The kiln opening that explained three months of pinholes
Since September, every piece I have glazed in the studio’s white satin has come out of the kiln with pinholes, tiny craters the size of a pin tip, mostly on the flat surfaces and the inside of bowls. Other students using the same bucket were getting clean surfaces. So the glaze was not the problem, and the firing was not the problem, which left me.
The instructor and I went through the possibilities over two Tuesdays. Glaze thickness: mine was about the same as everyone else’s by the scratch test. Firing schedule: identical, same kiln load. Clay body: same buff stoneware as half the class. Application: I was dipping, like everyone else. The one difference, when we finally got to it, was what I did before glazing. I was wiping my bisque ware with a damp sponge to get the dust off, and I was doing it thoroughly, the way you would clean a plate.
That sponge was the cause, in two ways. A wet bisque surface takes up less glaze, so my coat was thinner than I thought in places, and thin satin glazes pinhole more easily. Worse, the sponge was pushing fine dust and a film of water into the pores of the bisque rather than off it, and as the glaze melted over those spots, the trapped air and burning dust blew tiny bubbles through the surface that did not have time to heal before the kiln cooled.
The fix was to stop cleaning so hard. Now I dust the bisque with a dry brush, blow on it, and leave it alone. If a piece needs a wipe because it has been sitting for weeks, I use a barely damp sponge and then let the piece dry for a full day before glazing. I also hold the dip a second longer on the flat surfaces where the pinholes were worst.
The first kiln load with the new routine came out on Tuesday: two mugs and a small plate in the white satin, all clean. Not a single crater. Three months of pitted pots, and the cause was a habit I had picked up from washing dishes. I have put the pinholed pieces on the shelf at home where I can see them, as a reminder that the problem is usually me.