(2004 March 13)
The Chinese have been making wonderful red glazes with copper in them since at least the mid-1400s. There have been various versions at different times, and the history is quite interesting. (There is a fine chapter on copper colorants in Chinese Glazes, by Nigel Wood.) It turns out that the red color is an early example of nanotechnology: it is caused by a colloidal suspension of copper nanocrystals, some tens of nanometers on a side. (I think they range from ~20 to ~200 nm across, but don’t quote me.) Richard Zsigmondy got a Nobel prize in 1925 for achieving a systematic understanding of colloids; he invented the ultramicroscope in the process of his research.
Copper Red glazes are notoriously difficult and fickle. It’s debatable whether this is inherent, but there’s no question that it’s The Usual State of Affairs. I started trying to make a decent, noncrazing Copper Red in 1996 or 1997, and have made Copper Gray, Copper Grayish-Pink, a color that is probably the one the ancient Chinese called “Mule’s Liver”, and various other wretched failures. I’ve even made one that wasn’t too bad -- it came out a rich purplish red. (Darker and more purplish colors are common at higher temperatures.) More recently, I made something that looks about like strawberry jam. Not bad, but a bit pebbly, and not as rich a color as I might like.
A few nights ago I revised the “strawberry jam” formula, to see whether I could get something with a smoother surface. Here are photos; from left to right: the purple glaze I mention above, strawberry jam, and the result of the most recent revision --
The new glaze is a fair match to the bright red on some Chinese pieces, and I’m extremely pleased. Unfortunately, it is crazed, which is something I’m expressly trying to avoid, and it has some sort of crystalline scum on its surface, on the drip at the bottom where it has slid a little (these glazes are notorious for running off pots), so I’ve revised it yet again. I’ll present that result when I have it, if it’s worth looking at.
Various people have recommended various screwball firing protocols for Copper Reds, but the folks who seem to get excellent results are (mostly) agreed: start reducing fairly early [cone 012 is typical], and reduce at least until cone 6. Reduction can be relatively light; hard reduction is unnecessary and may cause carbon-trapping, which results in grayed-out colors. Allow a period of oxidation at or near peak temperature.
If you mix your glaze correctly, it may even be just that simple; but it’s nontrivial to work up a good glaze, as I’ve discovered by trying a lot of bad ones, and so far it has seemed impossible to make a good one that doesn’t craze. My current glaze makes use of two small tricks that I’m not quite ready to discuss (I may write an article about them, and I don’t want to spoil my chances of getting it published); suffice it to say that if you read Robert Tichane’s book on the subject, and you attend carefully to what you find in it, you’ll notice something extremely peculiar. I’m not referring to the fact that he contradicts himself, nor the fact that other people he talks about contradict each other and occasionally themselves — that’s only too common — but rather to the fact that he ignores the most important of his own experimental results.
More as it transpires...
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Last modified: Wed May 10 11:5525 EDT 2017