Slumping is one broad technique of warm glass working, for the forming of glass 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. The slumping of a pyrometric cone is often used to measure temperature in a kiln.
[edit] 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 steel[1] 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.
[edit] 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[2]. 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[2].
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