One thing about pottery, it is not quick. Just figuring out what happened to cause all those 'dots' I talked about last post took me a week, and I was lucky it was only one week. I found the problem right away.
I re-fired several of the mugs, each on a different shelf in the kiln. I made sure to have large witness cones near each one.
The bottom shelf was almost a full cone over-fired. The witness cone for Cone 7 was fully over. The other shelves were the Cone 6 as I expected.
So I'm figuring the problem is the thermo-couple at the bottom of the kiln. This problem of 'dots' has been creeping up on a me a bit so I guess the thermo-couple has been failing gradually. I just didn't twig to the fact that it was always the pieces on the bottom that were acquiring a 'dotty' nature in the firing.
Well. Lesson learned. From now on I will put in witness cones at least every 3rd or 4th firing. I've been slack about that. They tell you to use them every firing, but with a computer-controlled kiln it is hard to convince yourself to actually do it. Especially in really full loads.... and especially when you stop to think of how much those darn things cost.
Re-firing didn't affect the dots, by the way. They neither got worse nor got better. And I even tried applying dabs of colour to the dots and re-firing and that didn't work because I couldn't get enough on to really cover the white.
Another learning experience.... if I learn from my mistakes, I'm going to be awful smart some day.