.
Forming Techniques
Free Blowing
The shaping of glass entirely by the breath and manual manipulation of the glassblower, without the use of a mould. Free blowing requires considerable skill and allows for fluid, organic forms. It is the dominant technique in the Venetian and Murano traditions and in studio glass. Subtle irregularities of form produced by free blowing are a characteristic mark of hand craftsmanship and should not be confused with defects.
Lampworking (Flameworking)
A glassworking technique in which glass rods or tubes are softened and worked in the flame of a bench-mounted torch rather than in a furnace. Lampworking allows the creation of extremely fine and detailed work — miniature figures, paperweight canes, scientific glassware, and beads — that would be impossible at furnace scale. The technique requires a different set of physical skills from furnace glassblowing and uses borosilicate or soft soda glass depending on the application.
Mould Blowing
A glassblowing technique in which a gather of molten glass is inflated inside a pre-formed mould of wood, iron, or clay, causing the glass to take on the shape and any impressed decorative pattern of the mould interior. Mould blowing was widespread in Roman glass production and continued through to the industrial era, enabling consistent forms and repeating patterns. Dip moulds shape the form only; pattern moulds impart decoration and may be opened to allow the piece to be further expanded and finished freehand.
Pressing
The formation of glass by pressing molten glass into a metal mould using a mechanical plunger. The process was mechanised in the United States in the 1820s and enabled the mass production of complex patterns at low cost. Pressed glass is identified by its slightly rounded, fire-polished interior surfaces and mould seam lines, in contrast to the sharp, polished facets of cut glass. American Brilliant Period pressed glass and British Victorian pressed glass are collected in their own right.
Decorative Techniques
Acid Etching
A technique for decorating or matting the surface of glass using hydrofluoric acid or acid-bearing compounds. Acid etching may produce an overall frosted finish, or be combined with resist materials to create decorative patterns and pictorial designs. It was employed extensively in the late 19th and early 20th centuries for cameo glass production, particularly by the Daum factory and by Thomas Webb and Sons, as an alternative or supplement to wheel cutting. The depth and evenness of the etch, and the precision of the resist work, are indicators of quality.
Air Twist
A decorative technique used in the stems of English drinking glasses of the mid-18th century, in which columns of trapped air are drawn out and twisted into spiral threads within the glass. Air twists are produced by pressing a pattern of indentations into a gather of glass, trapping air pockets, and then elongating and rotating the gather. The resulting stems are classified by the arrangement of their threads — single series, double series, and compound — and are a defining feature of mid-Georgian English tableglass. See also the companion section on Glass Drinking Vessels.
Casing
The enclosure of one or more layers of glass within another of a different colour. The layers are gathered separately and fused in the furnace, then blown together so they expand and bond as a single unit. Cased glass allows for rich colour contrasts and forms the structural basis for cameo glass. It is distinct from flashing, where a very thin film of coloured glass is applied to the surface by dipping or trailing rather than by structural encasement. The number of layers and the thickness of each are significant in assessing cameo pieces.
Engraving
The decoration of glass by cutting into the surface with a rotating copper wheel fed with an abrasive slurry, or by using a handheld diamond-tipped or hardstone point. Wheel engraving allows for detailed pictorial, armorial, or naturalistic designs and was practised with exceptional skill in Bohemia, Silesia, and Germany from the 17th century, and in England from the 18th. Intaglio engraving cuts below the surface; relief engraving (Hochschnitt) cuts away the background to leave the design standing proud. Stipple engraving, which creates tone through a pattern of tiny diamond-point dots, is associated particularly with Dutch craftsmen of the 18th century, including Frans Greenwood and David Wolff.
Flashing
A thin coating of coloured glass applied to the surface of a clear or differently coloured glass base, typically by dipping the piece into a pot of contrasting molten glass. The flashed layer is thin and susceptible to wear, and may be cut or engraved through to reveal the base colour beneath. Flashing is widely used in stained glass and in Bohemian decorative glass of the 19th century, where it allowed the production of richly coloured pieces at lower cost than full casing.
Gilding
The application of gold decoration to the surface of glass, either by painting gold leaf or gold powder mixed with a flux onto the cold surface and firing it, or by cold gilding using an adhesive without firing. Fired gilding is more durable. Gold decoration on glass is found across many traditions, from Islamic enamelled and gilded mosque lamps to Bohemian Zwischengoldglas (gold-sandwich glass) and the richly gilt pieces of the 19th-century Stourbridge factories.
Inclusions
Decorative or structural elements deliberately embedded within the body of a glass object during its making. Inclusions may comprise trapped air bubbles (controlled or random), metallic foils (gold or silver leaf), coloured glass threads, aventurine (glass fused with copper or gold particles to produce a sparkling effect), or millefiori canes. Inclusions are a deliberate and valued feature in many art glass traditions, from Venetian work of the Renaissance to Scandinavian studio glass of the 20th century.
Iridescence
A surface effect producing a shifting, rainbow-like play of colour across the glass, similar to the surface of a soap bubble or the wing of a beetle. In ancient and excavated glass, iridescence results from long-term chemical weathering and the formation of thin surface layers that diffract light. In art glass of the late 19th and early 20th centuries, the effect was deliberately produced by exposing hot glass to metallic oxide vapours — tin, iron, or titanium — that deposited thin films on the surface. The technique is most strongly associated with Tiffany’s Favrile glass and with the iridescent wares of Loetz of Bohemia.
Specific Glass Types and Traditions
Cameo Glass
A type of cased glass in which two or more layers of differently coloured glass are fused together, with the outer layer subsequently carved, acid-etched, or wheel-cut to reveal the contrasting layer beneath. The technique derives from ancient Roman work in hardstone cameo and was applied to glass in the Roman period, as exemplified by the Portland Vase. It was revived with great refinement during the late 19th century, most notably by Émile Gallé and the Daum Frères factory in Nancy, and by Thomas Webb and Sons and Stevens and Williams in England. The quality of cameo glass is assessed by the precision of the carving, the depth of relief, and the complexity of the design.
Favrile Glass
A proprietary art glass developed and produced by Louis Comfort Tiffany from the 1890s, characterised by rich iridescent surfaces in amber, gold, peacock blue, and green, and by organic, naturalistic forms. The iridescence was achieved by exposing the hot glass to metallic oxide vapours in the furnace. The name derives from an Old English word for handmade. Favrile glass was produced in a wide range of forms including vases, lamps, bowls, and paperweights, and is among the most collected of American art glass. Pieces are typically signed and sometimes numbered.
Millefiori
Italian for “a thousand flowers.” A decorative technique of ancient origin in which bundles of differently coloured glass rods are fused together and drawn out into long canes, each of which reveals a floral or geometric pattern in cross-section. The canes are sliced into thin wafers, arranged in patterns, and fused into paperweights, vessels, or decorative plaques. The technique was revived to great effect in Murano during the 19th century, particularly by the Fratelli Toso and Pietro Bigaglia, and remains central to the Murano glass tradition. French paperweight factories at Clichy, Baccarat, and Saint-Louis produced millefiori weights of exceptional quality in the mid-19th century.
Murrine
Cross-sections of fused and drawn glass canes, each revealing a pattern — typically floral, geometric, or figurative — when sliced. Murrine are used individually or in combination to create millefiori and mosaic glass. They are made by bundling coloured rods together, fusing them with heat, and drawing the fused bundle out until the desired scale is achieved. The term is Italian: the singular is murrina, the plural murrine. Contemporary Murano artists have extended the murrine tradition into large-scale sculptural work.
Pâte de Verre
French for “glass paste.” A technique in which powdered or finely crushed glass is mixed with a binding agent and colourants, packed into a mould, and kiln-fired at a temperature sufficient to fuse the particles without fully melting them. The result is a dense, slightly granular, opaque or translucent material of great sculptural subtlety. Pâte de verre was developed in France in the late 19th century, most notably by Henri Cros and Amaury-Duval, and was later used with distinction by François-Émile Decorchemont and Gabriel Argy-Rousseau.
Sommerso
Italian for “submerged.” A Murano glassblowing technique in which one or more layers of intensely coloured glass are encased within a thick outer layer of clear or lightly tinted glass, creating a deep, jewel-like effect in which the colours appear suspended within the body of the object. Sommerso was developed at the Seguso Vetri d’Arte factory in the late 1930s and became closely associated with the work of Flavio Poli in the 1950s and 1960s. The technique demands precise control of layer thickness and colour combination and has been widely imitated.
Paperweights
Carpet Ground
A paperweight background composed of closely packed, uniform millefiori canes arranged to cover the entire base of the weight, creating a dense, even surface from which a central motif — typically a flower or complex cane — is set. Carpet grounds are associated particularly with the French factories of Clichy and Baccarat and are among the most technically demanding of all paperweight backgrounds to execute consistently.
Faceting
The cutting of flat, polished faces into the exterior of a finished glass paperweight, either as a decorative treatment in itself or to provide lenses through which the interior design can be viewed from different angles. Common facet arrangements include top windows, side printies (circular or oval concavities), and strawberry diamonds. Faceting was carried out after annealing and requires a separate cutting and polishing process.
Lampwork (in Paperweights)
The creation of decorative elements — typically flowers, fruits, insects, or reptiles — for paperweight interiors using the lampworking technique, in which glass rods are softened in a flame and manipulated with fine tools to build up naturalistic forms. Lampwork paperweights from the classic period (approximately 1845–1860) produced at Baccarat, Saint-Louis, and Clichy, and in England at Whitefriars, are among the most highly regarded in the field. The complexity and naturalism of the lampwork elements are primary indicators of quality.
Pontil Mark
A scar, rough area, or polished depression on the base of a glass object, left when the pontil rod is detached after the piece is finished. In paperweights, the pontil mark is typically ground and polished to a smooth, flat or slightly concave finish. The character of the pontil mark provides useful evidence for dating and attributing a piece: an unground, rough pontil is typical of earlier or more utilitarian production; a ground and polished pontil indicates finer quality work. The size and form of the mark vary by period, factory, and maker.
Condition and Assessment
Bruise
An area of internal fracturing within a glass object, caused by a sharp impact that has not penetrated the surface. Bruises appear as a white, cloudy, or star-shaped opacity within the body of the glass and are distinct from surface chips or cracks. They cannot be polished out and represent a permanent structural weakness. In the assessment of paperweights, a bruise to the dome is considered more significant than one to the base.
Crizzling
A progressive form of deterioration in glass caused by an imbalance in the chemical composition of the batch, typically an excess of alkali relative to the silica and stabilising agents. It manifests as a network of fine internal cracks, surface weeping (the leaching of alkali to the surface as a sticky film), or a cloudy, iridescent appearance. Glass made in the 17th and early 18th centuries is particularly susceptible, including some pieces attributed to the workshop of George Ravenscroft during his early experiments with lead crystal. Crizzling is progressive and irreversible; storage in stable, low-humidity conditions slows but does not halt the process.
Flea Bite
A very small chip to the rim, foot, or edge of a glass object, typically the size of a flea’s bite. Flea bites are extremely common on antique glass and are generally considered minor condition issues in the trade. They may be polished out on the rim of a vessel without significantly altering the profile, though any such intervention should be disclosed.
Sickness
A term used in the trade to describe a cloudy, milky, or iridescent film on the interior or exterior surface of a glass vessel, typically caused by moisture, mineral deposits, or chemical reaction with the contents over a long period. Sickness ranges from light haziness, which may sometimes be partially removed by careful cleaning, to severe etching of the surface, which is permanent. It is a common condition issue in decanters, carafes, and other vessels used to hold wine, spirits, or water.
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