Joss Research: Ceramics: Glazes: Glaze Tests and a Bowl, Spring 2004

A Joss Research Interim Report: Ceramics

Firing Results: 28 April, 5 May, and 27 May, 2004

(May, 2004)

I have recently fired several glaze tests, some of which returned results that I consider interesting enough to report, and a pot or two. Here are some pictures and explanations.

  • I’ve been working on replicating the ancient Chinese “Tortoiseshell Tenmoku” glaze for a while; these tests provided the best results I’ve gotten so far:


    I took the firing to cone 9, but with a fairly long soak that started when cone 8 just began to move, with the temperature held almost constant until cone 9 slumped. Cone 9 is a bit on the hot side for tortoiseshell, so the base glaze is darker and both the base and the spots are more transparent than they would probably be if I’d fired them lower. (Just for reference: under common conditions, cone 8 roughly approximates to 1240° celsius, and cone 9 to 1260°. They don’t really measure temperature, though, and in this case I was probably holding the kiln at 1225 or so for two and a half hours.)

  • Another ancient Chinese glaze I’ve been working on copying is the black that was used on occasional pieces of Ding ware. This is a deep rich glossy black that is actually very dark brown: it is colored only with iron oxide. (It’s dark enough to appear quite black to the eye, even in direct sunlight, but also sufficiently transparent to have quite a bit of depth.) I’ve been pursuing this glaze for a while now, and have had fairly good luck with it. I can’t even begin to do it justice in a photograph, unfortunately, but here’s the latest test tile of what I’m tentatively calling “Mirror Black”:

    As I say, the photo doesn’t really show you what’s going on here. This is really a glaze that has to be seen “up close and personal”.

  • I have a bucket of a previous formulation of the Mirror Black, which I’ve allowed to mutate a bit, and which is currently a bit easier to photograph. Here’s a bowl from the firing, and a closeup of what the glaze looks like in bright light. (The spots are about 2 mm across.)


  • From time to time, I find materials out in the field that seem to be worth investigating. In southern Pennsylvania and the “panhandle” section of Maryland, there is a huge exposure of what looks to me like some sort of compressed red ochre, basically low-grade iron ore. It varies from dark brown, through a brownish purple that is reminiscent of the “zisha” clay of Yixing, in China, to a surprisingly rich red. I collected a small sample of the purplish-brown version, crushed some of it, ran it through the jar mill, and dipped a test tile in the resulting slip. Here’s the plain material, and what happens if I put various washes on it:


    “163 AWQ” is the same wash I used on the Tortoiseshell Tenmoku test tile, shown above. It contains a lot of calcium, as one might expect from analyses of Jizhou Tortoiseshell. The brown stripe is a wash I often use on reduction Tenmoku glazes; it’s about equal parts (by volume) of Ceramic Rutile and Gerstley Borate. The gray wash is something I’m just beginning to test; it contains 8 parts Red Iron Oxide, 1 part Manganese Carbonate, and 1 part Kaolin. I’m not yet sure whether it’s a keeper. (I’ll talk more about “163 AWQ” when I’ve written the article I promised Gerry Williams at the NCECA conference, back in March.) This clay is just melting at cone 9, but is easily turned into a glaze by the addition of a little flux, as you can see from the clear spots.

    I fired another test of this stuff, at cone 10 in light reduction, and it’s smoother and shinier, but still opaque where it’s just plain. I need to get my hands on some of the redder material and try that. I expect that a little wood ash will turn it into a fine Tenmoku.

  • I spoke briefly with Warren MacKenzie when I was last in Minnesota, a few weeks ago. He expressed a desire for a tenmoku glaze with a rough, orangepeel texture. (There was more to it than that, but I don’t have the time to provide the entire conversation, even if I could remember it all.) I thought I could probably come up with such a thing, so I figured out a first test, mixed it up, and fired a tile in the electric kiln just for yucks. (I often do that; surprisingly often I get interesting or informative results.) In this case, the glaze was a miserable failure as a tenmoku, but it fired out to a beautiful rich deep amber. It’s too dark to show up well in a photo, but here’s roughly what it looked like, first in sunlight and then under a high-intensity source:


    You can see that it “slid” just enough to thicken slightly at the base. I think that will prove to be bearable; but I also think it may move a bit less when I decrease the amount of iron oxide in it to lighten the color. I should probably mention the fact that I’ve been working on amber largely at the behest of Dick Roepke, who works mainly in stoneware, at Columbia Arts Center, which is (no real surprise) located in Columbia, MD.

  • I also fired a clear glaze on my translucent porcelain, but I don’t have a photo. It’s about like other translucent porcelain that I’ve photographed, which is to say that a test tile of it is not particularly striking. Moreover, one of the most important characters of this particular glaze is its texture, which is most easily appreciated by touch. (See the next item, which is also the translucent porcelain.)

  • Here’s something a bit out of the ordinary. It turns out that translucent porcelain has very few impurities, which makes it a very good substrate for something I’ve been working on. Here’s a little white saucer:

    Here it is again, with a blacklight shining on it:

    Now, that’s in broad daylight. (You’ll notice, if you compare the first of those two photos with the first of the next two, that there is perceptible blue color just from the UV coming out of the sky and causing the porcelain to fluoresce. Most of the blue is just the reflection of the sky, but the edge closest to the camera is reflecting clouds, and it’s still blue.)

    Here it is again, this time in reduced light, without and with the blacklight:


    To show you how translucent this stuff is, here’s the saucer with the blacklight behind it — the glow you see (except at the left and right edges, where the lamp is shining on the outside) is coming through the porcelain…

    I’m writing an article about this work, so I’m not going to go into details yet. (I intend to lay it out in full detail in the article.)

This work is supported by
The Joss Research Institute
19 Main St.
Laurel  MD  20707-4303   USA

Contact Information:

email:, where a is replaced by my first name (just jon, only 3 letters, no “h”, and b is replaced by joss.

My phone number is +1 240 604 4495.

Last modified: Sat Dec 23 20:50:56 EST 2006

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I am a Researcher of the Joss Research Institute. I work primarily on lasers and ceramics, with occasional excursions into other areas.

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