In all foundry work a model or pattern is necessary to start the process in casting that form in some material. Usually a rubber mold is made or "pulled" of the object or model. Once the rubber mold is pulled, a wax form of the object can now be made by pouring molten wax into the rubber mold. Once the hot wax is cooled, the design in wax is removed from the rubber mold and cleaned up. The new wax form then will have the high temperature mold material (called investment) applied to it. 
Once the wax form has the investment applied to it, the invested form has the wax "steamed out" to lose the wax and leave a negative shape in the investment of the design. This is where the term "lost-wax" comes from.
The steamed out investment is placed into a kiln, the desired glass billets (ingots) are placed into a reservoir for the glass billets to melt  and later to slowly flow into the negative shape in the heated investment. We do not pour molten glass into an invested form as they do in bronze casting.
After the glass has filled the shape to be cast, the glass must be annealed properly to make sure it has no undue stress. The amount of  time to successfully cast a form in glass and anneal it in the kiln can last from 4 days to 4 weeks, depending on the thickness of the glass and  complexity of a design.

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A rubber mold being taken of a Buffalo Skull.
There are two coats of silicone rubber on this skull.
More coats will be applied as needed.
Binh Pho, wood original and the cast glass duplicate, right out of the kiln. The invesment  still needs to be cleaned off of the cast glass form.
Binh Pho, rubber molds, waxes and glass casting
of his designs.
Two large invested castings that have cooled and are ready to come out of the kiln.
You can see the reservoirs for these castings.

Modern foundry technique can also start with 3D computer files instead of a model or pattern. Objects can be designed completely with  computer software and then with the appropriate 3D file, a rapid prototyper can build the object in a variety of materials, usually plastic. The design potential and accuracy of this modern method is phenomenal. We work with 3D files and rapid prototypers  and they are necessary to produce accurate or production no-wax models.