I fused two layers 12" diameter. I then tacked fused a piece on top approx 4" x6", 1 layer thick. All was fine after the tack fuse. I then slumped the piece in a shallow platter mold, when I opened the kiln I had a crack in the plate, running under the tacked piece only. I peeked in the kiln during the last few minutes in the slumping phase and there was no crack then. My schedule for the slumping was 200ph to 1000 hold 20min, 300ph to 1150 hold 15 min, 400 to 1250 hold 20min., afap 950 hold 60min, 150ph to 750 hold 15min then off.
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Just killed a long post - I will try to recreate it.
I have to assume the annealing of the initial and the tack fused piece was the same as listed here. My comments are made on this assumption.
First the general.
Tack fused pieces are much more difficult than full or profile fused pieces. The tack fused piece makes the thickness of the piece uneven. All published schedules assume the piece is of uniform thickness.
The tack fused piece is only barely attached, and so to some extent acts independendently from the main base, so requiring more annealing care.
The tack fused piece(s) shade the heat from the main piece, so inducing stress. The shape of the tack fused piece has a significant effect on the stresses. The easiest is circular, the most difficult is pointed.
Second the specific.
Your piece's maximum thickness is 9mm, but annealing for that would be inadequate as it is not a uniform thickness. One general solution for full and profile fusing is to go up one thickness - in this case go to annealing for 12mm.
However this is a tack fuse. A person who has considerable experience suggests that with a circular tack fused piece, you need to double the thickness to calculate the annealing schedule. BUT you have a rectangular piece, so her suggestion is that you anneal for something FOUR times the thickness of your piece - in this case 36mm.
As an indication of what this means, you would need to soak at annealing temperature for 6 hours, followed by the annealing cool of 12F/hour for the first 100F, then 22F for the next 100F and finally 72F/hr to room temperature.
This would need to be done on both the tack fusing and the slumping stages. You also need to think about the rate of increase to the working temperture.
Finally, the annealing soak is to ensure the glass is an even temperature throughout its thickness. The annealing requires a regular, slow cooling. The soak at 750F does nothing useful, and in some circumstances could induce some stress. Do not soak for long periods below the annealing soak. Instead slow down the annealing cool. For example, if you cool at 50F/hr to 750 and soak for an hour, you can change that to cooling at 40F/hr and achieve the same object, but more safely. They both total 5 hours.
