This jar, which had a nicely fitting lid,
received underglaze decoration in the raw state
and I glazed it with Clear after I bisque-fired it, and fired it to Cone 6 (about 2232 Fahrenheit).
The lid was placed separately on the kiln shelf to fire in the glaze kiln, and it warped as it was heated to maturity. It no longer fit. What a shame! It was, in fact, one of 3 lids that did this. Here is another jar with a lid that warped...
Second warped lid. This one was spoken for, too. Back to the drawing board. I mean, the wheel.
And another one...
Off-round, jar 3.
The reason the lids warped was probably twofold. One, my clay is a porcelain I only started using in the last year, and these are new lid forms that I may have made slightly off-round on the wheel. Porcelain is a delicate beast. Even if I corrected the lid shape while it was still soft on the wheel, it may have "remembered" (clay does) and reverted to off-round in the firing. This porcelain may "remember" more actively than my previous stoneware clay did.
Or, Two, while firing, the lids may have caught on minuscule rough spots on the much-used kiln shelf as they moved slightly during shrinkage.
If the problem was Two, about catching on rough spots on the shelf, maybe I could fix it by putting something on the kiln shelf under each lid, to allow it to move in the fire, without catching on anything and thus distorting.
I tried it. I have a small bag of kaowool, a refractory fiber that withstands great heat. So I made four little wads of kaowool fiber and sat the first jar lid on it, and re-fired it in the next glaze kiln.
Amazingly, it worked! While the lid has to be put on the jar in a specific way in order to fit properly, it does work. There was a downside, in that some kaowool touched the glaze inside the lid and fired onto it, and I had to grind those spots where it stuck. So I'm thinking maybe I'll try something else as the "gliding agent" next time. And I will use the gliding agent (maybe a fine dusting of dry alumina, and only on the lowest shelf to avoid sifting down to other pots?) under the freshly glazed lids the first time they go into a glaze kiln. Who knows, maybe it will cut down on re-firing, which is a waste of my efforts and electrical resources, and an iffy proposition.
The re-fired lid fits the jar now. Kooky, rehabilitated Kookie Jar!
Learning, lifelong!