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I am using a Paragon small SC-2 kiln 7" x 7" shelf, computer controlled.
I have many times in the past take a base coat of black maybe 6" x 6" load it up with scraps of dichroic and clear glass, fuse to a slab then cut it into pendants etc.
I fused a slab pulled it out decided to top it with a scatter layers of clear and dichroic for more depth. At 1400 I open the door the slabbed had busted in about 9 pieces. Okay I put another base coat put the busted pieces back on it at 900 I hear a ping the base shattered into 15 pieces. Determined to get this back together I put all the busted pieces back in and fused it back into a slab with no base coat under it, it did. Put clear and dichroic on top of the slab, at 600 ping, busted up again. Being a jewelry kiln I fire fast 1400dph to 1380 hold 5 min then 855 to top temp. It always busted going up. WHen I finally got it to fused the second time I even had it sit at 965 and soak for 45min. The first fuse that worked I did not do a soak but I never had whenever I had done this before probaly 5 ot 6 times was I just lucky then?

If glass isn't annealed right the first time is ever good if it was annealed in a second or third firing? If it was the annealing why did the base bust in the second attempt at 900.
This is all BE glass and cbs dichroic 90coe.

Baffled at this and could use some input. Thought for sure it could be the annealing but don't understand why the base that the glass pieces was on broke at 900

tom suter wrote:I am using a Paragon small SC-2 kiln 7" x 7" shelf, computer controlled.
I have many times in the past take a base coat of black maybe 6" x 6" load it up with scraps of dichroic and clear glass, fuse to a slab then cut it into pendants etc.
I fused a slab pulled it out decided to top it with a scatter layers of clear and dichroic for more depth. At 1400 I open the door the slabbed had busted in about 9 pieces. Okay I put another base coat put the busted pieces back on it at 900 I hear a ping the base shattered into 15 pieces. Determined to get this back together I put all the busted pieces back in and fused it back into a slab with no base coat under it, it did. Put clear and dichroic on top of the slab, at 600 ping, busted up again. Being a jewelry kiln I fire fast 1400dph to 1380 hold 5 min then 855 to top temp. It always busted going up. WHen I finally got it to fused the second time I even had it sit at 965 and soak for 45min. The first fuse that worked I did not do a soak but I never had whenever I had done this before probaly 5 ot 6 times was I just lucky then?

If glass isn't annealed right the first time is ever good if it was annealed in a second or third firing? If it was the annealing why did the base bust in the second attempt at 900.
This is all BE glass and cbs dichroic 90coe.

Baffled at this and could use some input. Thought for sure it could be the annealing but don't understand why the base that the glass pieces was on broke at 900

Your glass is breaking on heatup. Stress during heatup can be caused by uneven temps inside the glass or by previous poor annealing or by glasses of differing COE's. Slow down your heatup to begin with.

Once you are above the strain point speed does not matter any more (relative to breakage), nor does previous annealing.

If your glass appeared to break at 900 something is screwy. It could be that the thermocouple reads 900 but the glass is still down at 600 or 700. At any rate slow down the heat up. I don't actually know, isn't the strain point for BE and Spectrum somewhere around 820ºF?

According to Graham Stone's book on schedules you can take a single layer of 1/8" glass at roughly 1800 DPH ramping up but as soon as you go to two 1/8" sheets that maximum ramp slows to 430 DPH. I've taken BE up as fast as 500 to 550 DPH without a problem but I would not go much faster than that. To Bert's comments and mine, you are going way to fast going up.

Phil

The key to what's happening is that the first time you fire you are heating up a bunch of little pieces of glass. Some of them might be breaking down in the bottom of the pile and you wouldn't know it anyway. Thermal shock happens when one part of a piece of glass gets too hot relative to the rest of that piece of glass. To break little pieces with thermal shock you almost have to put one end in a flame, since the heat doesn't have to go very far to warm up the rest of the piece.

But once they are fused they are all now one large piece. You can't go up the second time nearly as fast as the first time.

ch

tom suter wrote:I am using a Paragon small SC-2 kiln 7" x 7" shelf, computer controlled.
I have many times in the past take a base coat of black maybe 6" x 6" load it up with scraps of dichroic and clear glass, fuse to a slab then cut it into pendants etc.
I fused a slab pulled it out decided to top it with a scatter layers of clear and dichroic for more depth.

Here is the key Tom: when you fire a piece a second time and cover it with smaller pieces in some spots (as you described in the second firing) you are insulating some areas and not others, creating uneven heating across the piece. Factor in the rapid heat and BOOM! you get thermal shock. Plus, I agree with Charlie that you are heating up too fast whether or not you have added the new glass elements.

Heat it up as slowly as possible. I reheat glass with added elements at no more than 200 degrees per hour. My pieces are probably much larger than yours, so you might not need to go all that slowly. But 1400 dph is too fast for subsequent firings.

Barbara