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.
"Warm glass" is in contrast to the many cold-working glass processes, such as leaded glass. "Hot glass", glassblowing, or lampworking is the working of glass in a direct flame, such as for laboratory glassware and beadmaking.
Warm glass working uses a variety of processes, according to the working temperature and the time the glass spends at this temperature. The glass becomes progressively softer, less rigid, and less viscous with increasing temperature. Kiln-worked glass (unlike lamp working) responds slowly though, and so the amount by which this affects the glass depends on the time it spends at working temperature.
There are three main processes, with variations within them. The broad process depends on the temperature, the variation within it depends on the time and also on slight variations of temperature. These processes are:
It is common for one piece to use several of these processes in turn. Coloured glass may be fused together to make a composite multi-coloured sheet. This glass is then cut cold and re-assembled in pieces, which are then fused back together. The piece is finally slumped into a mould to shape it.
Glass is usually worked for only one process in a heating cycle. Where a piece requires multiple cycles, it is returned to a lower temperature between them.
Fusing is the use of heat to join the glass by fusion welding, either with or without an associated change in shape, depending on the temperature. [1]
Tack fusing is the joining together of glass, with as little change to the shape of the pieces as possible. [2] Tack fusing may be used either decoratively, or to assemble a large piece of glass from laminations.
Where tack fusing is used to apply small decorative details to a larger piece, it is often desired to partially melt the small pieces so that they change shape (usually becoming more spherical, under the influence of surface tension), but without changing the shape of the carrier piece. This can be done by using an increased temperature, but only briefly. The large piece, of large thermal mass, heats up more slowly than the small decorations.
Full fusing is like tack fusing, but the temperature is higher so that the fused pieces begin to coalesce. [3] In the complete case, decorative additions to a surface are absorbed entirely into it and the surface becomes flat again. It is usually done for decorative effect.
Slumped glass is heated to the temperature at which the glass softens and begins to deform. It may either bend along a single curvature or, if heated sufficiently, may become elastic enough to stretch and curve to follow a compound curvature, such as a bowl.
Mould slumping begins with a sheet of flat glass placed above a ceramic mould. When heated, the glass slumps into the mould under its own weight.
These moulds are usually commercially made and are offered in a range of standard shapes and sizes: bowls, trays etc. For custom pieces, a glass worker may also make a specialised or temporary mould as a one-off.
To avoid trapped air, the mould is perforated with a small vent hole. The hot glass otherwise forms a good seal with the lip of the mould and an air bubble is trapped. Such a trapped bubble often causes problems - when cooling this air may contract to form a partial vacuum that is enough to break the glass. As the glass is not heated enough to become liquid, this air cannot escape as bubbles and so venting is required.
Kiln wash or ZYP Coatings' Boron Nitride Aerosol Lubricoat is used beforehand, to prevent the glass sticking to the mould. [4] [5]
A mould for free-fall slumping is in the form of a ring with a central opening. When heated, the glass falls through this opening and forms a bowl. Depending on the temperature and time, this bowl may be shallow or deep. If a kiln shelf is placed beneath the ring mould, this catches the falling glass and gives a vessel with a flat base.
Free-fall slumping is used to make taller vessels with steeper sides, such as vases.
Draping is a variety of free-fall slumping, where the mould former is placed in the centre of the piece and the outer edge falls under the heat. As this outer edge is unconstrained, it tends to fall in large folds. The edge is thus highly uneven, although a carefully draped piece may still retain perfect symmetry. For this reason draped pieces are often used as vases or wavey-edged bowls, but are difficult to use as a more functional vessel.
Draped pieces are sometimes sawn down and smoothed when cold, so as to reduce the unevenness of their edge.
Casting is the process where the glass begins to melt and behave as a liquid. Its shape is now constrained entirely by the mould and the previous shape is lost. Glass is viscous though and unlike metal casting, the soft glass does not flow through the mould. Variations in the glass are thus preserved in the final piece, so colours and inclusions present beforehand may still remain in the cast item.
Glass may be cast from either billets (solid ingots), sheet, loosely stacked pieces of glass (these are usually used with a low-temperature casting, so that their boundaries remain deliberately visible afterwards) or frit, ground or powdered glass.
Moulds for casting may be either re-usable ceramic moulds, or else a one-use investment mould of plaster. This mould may in turn have been formed by the lost wax process.
Pâte de verre (literally glass paste) is cast from powdered frit, mixed with a glue binder. This allows the paste to be applied to the sides of a large mould in a thin layer. When fired, a thin-walled vessel is formed. The transparency of the finished casting depends on the size of the frit used: fine powder produces an opaque cast, medium or coarse frit may be used to cast a transparent piece.
In fire polishing, the edges are briefly heated in a flame to smooth and round them. [1]
This is one of the few processes that involves manual work on the hot glass while still in the kiln. In a similar manner to slip trailing in ceramics, a pattern is formed on the surface, then trailed into feathers with a pointed metal rake. [1]
Warm glass working is similar to that for ceramics, in that a piece is assembled, placed into a cold kiln and then heated through a pre-defined cycle, including a slow cooling phase afterwards. Unlike hot glass, warm glass is rarely worked manually whilst hot.
Contemporary warm glass work is almost universally done in an electrically-heated kiln, although some gas or oil-fired kilns are still used. The reason for this is the extra control accuracy and programmability available with electric heating, [6] as well as their lower capital cost and convenient installation.
All kilns for glass work require a pyrometer, usually based on a thermocouple, as knowledge of the kiln temperature is essential for controlling the process.
Electric kilns have controllers with a variety of sophistication: the simplest is the "Infinity Control", [7] a simple open-loop power regulator. As this only controls power, rather than temperature, such a kiln must be manually controlled throughout the cycle. As firing cycles extend over several hours, potentially days for large architectural pieces, automatic unattended control is obviously important. Automatic temperature control uses a PID controller that maintains a constant set temperature. More sophisticated controllers allow the ramp heating and cooling rates to be controlled too, an important factor in glass heating. [6] Controllers dedicated for glass kiln use have their entire heating cycles defined before use with multiple set temperatures, hold times and ramps between them. The most sophisticated controllers of all are dedicated to glass use and allow pre-defined cycles such as "Fuse" or "Slump" to be selected from a simple menu, without their operator needing to be aware of the precise temperatures required.
Glass' behaviour when heated is highly complex. [8]
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 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.
Forging is a manufacturing process involving the shaping of metal using localized compressive forces. The blows are delivered with a hammer or a die. Forging is often classified according to the temperature at which it is performed: cold forging, warm forging, or hot forging. For the latter two, the metal is heated, usually in a forge. Forged parts can range in weight from less than a kilogram to hundreds of metric tons. Forging has been done by smiths for millennia; the traditional products were kitchenware, hardware, hand tools, edged weapons, cymbals, and jewellery.
Injection moulding is a manufacturing process for producing parts by injecting molten material into a mould, or mold. Injection moulding can be performed with a host of materials mainly including metals, glasses, elastomers, confections, and most commonly thermoplastic and thermosetting polymers. Material for the part is fed into a heated barrel, mixed, and injected into a mould cavity, where it cools and hardens to the configuration of the cavity. After a product is designed, usually by an industrial designer or an engineer, moulds are made by a mould-maker from metal, usually either steel or aluminium, and precision-machined to form the features of the desired part. Injection moulding is widely used for manufacturing a variety of parts, from the smallest components to entire body panels of cars. Advances in 3D printing technology, using photopolymers that do not melt during the injection moulding of some lower-temperature thermoplastics, can be used for some simple injection moulds.
Studio glass is the modern use of glass as an artistic medium to produce sculptures or three-dimensional artworks. The glass objects created are intended to make a sculptural or decorative statement. Though usage varies, the term is properly restricted to glass made as art in small workshops, typically with the personal involvement of the artist who designed the piece. This is in contrast to art glass, made by craftsmen in factories, and glass art, covering the whole range of glass with artistic interest made throughout history. Both art glass and studio glass originate in the 19th century, and the terms compare with studio pottery and art pottery, but in glass the term "studio glass" is mostly used for work made in the period beginning in the 1960s with a major revival in interest in artistic glassmaking.
Lost-wax casting – also called investment casting, precision casting, or cire perdue – is the process by which a duplicate sculpture is cast from an original sculpture. Intricate works can be achieved by this method.
A hot plate is a portable self-contained tabletop small appliance cooktop that features one or more electric heating elements or gas burners. A hot plate can be used as a stand-alone appliance, but is often used as a substitute for one of the burners from an oven range or a kitchen stove. Hot plates are often used for food preparation, generally in locations where a full kitchen stove would not be convenient or practical. They can also be used as a heat source in laboratories. A hot plate can have a flat surface or round surface. Hot plates can be used for traveling or in areas without electricity.
Lampworking is a type of glasswork in which a torch or lamp is 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 BCE. 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.
Soft-paste porcelain is a type of ceramic material in pottery, usually accepted as a type of porcelain. It is weaker than "true" hard-paste porcelain, and does not require either its high firing temperatures or special mineral ingredients. There are many types, using a range of materials. The material originated in the attempts by many European potters to replicate hard-paste Chinese export porcelain, especially in the 18th century, and the best versions match hard-paste in whiteness and translucency, but not in strength. But the look and feel of the material can be highly attractive, and it can take painted decoration very well.
Slip casting, or slipcasting, is a ceramic forming technique, and is widely used for shapes that can not easily be formed by other techniques. The technique involves a clay body slip, usually prepared in a blunger, being poured into plaster moulds and allowed to form a layer, the cast, on the internal walls of the mould.
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.
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.
Investment casting is an industrial process based on lost-wax casting, one of the oldest known metal-forming techniques. The term "lost-wax casting" can also refer to modern investment casting processes.
A muffle furnace or muffle oven is a furnace in which the subject material is isolated from the fuel and all of the products of combustion, including gases and flying ash. After the development of high-temperature heating elements and widespread electrification in developed countries, new muffle furnaces quickly moved to electric designs.
Fusible core injection molding, also known as lost core injection molding, is a specialized plastic injection molding process used to mold internal cavities or undercuts that are not possible to mold with demoldable cores. Strictly speaking the term "fusible core injection molding" refers to the use of a fusible alloy as the core material; when the core material is made from a soluble plastic the process is known as soluble core injection molding. This process is often used for automotive parts, such as intake manifolds and brake housings, however it is also used for aerospace parts, plumbing parts, bicycle wheels, and footwear.
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.
This is a list of pottery and ceramic terms.
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 15th century BCE in both Ancient Egypt and Mesopotamia. Modern cast glass is formed by a variety of processes such as kiln casting or casting into sand, graphite or metal moulds.
A glossary of terms used in glass art
Precision glass moulding is a replicative process that allows the production of high precision optical components from glass without grinding and polishing. The process is also known as ultra-precision glass pressing. It is used to manufacture precision glass lenses for consumer products such as digital cameras, and high-end products like medical systems. The main advantage over mechanical lens production is that complex lens geometries such as aspheres can be produced cost-efficiently.