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I recently fused a couple of 12"x12" 96 pieces of clear, with colors on top. There may be a problem with my thermocouple, which I'm trying to figure out with the programming manufacturer by doing a few tests, but in the meantime, the base sheet created four large blow-out holes in one sheet. On another trial, I went to a lower temp, and the sheet tack fused, but the beginnings of some bubbles were evident.

I wonder why, if I'm only tack fusing, and holding at 1225 for 30 minutes, there would be bubbles trying to surface? I don't use fibre paper, just a kiln washed shelf.

The schedule was
300 – 1000– 10
400–1225–30
500–1465–10
9999 –1000 – 25
90–960 – one hour
200 – 800–5

Also, if I desired bubbles, is there something I can put on the kiln shelf to create them?

I believe thermocouple degradation results in under fired glass.

Take a straight edge and put it over the part of the shelf where it bubbled. A dip in the shelf can cause this.

jerry keller wrote: The schedule was
300 – 1000– 10
400–1225–30
500–1465–10
9999 –1000 – 25
90–960 – one hour
200 – 800–5

Also, if I desired bubbles, is there something I can put on the kiln shelf to create them?

You're going way too fast above 1100F, and especially from 1225 to 1465. Slow down to 200 dph. You may also want to lengthen the bubble soak time. This alone will probably eliminate the bubbles.

The bubbles are more likely if you have less than 6mm total of glass. Two layers of S96 glass are often less than 6mm, another reason to slow down.

Larger pieces (such as 12" square) are much more likely to have this problem than smaller ones, especially if the piece is only a couple of layers of glass without elements on top.

Some kiln washes make this more likely as well.

The bubbles are caused by small pockets of air/gas between the glass and the shelf (which may have warped during the firing). When you fire too fast above 1240, the gas expands and doesn't have time to escape. If the glass is heavier, it can help push the gas out, but too little glass and you don't have enough weight.

If you wanted to do this intentionally, you'd have to know where the depressions in your shelf are and then fire like a bat out of hell from 1100 to your top temperature.