Glass slumping is one of the most satisfying entry points into kiln-formed glass — you start with flat fused sheet glass or pre-made blanks, and the kiln does the shaping work over a mold. But getting clean, consistent slumps requires understanding how heat and time interact with your glass and mold. Here's what experienced slumpers know that beginners usually have to learn the hard way.
When glass is heated to slumping temperature (typically 1225°F–1275°F), it becomes soft enough to move under its own weight but doesn't fully flow. Gravity pulls the glass down over or into a mold, and surface tension keeps it from sagging unevenly. The result: a shaped piece that retains the surface texture and color of the original flat blank.
The key word is "weight" — slumping is gravity-driven, not pressure-driven. You can't rush it by going hotter. Going too hot causes the glass to flow instead of slump, losing surface texture, thinning at edges, and potentially sticking to the mold.
In a slump, glass sinks into a concave mold (like a bowl mold). Gravity pulls the center down while the edges rest on the mold rim. In a drape, glass hangs over a convex form (like an upturned bowl). Both use similar temperatures but the mold position and glass support are different.
For beginners, slumping into a concave mold is more predictable. The edges are supported throughout the process, reducing the risk of the piece shifting during firing.
Uneven heat distribution is the number one cause of asymmetric slumps and cracked pieces. If one side of your kiln runs hotter, the glass on that side slumps first while the other side is still stiff — creating internal stress and uneven form.
Top-fired kilns with a single central element can produce a slight hot spot in the center of the shelf. For smaller pieces placed centrally, this isn't usually a problem. For larger pieces that span the full shelf, the edge-to-center temperature differential can create issues. The Hot Shot 12G and Hot Shot 16G are designed for even heat distribution to minimize this.
Ceramic fiber, stainless steel, and ceramic molds are the most common choices. Whatever material you use, the mold must be coated with kiln wash (boron nitride spray or traditional kiln wash) to prevent the glass from sticking. Reapply kiln wash every few firings or whenever you see it flaking.
Mold depth also affects slump time. A deeper mold requires a longer hold at slumping temperature to allow the glass to conform fully to the shape. Rushing the hold leaves flat spots in the bottom of bowls or incomplete contact with the mold walls.
A basic slump schedule for 6mm fused glass in a medium-depth bowl mold:
Adjust hold time based on mold depth and glass thickness. Thicker glass and deeper molds need more time at slumping temperature.
Ready to start slumping? Browse our glass kiln lineup — the Hot Shot 12G and 16G are both excellent for slumping work and ship fast.
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