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Firing Log

ancient kiln | 21st century logbook

March 28, 2006

Test Clay for Firecolor

Filed under: anagama, 6th,Clay Bodies,Firing — odin @ 1:22 am

I made up a test clay for the current firing. I mentioned in a prior entry that sometimes the best color on anything from the kiln has been on my wadding. What could be more obvious than to make up a batch of clay with similar materials? Why haven’t I done it sooner?

First, let me describe more fully what I want. Furutani fired pieces with incredible firecolor. I have wanted the same result for a long time, but the closest I ever came was orange wadding. I have brown and blue coming out my ears, but no reds.

I can’t speak for all anagamas, but in my firings of unglazed pottery, color is most directly related to clay. Porcelain is generally white. B-mix is sea green on the front, and rust colored on the back (I don’t like the rust). Clays from Seattle Pottery Supply are varying degrees of brown (Sea Mix (brown/tan), Delta White Stoneware (dark tan), Sclupture Buff (very dark brown). Never a red.

At one point in time, I was trying to concoct clay mixtures that mirrored the chemical composition of Shigaraki clays. There were some interesting results, but never a red. I feel my failures were related to the fact that clay is a suspension of particles rather a mass of disolved elements. Think of it this way: let’s say I’m trying to make a clay that has a chemical composition of 1a, 2b, 3c, and 4d (numbers indicate amount of mineral, letters refer to different minerals). Potters make the original clay out of two rocks, rock-one’s chemical analysis is 1a and 2b; rock-two’s is 3c and 4d.

I have at my disposal different materials: rock-a’s chemical analysis is 1a, 2b, and 1c. The other is 2c and 2d. Then I add some pure “d”, two units, and end up with the same chemical composition as the original, but a completely different clay.

You should have seen the spreadsheet — it was pretty nifty. But all that nift didn’t give me firecolor. So, back to the wadding. For whatever reason, the fireclay/silica/grog/rice-bran mix often turns bright orange. Two nights before loading day I suddenly decided to make a wadding like clay. It was two parts Lincoln Fireclay, two parts silica, and one part 10 mesh custer feldspar.

I think it was a good idea to finally give this a shot. But I made a mistake in my haste and exhaustion.

Firecolor does not come about on pieces with lots of ash glaze. Feldspar granules tend to melt and make a glassy smooth glaze. What this means is that my recipe will work against itself. The high silica content should resist glaze formation while the grains of feldspar will be slobbering glaze all over. I suppose it could be interesting, and we shall see, but I’ll have to make another go at the wadding-like clay for the next firing — one that doesn’t work against itself.

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