Start with a low relief sculpture of your choice, modeled in clay not more than 1 ½" thick (the relief should not be any thinner than 1" thick at its thinnest point). The finished glass will exactly reproduce the clay surface, so you can leave it rough or smooth it out as you prefer. In order to calculate the weight of glass pieces which will be needed, multiply the weight of the sculpture and multiply by 1.16. Make a mold from a cardboard box: cut out a piece of cardboard whose length is exactly the same as the perimeter or circumference of your sculpture; and whose height is 2" taller than the piece. Place the sculpture on a flat piece of glass, wood, or plastic and place the cardboard mold around it, sealing the bottom of the cardboard to the base with clay (so that casting mix doesn't seep out).
Next, make the casting mix to make the mold. This mold is a waste mold, which is used only one time (it is destroyed in the casting process). Two quarts of water are put in a two-gallon bucket, and silica casting mix is added (since silica is carcinogenic, a NIOSH mask must be worn during this process). The casting mix should be mixed slowly to avoid entrapping air bubbles. Then, the mix is poured into the mold at the sides (not directly over the sculpture, to avoid trapping air). When the mold is full, its sides are tapped to release air bubbles; and then it is left to cool overnight. The mold is disassembled and the clay original is removed (it may be necessary to break it). The mold is cleaned with a damp sponge and allowed to dry. It is a good idea to spray the dry mold with graphite film lubricant, which is available at auto supply stores, to make it easier to remove from the glass. Place a piece of plastic in the cavity of the mold and fill it with water to the thickness of the finished piece. Weigh this water, and multiply by 2 ½. The result is the weight of glass necessary to fill the mold to the correct thickness. Put a refactory shelf in the glass kiln on 1" posts. Put ½" of sand on the shelf and place the mold on this sand, using a level to insure that the mold is level in all directions.
Wash the glass pieces with soap and water, then with window cleaner and allow it to dry. Place the glass pieces on edge in the cavity of the mold, with taller pieces at the center and shorter ones outside. Don't permit glass pieces to touch the mold itself, and take care not to chip the mold. The firing should be done as follows: temperature rise at 250° per hour until 1250° is reached; then hold for ½ hour. Temperature rise at 250° per hour until 1450° is reached; then hold for 2 hours. Crash the cooling at 950° per hour and hold for 6 hours (to anneal the glass). Cool at 60° per hour until 500° is reached. Cool at 100° per hour until 200° is reached. Then let the kiln cool on its own to room temperature. It is easiest to regulate this firing using electronic kiln controls or kiln sitters. When cool open the kiln and let the glass cool down to room temperature. Remove the mold and soak it in water, which will facilitate removal of the mold. Grind off any sharp edges with a hand-held grinding stone. Voila!
The pate de verre technique is enjoying a modern revival, thanks to the availability of inexpensive glass kilns and modern technological improvements such as kiln sitters. Owning your own glass kiln opens the door to a new world of creativity.