My big kiln has had a number of difficulties in the last month or so.
First I began blowing bubbles in and through a lot of pieces. I had not changed any of my practices (however unscientific they may be) and could not see what was happening. I tried a number of tests: firing slower, firing lower, venting the kiln, firing on batts instead of the sand, testing the temperature distribution round the kiln.
The last showed that I had hot perimeters in the kiln, very unusual. I checked with the manufacturer who suggested I take the elements out of the ceramic tubes and space them out at the ends. They forgot to mention that they are wired in a single run of 6 metre long length, through 6 tubes. I got the heating of the elements OK on the first one's ends so they were evened out with the rest of the element. Then the problem was to heat the bend of the element and straighten it so it would pass through the second tube. Well, I wasn't up to it. The element broke.
Now the question was whether to try to buy the appropriate amount of wire and wind it myself and then insert it into the tubes, or hire someone else to do it. I decided on the latter. After some consultation we decided to establish the elements in three pairs for each phase. This means the element tails are at the back and I only need to replace one pair instead of a complete run of 6. Much cheaper to repair if needed.
Fired away confident that all was well. And it was. Until the third firing when things seemed a little odd, as the kiln seemed cooler at one end than in the centre and other end. On the fourth firing it was clear that one phase of the kiln was not working. So another large piece lost.
The people came and repaired a broken element, since there was a loose connection that caused the element wire to overheat and then break. Fixed. Not quite. fifth firing was fine, but sixth firing showed only one phase was firing. Now not only the original problem phase but the centre too.
Well, the company has a couple of people who are knowledgeable about glass kilns and they were away in Inverness, and of course, there is a holiday too. So the kiln sat idle for just over a week waiting for repair.
Today the man came and found two other wires leading from the main connections to each of the phases had corroded and failed. Well, they hadn't replaced them when doing all the other work on the elements and wiring. Certified as working as of 11:30 this morning.
Now it has somebody else's tests in it so at least if there are problems, it will not be mine. Then, of course, the first firing after repair has been fine. It has only been later when problems arise. I wonder when I will trust the kiln again.
Oh yes. The bubble problem was an accumulation of dust from sieved thinfire over four years. The sand bed becomes less porous with lots of dust and so does not let the air from under the glass as well as a mix with less dust.