Thursday, March 25, 2010
Two in the Morning and all that!
I had odd mixture of things go wrong. Some problems I think were caused by firing the last few hundred degrees a little bit too slowly, which resulted in too few crystals in some pieces, but I had an odd thing happen to what had been a very reliable glaze in past firings where the copper colour came out a lot more blue green than previously, as if one of the glaze materials was a lot more alkaline than before. In fact the glaze looked quite different in character as well as in colour, so I have had to run a whole bunch of tests which I am firing now, to try to determine what caused the change. The cause could range from one of the glaze materials being from a newer bag, to the fact that the clay that the pots were made of was from a different batch than the ones before (and I have found that the clay body does have quite an influence on the glaze).
The most spectacular failure of all was the collapse of one of the little round insulating fire brick disks that one of the pots was sitting on in its glaze catching saucer. This was caused by the run off glaze penetrating through cracks in a coating of alumina that was on the fire brick, and simply eating away at most of the fire brick. I found that several of the other pots nearly suffered the same fate, and I was lucky to have not had a far worse disaster than I actually had. The pot that lent over very nearly came into contact with the kiln wall and the elements, and would have caused costly damage if it had touched them. It was a good reminder of what can go wrong. The break down of the alumina was most likely because I tried to economise by recycling some from the firing before, with the thought that the calcined alumina would in fact be more reliable as it would not shrink as much as the new stuff, but it obviously did not coat as well. We learn don't we!
Got one really lovely little glaze test out of the bad firing, and I am using that glaze on a slightly larger piece in this firing in the hopes that it will turn out well again.
Must go and shut off the kiln about now.
Thanks to all of you for your comments on the previous post. I will respond to them when I can open my eyes again! Laura appreciated all the nice things you said about her efforts too. Nice to hear from Joseph, a first time commenter. I also had an inquiry from Minna on one of my previous posts (about tiles and glazes). I have left a detailed response there, complete with glaze recipes for Minna as she asked for some advice and recipes for chun glazes.
2.30am Must go to bed,
Best Wishes to you All... P.