Step 3: Making the Outer Wall of the Mold
I cut a circular disk 12.5 inches in diameter out of thick Styrofoam using a hot-wire foam cutter. This disk serves as a template for assembling the arcs to make the dam, and keeps the finished product reasonable round. The pieces are assembled as tightly as possible around the Styrofoam disk. High-temperature furnace cement is used to glue the pieces together.
Molten glass is dense, and exerts considerable hydrostatic pressure on the inside of the dam. A failure will cause molten glass to run out all over the inside of the kiln. So after the cemented ring sets up, I usually reinforce the joints by cementing on short scrap pieces of firebrick left over from cutting out the arcs. Several wraps of thin stainless steel wire wound tightly around the mold and held in place with dollops of cement will help prevent dam from opening up and spewing out molten glass if it should crack or if one of the joints fails. These features are shown in the last photo below, taken at a later stage of mold construction.
The inside surface of the mold will get several coats of kiln wash before firing. Kiln wash prevents the molten glass from adhering to the mold.