Warm GlassANNEALING AND COOLING (KILN CASTING)
Once the casting has been formed, the mold and glass should be crash cooled (just as in fusing and slumping) to around 1100 degrees F. From this point on, however, annealing and cooling take much longer than do the same processes during fusing and slumping.
This is for three main reasons:
- Kiln castings tend to be thicker than fused and slumped items.
- Kiln castings tend to be mostly surrounded by the mold, rather than mostly open to the air (as in a fused or slumped item). This means that heat can’t escape as easily.
- Kiln castings are surrounded by a thick, often fragile mold. As a result, the temperature variations within the casting are greater than in fusing and slumping. Extra time is needed to allow for temperature to stabilize.
How long do you anneal and cool? It is not unusual for annealing of large items to take several days, or even weeks. Even small cast items will generally require several hours to anneal. When figuring the length of time, you must consider both the thickness of the casting and the thickness of the mold. You should calculate conservatively; there’s nothing worse than losing a casting during the annealing and cooling phase.
Once the glass has been annealed, cooling to room temperature follows. Here caution is also in order. The thickness of the glass, combined with the thickness of the mold (which acts to shield the glass from temperature changes), means that cooling should also take (no surprise here) longer than with simple fused or slumped items. Even when the item has apparently cooled to room temperature, it should be left to sit another day or so. This is because the inside of the mold can still be hot even though the mold itself feels cool to the touch.
Click here to go to the next step in the Kiln Casting process, "Removing and Cleaning the Casting."