Thursday, March 29, 2012

Porcelain: thrown vs. rolled

No, I'm not committing violence upon my pottery as I make it. I'm examining the behavior of a new clay as it goes through my two basic forming processes.

Standard Porcelain 551, a new venture in fine white clay, threw well on the wheel, and trimmed well (that's where you use tools to refine the forms after they have been thrown on the wheel and firmed up). Goblets in various stages of trim:

But slab work (rolled between canvas sheets, textured and draped into a form) cracked as it dried. It does not want to be bent and manipulated. It probably wants to be covered and dried verrrry sloooowly. I didn't treat it to the right tender care, apparently, and it misbehaved afterward as it dried.

Well, this project is experimental just now, so anything could have happened. Different clays behave differently. Porcelain has a very fine particle size and this one has a large shrinkage rate. I lost all three of my sample projects, two 17" long by 12.5" wide platters and one square, textured bowl. The bowl, which I handled more in the raw, flexible state, cracked in four places. Here's a platter, in the process of objecting to my handling. It has two cracks forming, one at the top and one at the bottom. They will be much larger before they are done spreading:

The cracks from this one are mostly from settling under its own weight as it dried unevenly. You can see that the middle is damper (darker) than the edges. In short, it has poor standup when it's taken from the form it's draped into before it has dried completely. It can't support itself unless it's dried slowly and evenly. I can try that next, but I won't spend too much time on it. I prefer a clay for slab work that doesn't need so much pampering. It might only be worth the extra effort if glazes show up very beautifully on it. Even then, a high rate of loss of product isn't very good business.

At least I know I can throw with Porcelain 551, and I like it! It responded very well to the wheel process and felt mighty nice and non-gritty in my hands. The thrown forms seemed to survive much better. I am going to dry them slowly, and see if they want to crack too! Hope not. More tests to follow after Passover.

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