Slumping is a technique in which items are made in a

kiln 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, chemical changes. Kilns have been used for millennia to turn objects m ...

by means of shaping

glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...

over molds at high temperatures. The slumping of a

pyrometric cone Pyrometric cones are pyrometric devices that are used to gauge heatwork during the firing of ceramic materials in a kiln. The cones, often used in sets of three, are positioned in a kiln with the wares to be fired and, because the individual c ...

is often used to measure temperature in a kiln.

Technique

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 A ceramic is any of the various hard, brittle, heat-resistant, and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcela ...

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...

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 Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula B N. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. The hexago ...

, 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 In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...

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 Thermal expansion is the tendency of matter to increase in length, area, or volume, changing its size and density, in response to an increase in temperature (usually excluding phase transitions). Substances usually contract with decreasing temp ...

) 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.

History

During the

Roman period The Roman Empire ruled the Mediterranean and much of Europe, Western Asia and North Africa. The Roman people, Romans conquered most of this during the Roman Republic, Republic, and it was ruled by emperors following Octavian's assumption of ...

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.Allen, D., Roman Glass in Britain, ed. J. Dyer. 1998, Princes Risborough, Buckinghamshire: Shire Publications 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.

References

{{Glass forming Warm glass Glass art Glass production

Technique

History

See also

References