(2002.01.06)
Take a hint from the word "Muddy" in that title, and if you lack interest in pottery, please do yourself a favor -- skip to the next biscuit, once there is one.
As I have mentioned a few too many times already, I have a peculiar fascination with what I've been calling "Red Tenmoku". Here's my admittedly defective understanding of what that means: the ancient Chinese had lots of pottery styles and glazes, among which were various types of teabowls. According to at least one Web site I have read, several of these styles were (and are) known in Japanese as Tenmoku (or Temmoku), so this term does not refer only to a single glaze type, the way I came to understand early on. It's not a glaze type at all, in fact.
Well, okay: at least some tenmoku bowls are covered with glazes that seem to belong to a family. These tend to have a dark ground, on which are lighter features of one or another sort. You can see a few examples here if the page is still up. (Some of these are truly lovely, btw.)
As it is used in most studios I've been around in the US, the term "Tenmoku" refers to a type of glaze that is glossy black where it is thick, and breaks to an opaque milk-chocolate brown on edges and ridges (and anyplace else where it is thin; if you don't leave a piece in the glaze long enough when you're dipping it, or if someone has cleverly added too much water to the glaze slip, you get a very blotchy appearance that some people like and some don't). This does seem to correspond to some of the ancient Chinese and Japanese glazes, but it's quite limited. Moreover, this type of glaze is generally fired in reduction in the US, but if you read Robert Tichane or various other people on the subject, you'll find that at least the Jian ware bowls [the one on the page I link to above is listed as having been made around the 12th or 13th century] were clearly fired in oxidation: the clay from which the bowls were made has lots of dark particles in it that are rich in iron oxide, and if it had been fired in reduction it would have melted or at least slumped badly.
I'm not sure that there is any simple way to resolve the discrepancy here. I think that what I'm going to do for now is say "tenmoku glaze" when I mean the stuff I see in US studios, and "Tenmoku teabowl glaze" or some more-specific term when I'm referring to ancient wares.
Enough stage-setting for the ancient world. Let me set my own stage now --
(2002.01.06)
In 1999, having read a little bit of Robert Tichane's description of his investigation of one of the ancient glazes, I decided that I wanted something similar; but where Tichane says that his chip of glaze was brownish-yellow when he looked through it, I wanted deep cherry-mahogany red. After all, my clay bodies are white to buff, not dark brownish gray, and I like a variety of colors.
I went ahead and mixed up some tests, starting with brick clay and [unwashed] fireplace ash. 85% clay and 15% ash was brown, opaque, pleasant, and slightly crystalline. (Little deep-red lustrous crystals, almost metallic. In fact, it's not always easy to see the color -- the crustals appear to be a sort of gunmetal gray most of the time.) By the time I got to 35% ash or thereabouts the glaze was a transparent greenish/brownish color, and was leggy like wine on the wall of a glass. (This is a very common character of ash glazes.) Comparing these, I decided to add some silica (for glassy/glossy surface) and some iron oxide to darken the glaze.
In fairly short order I had my "Red Tenmoku". Unfortunately, even though I had measured the amounts of materials, I ran the glaze through a sieve after I mixed it, losing an uncertain amount of each item, so I no longer knew the composition. At that point I didn't have analyses of either of my main ingredients, so I was on thin ice in any event.
I glazed a few pieces, including a teacup that I gave to Vicka Corey, (It's hard to capture the actual look in a flash photograph; the areas that appear fleshy in the photo are mostly covered with burnt-gold crystals, where I used a rutile wash over the glaze. For the purposes of this discussion, the important thing is the red color just below the rim, which is lighter in the photo than it is when you actually hold the cup in your hand.)
Then I tried to add more glaze so I could dip larger pieces, and the color just wasn't the same. I spent about two years trying to get back to the original, and I learned quite a bit, but I'm only now getting close to what I originally had, and I'm not quite there yet.
On The Other Hand, what I'm currently getting may be of some passing interest.
After much fussing and fretting and trying of things, I've discovered that only certain fireplace ashes give me any red coloration. Others produce various browns with the brick clay that I'm using. Soon, I will have analyses to compare, and I may be able to determine the difference. This is not, of course, guaranteed, but at least there's a decent chance.
In the meanwhile, serendipity has struck. A few weeks ago I decided that I had too many little containers of glaze lying around, and I combined almost all of the test versions with the original. Then I ran the glaze through sieves until I got to 120 mesh. This mostly took out what appears to be sand, but that isn't a problem -- my impression was that the glaze had too much silica in it anyway. I dipped some test tiles for a typical reduction firing, and just for yucks I dipped one test tile to go into my electric kiln for an oxidation firing that was supposed to go to cone 9, but actually ended up running a bit hot. I figured that I'd use the results of the reduction tests to adjust the glaze and see if I could get something usable from it.
When I took the test tile out of the electric kiln, however, it was clear that I wasn't about to make any adjustments at all. I dipped almost a dozen pieces in the stuff and ran another firing, with rather happy results. Since then, I've fired a few more pieces, and they are all pretty decent. (Wait for it -- there's a photo a little further along if you want one.)
(2002.01.06)
...I've also been trying to reproduce a glaze I've never actually seen, which Nigel Wood has one single picture of in his book on Chinese Glazes. It appears to be a fairly plain glossy black, and shows up on a very few pieces of Ding ware from ancient China. If you want an index to what "very few" means, when Wood was writing, just a few years back, there were fewer than a dozen pieces of this ware known world-wide, all in museums, none of them in China -- the Chinese themselves had only shards. (I would guess that they're looking for more examples, but I have no idea whether they've found any yet.)
The photo, which is on page 155 of his book, is drop-dead gorgeous, and he provides one analysis of the glaze, so I tried to replicate it. My attempts looked like they might be proceeding in a vaguely useful direction, but they were slightly rumply and underfired, and the color was not quite right somehow. In addition, Wood suggests that the Black Ding ware, like the more common White Ding ware, was fired in oxidation; but my oxidized tests were a lot less promising than those I fired in reduction.
Then I got this stuff out of my electric kiln, and it changed my outlook -- on the back of the initial test tile, I had painted four stripes of wash, to see what effects they'd have. The first stripe is a mixture of Rutile (impure natural TiO2) and Gerstley Borate (mostly Ulexite, calcium borate, with other minerals; a flux). The second is Red Iron Oxide and Kaolin. The third is the same brick clay that's in the glaze. The fourth is the slip I'm now using to sign my pots, which contains Kaolin, Red Iron Oxide (“RIO”), Chromium Oxide, and Cobalt Carbonate.
Here’s that test tile. Note the mild rainbow effect on the upper part of the tile in the photo on the left. The dropout black stripe I mentioned is about in the middle of the other photo.
In reduction, the Rutile/Gerstley wash typically produces burnt-gold crystals. Either I put it on too thickly, or it works differently in oxidation, because on this tile it bloated and blistered. The other washes weren't much better, except the brick clay. That made a dropout glossy black, and sent me off on a tangent.
I'm now going through a new set of successive approximations, trying various combinations of clay and ash and iron oxide, working toward two goals: first, to approximate what's in my glaze bucket, so I can make more of it. (I'm also going to get it analyzed, which will be very helpful.) Second, the tangent I mentioned, my attempt to approximate the Ding Black in an oxidized glaze with ingredients that I have here, ingredients that are probably fairly similar to the ones in the original rather than trying to replicate the analysis with a complex mixture of things.
Here are some early results. The first photo shows three glaze tests and a teacup. The test on the left is 75% clay and 25% ash that was sent to me by Becca Levin. For the second test, I added roughly 5% Silica and an equal amount of Red Iron Oxide. I didn't mix the result very well, so it's spotty. The third test is 70% clay, 20% ash, and 10% iron. It is not quite fully glossy, and I'm trying to decide whether to add a percent or so of ash, a percent or so of silica, or maybe both. The final object has the "Mixed Tenmoku Tests" glaze on it. All of these were fired in oxidation, mostly at cone 9, though the teacup may have been fired at cone 10. (Doesn't seem to make much difference, actually -- the glaze is happy throughout that range.)
Here are separate images if you want 'em. You can see a certain reddishness in the glaze on the cup more easily here:
(2002 I 19)
I have been playing with Rutile Blue glazes for quite some time now (years, in fact), trying to get the thermal expansion adjusted so they'll fit regular porcelains without crazing, and trying to achieve decent colors at the same time. This has been a bit crazymaking, so I recently cut a spreadsheet that displays my results in the fond hope that I'll be able to correlate the composition of the glaze with its virtues and vices in something at least remotely approximating a systematic understanding. (We Have Our Fingers Tightly Crossed...)
I have, in fact, come across a thing or two that I find interesting, and as long as we're already in the mud here I think I'll put up my dukes and wave them around a bit. First, here is a picture of two test tiles with precisely the same glaze on them.
The only difference here is the clay body -- the one on the left is on a regular commercial throwing porcelain, and the one on the right is on one of my translucent porcelains. The amount of iron (and perhaps other things) that the glaze extracts from the surface layer of the body during firing is sufficient to create the differences in appearance and texture. Yeesh.
Let me emphasize that a bit: the next photo shows two glazes that are, in loose terms, nearly identical to each other; they are on the same commercial porcelain as the tile on the left in the photo above.
In this case, the difference is that the one on the right has an extra percent and a half of Rutile in it, and a little bit more clay. (I have one glaze that shows no Rutile Blue effect at all; it isn't all that different from any of these others, but it fires out to a copper red with a bit of flambé opalescence. So go figure.)
Alas, when I went to add this glaze to my little worksheet, I discovered that it was not in my copy of HyperGlaze. It is also not in any of my backup copies. I consulted my journal, and at the end of the volume that covers the first half of 1999 (which only goes through May of that year, for reasons I don't understand), I find a casual mention or two of having mixed this particular version and having fired test tiles. I don't find a formula. Worse, I don't find the volume that covers the rest of 1999, so I am entirely frustrated -- I was hoping I might discover that I had renamed this glaze. I clearly neglected to note that fact, if it is a fact, in the comments in HyperGlaze, but if I can find a written mention I'm home free.
I do have the formulas of all or nearly all of the other examples (I've got about 3 dozen test tiles of Rutile Blues of various sorts), so I'm in fairly good shape even without this one. I'm also about to fire a couple new tests at Glen Echo Park, in a continuing effort to enhance my comprehension. We Shall See how they turn out.
I've decided that Too Much Boron Is A Bad Thing -- it
seems to bleach the glaze out -- so I'm keeping it down
to a dull roar. Other than that, about all I've been
able to discern is that some things are unnecessary:
Zinc, Tin, Phosphorus. The glaze doesn't mind a bit of
copper if you like purples, but copper is not needed for
the fundamental effect. What I'm not certain about yet
is just how much Rutile is required. (Working on it.) As
usual, if I get anything resembling results (or even
anything so dumb it's funny) I'll try to post images
here.
(2002.02.23, Saturday)
I have also been working on my translucent porcelain.
This is a lengthy business, something I've been at since
about August of 1996, on and off. I don't think I will
do the Great Boring History here, but I should perhaps
mention some of the stages of development.
Having once decided to mess with the expansion
coefficient, I proceeded to do so. I'm fairly certain
that I've compressed the firing range of the mix in the
process, which is sad, but one probably can't have
everything all at once. What I do have is a clay body
that is about as easy to throw as your average
commercial porcelain, fires well at cone 9 in an
electric kiln, and nearly fits ordinary glazes. (I will
either adjust my glazes, or tweak the body mix a little
more.)
Here's a "stripes" test, where I've alternated it with
Southern Ice. My mix, which we will call "T9" for
convenience, is stripes 1, 3, and 5; these are actually
slightly thicker than the stripes of "SI".
As you can tell by the peculiar appearance of the piece,
all is not quite right here. It fell on its butt during
the firing, and stuck to the floor of the kiln; I had to
pry it off, and what you see in the photo is three
chunks that have been glued back together in their
original orientation. I can't complain too loudly,
though -- it was prepared in haste, about an hour before
I turned the kiln on, and was dried on top of a hot
bisque kiln. I was not at all surprised that it fell off
its base. The results are, in any case, sufficiently
gratifying to offset any annoyance: a light source
placed even half an inch from the edge produces a
delicate glow at the margin, and I am a very
happy camper.
Pseudo-mailto: jon [at] bazilians [put it here] org
Last modified: Sun Feb 24 19:26:24 PST 2002
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