To make kiln cast glass sculpture you first make a model in wax.
The wax is surrounded by a refractory mold material, which is a material that can withstand high temperatures. At the top of the mold we leave a sprue cup open as an entry point into the wax.
Once the mold has hardened, the wax is steamed out of the mold, leaving an empty space where the wax pattern used to be. The mold is then placed in a kiln upside down with the sprue cup up. Chunks of room temperature glass are placed in the mold and the kiln is heated up slowly to the melting temperature of the glass: 1400 - 1600 degrees Fahrenheit depending upon the type of glass used. The mold is held at that temperature for as long as it takes to melt the glass.
The kiln is then cooled slowly to avoid cracking the glass. This slow cool is called annealing the glass. The thicker the glass, the longer it takes. Very thick pieces can take weeks to anneal. Once the kiln reaches room temperature, the mold is removed from the kiln and carefully picked away and destroyed to reveal the kiln cast glass sculpture within.