Testing, one two three

[offcntr]

Testing clay bodies again.

The last time I did this, I was in crisis mode. The clay body I'd relied on for years--Pine Lake White, from Seattle Pottery--had started throwing out chips. This sometimes happens when the fireclay component has insufficiently-screened bits of limestone in it. They calcine down to quicklime in the bisque firing, then rehydrate to lime from atmospheric moisture, expanding just enough to pop a little chip out of the surface. You can tell that's you problem because there'll be a little white bit at the bottom of the hole, lime-pops they're called.

This time, there were all the symptoms of lime-pops but the lime. Mystery pits were showing up after bisque, sometimes even during the glaze firing, which is worse. (You can sand/grind down the pits to minimize them in your bisque. If a chip pops out in the glaze firing, it leaves a bare hole and, often as not, a flake of clay stuck to the glaze.) My potter friend Pete Meyer suggested the problem might be bits of lignite (soft coal), which would burn out entirely. Whatever it was, it was driving me nuts.

Worse was the supplier's reaction. They claimed no knowledge of the problem; apparently, I was the only potter in the Northwest who had this happening. I decided to take my business elsewhere. After testing a bunch of clay bodies that looked similar (based on sample chips on their website), I settled on Clay Art Center's DWS.

I've used DWS for a couple of years now. It throws and hand builds well, iron spots nicely, but it's difficult to trim. This summer, in particular, I've noticed that it's very hard to smooth out the pits left behind by the coarser particles, and that can sometimes lead to pin-holing in the glaze. I don't know if the particles are coarser, or I'm trimming differently--dryer, perhaps, since it's summer, or my recently re-sharpened tungsten carbide trim tool has different cutting characteristics. In any case, it's wasting my time and occasionally, my pots.

So I'm testing again. Two candidates this time: Dakota White, which is basically the same clay without the coarse bits (DWS is apparently short for Dakota With Sand), and CAC White, recommended by the salesman in Tacoma, which I suspect will be too white for my glaze palette. I want a warm tone with iron speckles in my glaze, and I don't think I'll get them.

So far, I've done shrinkage tests, flat tiles scribed with a 10 cm line that I'll then measure as they dry, are bisque fired, and glaze fired. I've also thrown and trimmed some bowls--both trim nicely smooth. I'll also need to do some tests throwing larger forms, to see whether I really need the sand or not. I could theoretically buy both Dakota and DWS, but I hate to complicate my life like that. One clay body, please.