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Setup. 14in diameter strip slab fused. Set kiln side down, place a circle of clear, build another strip slab design on top. Fuse to 1600, no hold, 900 for 1.5 hours, 100 to 700 ect. I have a 5/8 in x 14 slab set over a 10in hole in the shelf. Fire at 300 dph to 1250 and stop at desired depth. This setup has worked. Yesterday’s drop produced a split in the piece on the outside not going all the way to the inside. I have no clue why. Any guesses out there?

Photo? Complete firing schedule?

Brad, full schedule for the piece or slump? The site won’t accept my iphone or ipad photos. I could send you one if your address accepts my photos.

Doug Zawodny wrote:Brad, full schedule for the piece or slump? The site won’t accept my iphone or ipad photos. I could send you one if your address accepts my photos.

Schedule for the slump, isn't that where the problem happened?

Photo is probably too big, but you can email it to me and I'll resize and post for you. wg at warmglass.com

Slump was 300dph to 1250, hold till desired depth is reached afap to 900 hold for 1.50 hours 100 to 700 no hold afap to room temp. Ive had 2 successful drops using this schedule for a 5/8 in thick slab.

Looks like thermal shock from being too close to top elements. That kind of slit is characteristic of that situation. I usually see it in slumps.

Way to fast on the slump is my opinion
The other Jim

Jim, 300dph is too fast? What would you use?

Brad, my kiln has a separate control for the lid elements. For slumps I have it on low. Do you know if you Can you evenly heat a piece with just the side elements? My shelf is 5 in from the lid.

The faster the change in temperature and of course the more work that has gone into the piece, the greater the chance of a problem.
Speed kills.
There's temperature differentials where glass touches mold and when glass is in the air. Different colours of glass absorb heat differently.
Brian Blanthorn's term to describe the situation, 'Evenivity" is crucial
What is a little more time in the scheme of things?
Try heating at 100 degrees an hour.
Too long a heating and cooing cycle for your liking?
Your needs are not what the glass needs so live with it

I heat at 100 on the way up, never more than 200. Also, pause 10 minutes at 500 to allow the heat to even out a bit before you get to the range where thermal shock is more likely.

You can fire in a kiln with side elements if you can turn the top ones off. I routinely drop 3 to 4 layer pieces 4 1/2" (sometimes 5") in a 6" deep kiln with side only elements on, but you have to go slower than 300 to make it work.

Thanks you all for the help. I will slow down.

Follow up. Slowed the heatup to 100dph. Same thing happened. I now know Brad was correct in that the top elements were too close. The delamination was always on the bottom of the slab. Dropping the shelf solved the problem.