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.
Monday, June 10, 2013
Yesterday, I wandered into the Studio early, coffee cup in hand. It was Sunday and I used that as my excuse for not planning to work as usual. I'm old enough to remember when Sundays were Sundays, and sometimes I still use the day as an excuse to do what I feel like doing instead of what I should be doing. I had the vague idea that I'd look over some of the challenges I've been working on lately and maybe there would have been a miracle overnight, or else there would be some sort of inspiration, maybe a flash of light, a drum roll, a voice from above, something like that, solving my problems for me.
No such luck. The travel mugs I'd so looked forward to seeing out of the kiln on Friday were still 'spotty'. This is a problem with low-fire majolica, but sometimes I see it on my Cone 6 stoneware too. Usually I just see one or two tiny white dots, but in this case every mug has lots wherever there is colour.
Another problem looking for a solution is a little oval soap dish. We won't discuss the fact that this is a commission and I didn't insist on charging my usual development fee. Should have, then I wouldn't have to be futsing around trying to make very small oval dishes with closely matching lids. The customer was absolutely adamant about the design, the size, the shape, the weight. He had worked hard to get to my studio and I didn't have the heart to say 'no', but I should have.
Now, this would be dead easy in a round shape, but the customer wants to match an oval one he has. It is less than 4" long and weighs practically nothing. Earthenware, slip cast, decorated with a decal.... what was I thinking to even consider this?
I make my slab pieces over plaster molds. Lots of people make theirs into plaster molds, but I find that then I have to make a template, cut the clay carefully, ease it into the mold and try to shape it without making thick corners.... my system is easier, more flexible, less annoying and usually works great. I also like the fact that I can change the edges of the dish more easily. But the trouble with this little thing is that it warps in the firing.
If you look carefully, you can see that the top piece doesn't sit quite down on the bottom piece near the middle of the long side. And the two pieces were fired together.This is my third attempt and the dishes aren't getting any better. What to try next? I'll try making the clay slab a mite thicker. The customer might not like that as the dish will be heavier, but I can't think of much else to try. The two pieces fit perfectly at the leatherhard stage and seem to be OK at the bisqued stage, but they warp in the glaze firing. Any uneveness in the wall of the piece will cause warping in the final firing, and the shape itself has no built-in integrity. In my flat dishes I make the edges with a little 'wave' to them, and then a slight warp doesn't show. Can't do that here.
Anyway, I mooned around the Studio for a while looking at these things and also the rectangular butter dish I am trying to develop and finally decided maybe it should be a gardening day. After all, it was Sunday.
Posted by Lis at 2:32 PM