slumping disaster.

[bskirwin]

I had a disaster in my kiln the other day, and don't understand what happened. I fused two ( 15 inches) circles of glass together, black - 90 Coe, and rainbow iridized on black - 90 COE, and the fusion piece was fine. I then masked off the piece with vinyl resist and cut out a pattern on the iridized top coating, and then washed off the piece. I then placed it on an open footed platter ceramic mold I made, and slumped it at a rate of 350 per hour to 900, then held for 30 mins, then went up 350 to 1250 held another 30 mins, and quickly went up to 1350 to fire polish the sand blasted areas black, and then ramped down AFAP at to 1000, and ramped down to 750 at 100 per hour. When I opened the kiln, 4 other pieces were fine, but this piece had shattered all over the kiln shelve into pie shaped pieces and they were all over the mold, kiln shelf and brick walls of the kiln. Any ideas what went wrong......? Thanks. Barbara

[Lisa Allen]

Sounds like thermal shock.

The only piece I have ever had explode like that was thicker than yours, but as big and it was from ramping up too quickly. Try going slower if you do it again so that it can heat up more evenly. I don't go more than 300dph on a piece that size and now I'm down to 100 or 200dph if it is thicker than 2 layers.

Also, placement in the kiln can make a difference. If parts of it are closer to elements and others are not, you are risking uneven heat up and potential KABOOM.

[Ron Coleman]

I had a disaster in my kiln the other day, and don't understand what happened. I fused two ( 15 inches) circles of glass together, black - 90 Coe, and rainbow iridized on black - 90 COE, and the fusion piece was fine. I then masked off the piece with vinyl resist and cut out a pattern on the iridized top coating, and then washed off the piece. I then placed it on an open footed platter ceramic mold I made, and slumped it at a rate of 350 per hour to 900, then held for 30 mins, then went up 350 to 1250 held another 30 mins, and quickly went up to 1350 to fire polish the sand blasted areas black, and then ramped down AFAP at to 1000, and ramped down to 750 at 100 per hour. When I opened the kiln, 4 other pieces were fine, but this piece had shattered all over the kiln shelve into pie shaped pieces and they were all over the mold, kiln shelf and brick walls of the kiln. Any ideas what went wrong......? Thanks. Barbara

Your firing times look ok for this size piece, so I'm thinking the mold design may be the problem. Look at the broken pieces of glass, did they take on the shape of the mold? If they did, then the cracks came as the glass was cooling.

When you say "open footed" do you mean there are 3 or 4 depressions in the bottom of the mold? If this is the case, there is the possibility that the glass got trapped in the mold. If the feet were very deep and the glass slumped down inside then it would be trapped between the feet. The clay won't shrink as much as the glass when it cools so the glass between the feet would be under tension if the feet got caught on the mold. This would be very similar to slumping over a clay mold and having the glass crack as it cooled if it got stuck on the mold. Molds can be made to form feet but they have to be small and shallow and any foot edge facing the center of the piece needs to have a very gentle slope.

Ron