Breaks in Slumping

Despite not opening the kiln until it was 120 degrees, a ¾" crack formed in the middle of a 15" x 15" flat fused 1/4” slumped platter, fused, then slumped. The bottom layer was four squares, topped by a single layer of Tekta. How can I avoid crack at the meeting of the squares?

The fuse schedule was

• 300°F to 1110°F, 30’

• 200°F to 1240°F, 60’

• 600°F to 1475°F, 17’

• 9999 to 900°F, 60’

• 150°F to 700°F, 1‘

The slump schedule was

• 250°F to 1225°F, 60’

• 9999 to 900°F, 90’

• 100°F to 700°F, 1’.

Any ideas why the platter cracked? Thanks.

Hannah Gulick adds: “We see this phenomenon happen a lot with this kind of checker board pattern. I find it occurs more on the underside than the topside, but it's always at the point where the four corners are meeting. It's a perfect storm situation to do with the design, colors, mold, etc. Bullseye recommends doing checker board patterns in general at a slower initial rate of heat for slumping, usually 100°F/hour.”

This piece shows extreme stress in several places.

Stress generally is most concentrated at corners and points in any piece. It is logical to treat pieces with meeting corners and points more cautiously than normal. This will include a stress test of the full fused piece to determine what stress there may be at that point in the process. I would have annealed as for 9mm, resulting in an annealing schedule of:

• 1.5 hour anneal soak

• 125°F – 800°F, 0’

• 225°F to 700°F, 0’

• 600°F to room temperature

If the stress test shows significant stress, it needs to be fired again and annealed for at least one more layer thick, depending on the amount of stress showing, and checked again before slumping.

I have found that slumping a full fused piece needs to be fired for at least one layer thicker than usual. In this case, where known stress points are at corners, even if one rather than multiple points, more caution is needed. I would anneal as for two layers thicker, or at least for12mm, resulting in:

• anneal for 2 hours

• 100°F to 800°F, 0’

• 180°F to 700°F, 0’

• 330°F to room temperature