Glass furnace front panel
The crucible in the investment cavity offers a good cut-away look at the lower furnace construction. The crucible is elevated, sitting on a hard firebrick “split” (half-height brick). This ensures that the investment can be vibrated right down under the bottom, and the bottom of the pot is not sitting on soft firebrick. The 6″ of fiber and the 2 layers of K-23 bricks are also added to the front.. The shorter front “cage” is clamped on. These lighter density insulations are appropriate below the glass line when an investment is used around the glass crucible. The density of the crucible, it’s contents, and the investment will be nearly 500 pounds. The corners can be filled with soft brick to reduce the investment volume a little bit, but the function of the investment is to support the weight of the glass. In applications like this, I consider the crucible to be the container for the liquid glass, but I do not expect it to support the weight of the glass. This is the role of the investment. This brings up the debate of “free-standing vs. invested”. Free-standing pots have a much shorter life and must be heated and charged carefully. Invested furnaces have more density and therefore absorb more heat during warm-up, but offer the mechanical support and “peace of mind” that cannot be had with a free-standing pot.
Poured investment
upper chamber bricks
The crucible is filled with bricks to prevent it from floating up once the refractory cement is placed underneath it. The sides of the casting are trowelled down on a slight angle to the pot edge. This allows sloppy charging bits to flow down into the pot. If glass cannot sit on a horizontal surface, it is less likely to cause erosion. The angled sill trough is cast in place as well. It is a separate casting from the pot casting, isolated by a piece of wax paper. This sill is angled for the same reason, so sloppy gathers can flow back down into the pot, instead of accumulating on the sill and gluing the door shut. It is removable, so can be removed hot, then re-cast into position in a cold furnace. The following photo illustrates the positioning of the upper, heating chamber bricks. The outer row is K-23 bricks, and the inner row is K-26 bricks. Notice the orientation which ensures that all joints on the outer layer are overlapped by the inner row. These bricks are not cemented in place, just “placed” in position. The crown with it’s little “keeper” channel will prevent side-wall movement.