Written records show the casting process dates to the second millennium B.C, possibly in Mesopotamia to produce jewelry and glass sculptures. Kiln casting involves the preparation of a mold that is often made of plaster or plaster mixtures. A rubber form is made from the original sculpted clay model and then used to make a wax version of the original clay sculpture. The wax model is then surrounded with a jacket of high temperature refractory plaster in a process known as “investment.” After the refractory plaster hardens and dries, the wax is melted out of the refractory material using a steamer leaving an empty cavity that exactly duplicates the original sculpture.
The plaster, refractory mold must next be hand filled with layers of powdered or chipped glass. The heat resistant plaster mold is placed in a kiln with a funnel like refractory opening filled with more solid glass granules or blocks. The kiln is heated to a high temperature, normally 2300 degrees Fahrenheit (1400 degrees Celsius); the glass melts into a liquid filling the mold. The molten glass must then be cooled very slowly until cold in a process known as annealing. The plaster mold is broken to pieces and removed, revealing the cast glass piece it had contained. Several days are needed to clean and refine the piece using grinding and polishing materials. From start to finish small pieces take weeks and larger works take months to finish.