Sometime you want to have bubbles in your pieces. Sometimes you don't. But you never want what I am getting recently. Large burst bubbles.
Kiln forming is often an unpredictable art, but mostly the variations should be small. Withing the last month my big kiln has begun blowing big bubbles. Like this one.Most often these seem to come from underneath the piece but sometimes between layers. This is a piece I put in after I thought I had diagnosed and fixed the problem:
This has bubbles coming from under the piece and from between the layers. My practice has not changed as far as I can tell, from the past when there was no problem. I have checked on the density of the sand bed beneath the glass, by compacting it tightly. I have been very careful about level screeding of the bed. I have fired on both unfired and fired thinfire. I have fired smaller pieces and still got bubbles (see the piece at the top). I need my big kiln. I need to find out what is happening.
So I went back to basics and did a "Know your kiln" test from Bullseye . This showed me that the temperatures in my kiln are different than at the beginning of its short live (five years old now). The chamber is much h0tter at the back than at the front, rather than just a little. I phoned and found out that the elements are loose inside the ceramic tubes. This probably means that with me leaving the lid open when not in use, the elements have slowly compacted to the back of the kiln. This means that I have to take the top off the kiln, and remove 8 square metres of fiber blanket to get to the tubes that hold the elements. I then have to lift each of the 20 tubes and stretch the elements back into some sort of even-ness. I expect this will be a 2-4 day job, when I could be doing other things.
Even when this is done, I am not sure it will cure the bubble problem, as I am getting bubbles at the front and at the back both. Still, the elements must be sorted. And in the spirit of eliminating problems one at a time, I will get to the bottom of this!