Breaking problem

[Christyn Mattson]

I had something happen that I haven't experienced before and need your ideas.
I cut a 6" white tile (system 96) capped it with clear and put black zebra stripes on top of the clear with some black frit in between the stripes.
Following the 96 firing schedule, I ramped 600dph to 1100 and held for 10 minutes. For some reason I felt I needed to check it. When I looked, the white base layer had split into 2 pieces and had slid away from each other so it was sticking out from the clear on both sides about 1". I did not take the firing any higher and skipped to annealing at 950 for 30 minutes, then cooled to room temp.
The next day, I had to cut the edges of the xtra white off and piece in another section of white in the middle where there was a space under the clear. I used hotline fusing glue and added white frit in the cracks.
I re-fired at 400dph again to 1100 and held for 10 minutes. Then full power to 1450 for 20 minutes. Annealed and cooled.
Next morning, I opened the kiln and there were more cracks in the white under the clear that were different from the first cracks, although these were fused together. I also noticed that some of the black stripes had broke into 2 pieces and had fused with a very faint space between. I was really disappointed. I have a box lid I want to put this in and it didn't quite fit so I took it into the grinder and began to grind a little off and planned to fire polish it and all of a sudden it broke into two chunks! I cleaned it up and checked it under my stressometer and couldn't see any stress. What the heck happened? I want to make another one but don't want to waste the glass again. Any ideas? I'm dumbfounded.

Christyn

[Tony Serviente]

Everything points to thermal shock coming up, though it sounds like you did everything right. Is your kiln side fired? If so, I would be more conservative coming up, like 300dph, especially when using high contrast colors,and you can't get any higher than black and white. If your kiln is top fired, come up slower anyway. I suspect it broke apart when you went to grind due to cracks that were there that you may not have seen, that did not fully fuse back together.

[Cynthia]

It sounds like you had 9mm of glass and 600 dph is pretty fast for that thickness. Also, at 1100, I expect it was still close to 9mm thick in places, so it wasn't annealed long enough for that thickness. I agree with Tony that you thermal shocked it the first time, and probably the second time too.

The second time may be because you had stress because it wasn't fully annealed and it was ready to go pop anyway.

Breaking when you put it to the grinder says to me that it did indeed still contain stress. You can't see it in opals with a stressometer, and even if you could, once it broke the stress was relieved.

Since you buffered the white glass (a hard glass) from the black (a soft glass) with the clear, I would have guessed that the stress would have been lessened, but with such different colors, in my opinion the hardest to fire together, you should be much more conservative with your schedule (just like Tony suggested), and for thicknesses more than double, you need to slow it down as well. I would anneal such a piece for 45 minutes and descend down through to below your lower strain point at no more than 120 dph assuming that the piece is only 6mm thick once fired. This should be overkill to make sure it's annealed well for a 6mm piece. Check Spectrums site for times and temps for thicker pieces if it is going to remain 9mm thick.

That's my take anyway.

[Christyn Mattson]

Thanks guys! It all makes sense. I didn't know about the sensitivity between contrasting colors. I'm usually pretty conservative in firing. Just got impatient! Thanks! I'll try again!

Christyn

[steve_hiskey]

Hmmm... I have always considered 300dph aggressive. I was getting a lot of breakage... but that could be because I was using chunks of glass from glassblowing in my pieces.

White is definitely the "hardest" and black is one of the "softest" colors. Hardness, from what I know, translates to a slight difference in COE..

If it is still breaking after slowing everything down, make sure you have a real piece of fusible/compatible glass. Do you make beads? Take a welding rod and get a piece of the white stuck to the top in a torch and make a rod out of it. Lay a strip of Clear on one side of the white. Heat the whole thing and pull a stringer. If the stringer bends when it cools down, you have a different COE. It will bend a little... but if the COEs are not compatible, it will bust apart or REALLY bend.