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Slumping is a technique in which items are made in a kiln by means of shaping glass over molds at high temperatures. The slumping of a pyrometric cone is often used to measure temperature in a kiln.
Slumping glass is a highly technical operation that is subject to many variations, both controlled and uncontrolled. When an item is being slumped in a kiln, the mold over which it is being formed (which can be made of either ceramic, sand or metal) must be coated with a release agent that will stop the molten glass from sticking to the mold. Such release agents, a typical one being boron nitride, give off toxic fumes when they are first heated and must be used in a ventilated area.
The glass is cut to the shape of the mold (but slightly larger to allow for shrinkage) and placed on top of it, before the kiln is heated.
The stages of the firing can be varied but typically start to climb at quite a rapid rate until the heat places the glass in an "orange state" i.e., flexible. At that point, gravity will allow the glass to slump into the mold and the temperature is held at a constant for a period that is known as the "soak". Following this stage, the kiln is allowed to cool slowly so that the slumped glass can anneal and be removed from the kiln. If two differing colours of glass are used in a single piece of work, the same CoE (coefficient of thermal expansion) glass must be used, or the finished piece will suffer from fractures as the glass will shrink at differing rates and allow tension to build up to the point of destruction. To compensate for this, many glass manufacturers subscribe to make glass to the same CoE. Examples include Spectrum glass system 96 or uroboros 96 series, and the use of this glass will allow the cooling to remain uniform and ensure that no tension builds up as the work cools.
During the Roman period open vessels, such as bowls and plates, could be produced by forming a glass sheet over a core or former. This technique resulted in vessels with rough surfaces, which could then be ground or polished to a smooth finish. [1] An additional technique, used in the production of Roman pillar-moulded bowls, utilised a slotted tool to impress ribs on the glass sheet prior to slumping. This created a bowl with a ribbed exterior, and these were then polished around the rim and sometimes given horizontal cut lines inside for further decoration. [1]
A kiln is a thermally insulated chamber, a type of oven, that produces temperatures sufficient to complete some process, such as hardening, drying, or chemical changes. Kilns have been used for millennia to turn objects made from clay into pottery, tiles and bricks. Various industries use rotary kilns for pyroprocessing—to calcinate ores, to calcinate limestone to lime for cement, and to transform many other materials.
Raku ware is a type of Japanese pottery traditionally used in Japanese tea ceremonies, most often in the form of chawan tea bowls. It is traditionally characterised by being hand-shaped rather than thrown, fairly porous vessels, which result from low firing temperatures, lead glazes and the removal of pieces from the kiln while still glowing hot. In the traditional Japanese process, the fired raku piece is removed from the hot kiln and is allowed to cool in the open air.
Fiberglass, or fibreglass is a common type of fiber-reinforced plastic using glass fiber. The fibers may be randomly arranged, flattened into a sheet, or woven into a fabric. The plastic matrix may be a thermoset polymer matrix—most often based on thermosetting polymers such as epoxy, polyester resin, or vinylester—or a thermoplastic.
Glassblowing is a glassforming technique that involves inflating molten glass into a bubble with the aid of a blowpipe. A person who blows glass is called a glassblower, glassmith, or gaffer. A lampworker manipulates glass with the use of a torch on a smaller scale, such as in producing precision laboratory glassware out of borosilicate glass.
Art glass is an item that is made, generally as an artwork for decoration but often also for utility, from glass, sometimes combined with other materials. Techniques include stained glass windows, leaded lights, glass that has been placed into a kiln so that it will mould into a shape, glassblowing, sandblasted glass, and copper-foil glasswork. In general the term is restricted to relatively modern pieces made by people who see themselves as artists who have chosen to work in the medium of glass and both design and make their own pieces as fine art, rather than traditional glassworker craftsmen, who often produce pieces designed by others, though their pieces certainly may form part of art. Studio glass is another term often used for modern glass made for artistic purposes. Art glass has grown in popularity in recent years with many artists becoming famous for their work; and, as a result, more colleges are offering courses in glass work.
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Fibre-reinforced plastic (FRP) is a composite material made of a polymer matrix reinforced with fibres. The fibres are usually glass, carbon, aramid, or basalt. Rarely, other fibres such as paper, wood, or asbestos have been used. The polymer is usually an epoxy, vinyl ester, or polyester thermosetting plastic, though phenol formaldehyde resins are still in use.
Thermoforming is a manufacturing process where a plastic sheet is heated to a pliable forming temperature, formed to a specific shape in a mold, and trimmed to create a usable product. The sheet, or "film" when referring to thinner gauges and certain material types, is heated in an oven to a high-enough temperature that permits it to be stretched into or onto a mold and cooled to a finished shape. Its simplified version is vacuum forming.
Lampworking is a type of glasswork where a torch or lamp is primarily used to melt the glass. Once in a molten state, the glass is formed by blowing and shaping with tools and hand movements. It is also known as flameworking or torchworking, as the modern practice no longer uses oil-fueled lamps. Although lack of a precise definition for lampworking makes it difficult to determine when this technique was first developed, the earliest verifiable lampworked glass is probably a collection of beads thought to date to the fifth century BC. Lampworking became widely practiced in Murano, Italy in the 14th century. As early as the 17th century, itinerant glassworkers demonstrated lampworking to the public. In the mid-19th century lampwork technique was extended to the production of paperweights, primarily in France, where it became a popular art form, still collected today. Lampworking differs from glassblowing in that glassblowing uses a furnace as the primary heat source, although torches are also used.
Compression Moulding is a method of moulding in which the moulding material, generally preheated, is first placed in an open, heated mould cavity. The mould is closed with a top force or plug member, pressure is applied to force the material into contact with all mould areas, while heat and pressure are maintained until the moulding material has cured. The process employs thermosetting resins in a partially cured stage, either in the form of granules, putty-like masses, or preforms.
A foundry is a factory that produces metal castings. Metals are cast into shapes by melting them into a liquid, pouring the metal into a mold, and removing the mold material after the metal has solidified as it cools. The most common metals processed are aluminium and cast iron. However, other metals, such as bronze, brass, steel, magnesium, and zinc, are also used to produce castings in foundries. In this process, parts of desired shapes and sizes can be formed.
Glass fusing is the joining together of pieces of glass at high temperature, usually in a kiln. This is usually done roughly between 700 °C (1,292 °F) and 820 °C (1,510 °F), and can range from tack fusing at lower temperatures, in which separate pieces of glass stick together but still retain their individual shapes, to full fusing at higher ones, in which separate pieces merge smoothly into one another.
Murrine are colored patterns or images made in a glass cane that are revealed when the cane is cut into thin cross-sections. Murrine can be made in infinite designs from simple circular or square patterns to complex detailed designs to even portraits of people. One familiar style is the flower or star shape which, when used together in large numbers from a number of different canes is called millefiori.
In glassblowing, cane refers to rods of glass with color; these rods can be simple, containing a single color, or they can be complex and contain strands of one or several colors in pattern. Caneworking refers to the process of making cane, and also to the use of pieces of cane, lengthwise, in the blowing process to add intricate, often spiral, patterns and stripes to vessels or other blown glass objects. Cane is also used to make murrine, thin discs cut from the cane in cross-section that are also added to blown or hot-worked objects. A particular form of murrine glasswork is millefiori, in which many murrine with a flower-like or star-shaped cross-section are included in a blown glass piece.
Glass casting is the process in which glass objects are cast by directing molten glass into a mould where it solidifies. The technique has been used since the Egyptian period. Modern cast glass is formed by a variety of processes such as kiln casting, or casting into sand, graphite or metal moulds.
Devitrification is the process of crystallization in a formerly crystal-free (amorphous) glass. The term is derived from the Latin vitreus, meaning glassy and transparent.
A glossary of terms used in glass art
Hellenistic glass was glass produced during the Hellenistic period, from the conquests of Alexander the Great to the expansion of the Roman Empire in the Mediterranean, Europe, western Asia and northern Africa. Glassmaking at this time was based on the technological traditions of the Classical antiquity and the Late Bronze Age, but was marked by transition from limited production of luxury objects made for the social elite to mass production of affordable glass vessels used by the broader public to satisfy everyday needs.
Plique-à-jour is a vitreous enamelling technique where the enamel is applied in cells, similar to cloisonné, but with no backing in the final product, so light can shine through the transparent or translucent enamel. It is in effect a miniature version of stained-glass and is considered very challenging technically: high time consumption, with a high failure rate. The technique is similar to that of cloisonné, but using a temporary backing that after firing is dissolved by acid or rubbed away. A different technique relies solely on surface tension, for smaller areas. In Japan the technique is known as shotai-jippo, and is found from the 19th century on.
Warm glass or kiln-formed glass is the working of glass, usually for artistic purposes, by heating it in a kiln. The processes used depend on the temperature reached and range from fusing and slumping to casting.
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