Recently, I opened a kiln only to find disappointment. I glazed pieces with the same glaze I've been using for awhile now, but in a different electric kiln. About 2/3 of the entire kiln load had blistering and gross spots caused by bloating of the clay. After trouble-shooting with some of my colleagues and professors, I think there were several problems with this firing:
1. It got too hot. In the kiln I had previously fired in is much older and may not have been firing to a true cone 6 temperature. And/or the newer kiln over-fired.
2. It fired too quickly. Previously, my firings were extremely slow because it took so long for the old kiln to make it to temperature.
3. I didn't fire my bisque hot enough. The last bisque firing I did, we were out of 06 cones, so I just fired to cone 08. It's possible that cone 08 was not hot enough to burn some of the gasses off, and they were trapped and came through during the glaze firing.
My plan going into the next firing is to test the same glaze batch at cone 5 in the newer kiln and see what happens. I also plan to fire again in the older kiln since I have gotten consistent results. I will be sure to use witness cones in every firing now to see what temperature my pieces are actually firing to.
I must admit, it wasn't easy to make this post and admit failure. But I think that its healthy to talk about mistakes and is helpful for everyone. Seeing clay go through the process of dirt to permanent, useful object is one of the most beautiful things about working with ceramics, but it's also the most frustrating and difficult. During drying, pieces can warp and crack, bisquing can cause explosions, and glazes can blister, run, craze or just plain look ugly. Is all this pain and loss worth it? I think that the brutally unforgiving process makes it much sweeter when you open a kiln and find that one brilliant pot.
I'm trying to stay positive and use this failure as fuel to make, make, make. So for now, it's back to work!
Oh and p.s. - I welcome comments, questions, or discussion, so please feel free to leave comments below!