(11 October, 2004, with a followup)
Here is a Red Temmoku bowl, fired to cone 10r. The bowl is about
6" across. It is made of Loafer’s Glory, from
the glaze consists of Brick Clay from western Wisconsin;
Wood Ash (Oak, unwashed); and Red Iron Oxide (84% purity).
I strongly suspect that the yellow teadusty or
“corn-pollen” sprinkles, which show up
particularly well on the interior, result from the ~2.6%
MgO content of the Brick Clay; I get essentially the
same effect in other iron-rich glazes if I add enough
Mg. (Oddly, although all or nearly all of the ancient
Chinese high-iron Jianware glazes contain noticeable
MgO, very few modern Western “Temmoku”
glazes seem to use magnesium. Go figure.)
(Click any of the small photos to get one that is 1280x960 px. If you want something even larger, you can change the “.10c.” in the filename of the large image to “.22c.” for the full-size originals, 2272x1704; just be aware that those files are about 1.5 MB each, and may take a while to download if you are on a slow connection.)
This bowl was donated to a benefit auction in Minneapolis.
Note: The word “Tenmoku” properly
refers to Jian teaware styles of the Song dynasty, not
to glazes. I am using the more modern romanization of
the word, “Temmoku”, to refer to glazes. The
spelling difference is regrettably slight, but it seems
to me that the distinction is important I have
seen a claim, from someone who failed to understand this
issue, to the effect that there can’t possibly be
any White Tenmoku pottery. This turns out not to be the
case; I have held a White Tenmoku bowl in my hands. It
was grayish, to be sure, but definitely unlike the usual
dark or “Hare’s Fur” or teadust or
oilspot glazes you see on most Jian teaware.
The history of this glaze is complicated, and (for me, at least) fraught. When I first started making my own glazes, I read Robert Tichane’s book about Ash Glazes and decided that I wanted something like the typical Jianware glaze, but dark cherry/mahogany red rather than the yellowish color he described. This is because Jian teawares are made of a dark stoneware-type clay, and a yellowish glaze looks really dark on them; I am using porcelain, and a yellowish glaze is not anywhere near as interesting, at least to me.
My first attempt at one of these was successful, but as soon as I tried it again, I failed. This taught me two important lessons: you have to sift the clay and the ashes before you weigh them out, because otherwise you don’t know the actual proportions of the glaze. It also helps to keep a searchable record. Soon after that I started using glaze calculation programs.
Some years later, when I made the bowl in the photos above, I got the red color back again. I fired a few things, and then I decided to add more glaze to the bucket. The result was not red. I have been searching for the key to this for several years now, and every time I think I have it, I get another brown or opaque glaze test out of the kiln. This is discouraging and tiresome, but I have certainly narrowed down the parameters to some extent. I keep going for two reasons: first, the fact that I really want this color. Second, the fact that every once in a while I get a hint of it on a piece or a test tile. The issue seems to be that it is tweaky, and that the key parameters (whatever the turn out to be) must be within a fairly narrow range of values.
I initially thought that the particular type of Red Iron Oxide was key, but I have since seen some red on pieces that used ordinary synthetic high-purity RIO. (...But see below for more about this.)
I initially thought that a hot reduction firing was key, but I have some test tiles with red on them that were fired at cone 9 in the electric kiln, and I have seen some red on at least one piece that I fired to cone 9o. (...The key in that sentence is “some red”; see below for more.)
Recently, I have been digging back through glaze recipes and my notes on them, and I have discovered an odd thing: the percentage of brick clay that I use in the glaze has crept up stealthily over the years. As far as I can tell, the original was only about 40% clay, whereas recent tests have been above 70%. This would seem to be a key factor, except that the glaze on the bowl above seems to have been made with about 70% clay.
I continue to be puzzled, and I continue to test various conjectures. Recent tests have been more like a Kaki (“persimmon” rich crystalline brown, rather like the inside of a ‘Hyakume’ or ‘Chocolate’ persimmon), quite opaque, so my next step will probably be to revert to a 50-50 mix of clay and ash, which should be pale green and transparent, and start adding iron oxide to see what happens to the color.
Wood Ash being the highly variable material that it is, however, this means not one series of tests but rather at least 4. I have ash from a wood-fired pizza place, from Dr. Becca Levin’s fireplace, from a charcoal grill, and from a Black Locust tree that Edwin Gould burned. (I also have smaller quantities of ash from several sources including Fred Paget, and if I can’t get enough variation from the four I list above, I can always try some of the others. The problem is that if I want to mix a lot of the glaze, I need a fair amount of ash; I am fervently hoping that the ash is not the key here.)
There is also the obvious possibility that there are several keys, all of which have to be positioned accurately. It might help if I had analyses of all of my materials, but that is rather expensive, so at least for now I am proceeding on a grossly empirical basis. Eventually, however, I may have to grit my teeth and start saving for a battery of XRF runs. (I do have XRF analyses of the brick clay, performed by Mary Simmons some years ago, and they are extremely helpful; but without analyses of the ashes I am still to a large extent floating around like an untethered balloon.)
I will post more about this as it happens.
I am now beginning to suspect that the red I see in pieces that come out of cone 9 firings in the electric kiln may not be precisely the same red I see in pieces that are fired in reduction. My most recent test results suggest that high temperature is important at least one glaze that comes out red at cone 11+ seems to be brown at cone 10. (My test kiln fires so quickly, btw, that cone 10 is about 1305-1307° C, and cone 11 is more like 1320° C or a bit higher.)
I am also drawn to the conclusion that the RIO I use is important; with ordinary high-purity synthetic RIO I seem to get a nice rich brown, rather than red. That’s not 100% certain yet, and I need to do a little more testing; but either way I think I’m going to have to get the 84-86% pure material analyzed.
Most of my recent tests, btw, have been with mixtures
that are close to 60% brick clay, 30% wood ash
(unwashed), and 10% RIO. Different ashes are giving me
different degrees of redness, and I will probably have
to get some of those materials analyzed as well.
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Last modified: Wed May 10 12:12:40 EDT 2017