Slumping is one broad technique of warm glass

Warm glass

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....

 working, for the forming of glass

Glass

Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

 by applying heat to the point where the glass will soften. The increasing fluidity of the glass with temperature causes the glass to 'slump' into or onto the mold under the force of gravity.

Technique

Glass is most commonly heated in an oven, often using glass in a sheet form and 'slumping' it over a form or into a mold. Molds are generally made of high temperature plaster, clay coated with plaster or another release agent, graphite, sand mixed with a bonding agent, steel, stainless steelhttp://www.warmglass.com/molds_for_slumping.htm#stainless or other materials. At the point where the glass has achieved the desired form the heat is quickly vented and the temperature reduced to prevent further movement of the glass and then it is stabilized at its respective annealing temperature and annealed

Annealing (glass)

Annealing is a process of slowly cooling glass to relieve internal stresses after it was formed. The process may be carried out in a temperature-controlled kiln known as a Lehr. Glass which has not been annealed is liable to crack or shatter when subjected to a relatively small temperature change...

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History

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. 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.