As I think I explained in the first archived biscuit, a gaiwan is a device for brewing tea in the old-fashioned Chinese "gong-fu cha" style. The advantage of a gaiwan is that it is made of smooth-surfaced material (porcelain, usually, though I have seen a few jade ones), which makes it easy to clean. Thus, if I understand this correctly, you are slightly more likely to see gaiwan at tea-houses than in people's homes.
At home, people tend to use those cute little Yixing teapots when they are doing gong-fu tea. Crummy Yixing ware, like anything ill-made, is trash; but the good stuff (which is not cheap) is very special indeed, and once you learn how to deal with it, you can make really fine tea in it. This process is not quite like making tea the way we usually do, in a big teapot; see the first archived biscuit for a bit of an explanation of the method. If you want to explore the difference in the way it smells and tastes, I'd suggest that you find someone who is really good to make it for you the first few times, because there are several variables that need to be controlled accurately, and your chances of succeeding on your own if you have never seen it being done are about nil. You'll probably get tea, but you won't be anywhere near the limits of what the pot is capable of.
Yixing ware is made from a very special kind of clay that the Chinese call "purple sand". I have not yet gotten my hands on any of it, and until I do there is very little chance that I'll be able to approximate it at cone 11.5. Even if I could (and I am trying: as I write this, my first test is in the kiln), there's no guarantee that the result would be worth making tea in: there really does seem to be something special about the clay itself. Besides, I'm not a good enough teapot maker yet. Teapots are hard!
A gaiwan, on the other hand, is surprisingly easy: three pieces, no handle, no spout, and the fit is on the loose side. Of course, if the gaiwan itself wobbles badly in the saucer or if the lid rides too high (or, worse, too low) inside the gaiwan, it's a lousy one. But that ain't a patch on spouting a teapot, believe me, and a lid that merely has to go inside the upper part of a cup is easy.
On the other hand, your loyal author is a lab-klutz, and when I was sponging the dust off my first attempt at a gaiwan, in preparation for glazing it, I broke it. At that point, unfortunately, all three pieces had been bisque-fired, so when I threw the replacement gaiwan I didn't really know how big to make it. Fortunately, as it happens, my usual translucent porcelain mix doesn't shrink too much as it dries, and pottery doesn't change its size very much at all during bisquing, so I was able to get a workable replacement on my first try.
The next problem is that the lid still has to fit inside the gaiwan after they have been fired. All of my stuff warps during firing. This is almost unavoidable, especially with translucent porcelain, which has to be reasonably thin, and which must be brought fairly close to its melting point or it fails to achieve good translucency.
I say "almost unavoidable" advisedly -- there is, it turns out, one fairly good way to fire things without having them warp too badly, and that is to fire them rim-down. The drawback, of course, is that you have to keep the rim unglazed. Also, if your glaze wants to run when it is melted, you are in deep, deep trouble. Fortunately, my clear glaze is reasonably well behaved. Unfortunately, the lid wasn't domed high enough, and it collapsed into a little pancake with a knob on it. Such is life. I'll probably try to make another lid, because the cup part is decent, and the saucer is bearable.
A proper gaiwan should look like a delicate little flower with the saucer as calyx, the gaiwan itself as corolla, and the knob atop the lid as a pistil rising up out of the center. I'm afraid it wil be a while before mine meet that standard, but it's fun working toward it.
The clear glaze that I sprayed onto the gaiwan is one member of a largish family, most of the members of which will probably never exist because I'll never need them.
Different "clays" have different coefficients of thermal expansion. That is, if you take a ceramic rod that is, as nearly as you can measure it, just ten inches long at zero degrees celsius, and you heat it to one hundred degrees celsius, you will find that it is now somewhat longer. Cool it back down to zero, and once again it is ten inches long.
If you take the length change you've observed and divide it by a hundred, you'll get the average length change per celsius degree within that "zero to one hundred degrees" range. For almost all ceramics used in pottery, that number is between 40 and 150 parts in ten million. A few parts in ten million may seem small, but in fact a good flameproof body should be significantly lower, and I believe that Pyrex(tm) glass weighs in at about 32.5. Fused silica (that is, silica glass, which is very different from any of the crystalline forms of silica) is lower still.
So.
My original translucent porcelain appears to fall around 55.5, but you must take that with a grain or two of salt, because I do not have the correct instrument with which to measure it. Eventually I'll get the measurement made, because I do know someone else who has such an instrument (it's called a dilatometer), but that hasn't happened yet. Until then, what I've done is mix up a glaze that seems to fit the clay, and the number I'm giving you is the number that I get from the glaze calculation program I use (Richard Burkett's HyperGlaze).
Unfortunately, because my glaze contains rather a lot of lithium, HyperGlaze warns me that any number it calculates is going to be incorrect. This means that while I can use "55.5" for my own purposes, as long as I'm mixing glazes with lots of lithium in them and calculating them with HyperGlaze, it isn't a real number for the real world.
Fortunately, I do make a lot of high-lithium glazes, and I use HyperGlaze all the time, so I should be in good shape. (I will note here that Parmelee and Harman, in Ceramic Glazes, report that inclusion of lithium in glazes for porcelain tends to enhance both resistance to mechanical wear and resistance to chemical attack. This doesn't necessarily mean that lithium never leaches out of fired glazes, but at least it points in a happy direction. (I'm looking into the issue of leaching and the safety of lithium glazes. As far as I'm currently aware, only people who are already taking a nearly toxic level of lithium carbonate as medication have anything to worry about.) As I say, the main problem with lithium is that its effect on Coefficient of Thermal Expansion, or CTE, is nonlinear with concentration, which makes accurate calculation of the CTE of lithium glazes a nightmare that no current glaze-calculation software can surmount.)
In addition, the stuff itself sometimes behaves oddly. Ron Roy reports having seen a lithium glaze that crazed and shivered on the same piece, and I believe I've also seen that happen. On the other hand, my current glazes seem pretty well behaved.
Back to the general issue of thermal expansion, though:
I mixed a clear glaze that seems to fit two of the usual commercial porcelains that are available in the Seattle-Tacoma area: Miki Willis, and Kenzan. That glaze, according to HyperGlaze, has a CTE of just under 64 parts in ten million per celsius degree. HyperGlaze doesn't actually tell me what temperature range that covers, but I'm going to assume that it's somewhere around room temperature.
It took me a few days to realize that this was an important and empowering piece of information; but when I did, I promptly recalculated my rutile blue glaze.
Then I did a stupid thing: I built a likely version and sprayed it onto a nice big teamug as a test. This is an example of hubris, and in fact I got the glaze a bit thinner than I actually wanted to, so the mug doesn't look as nice as it might. But it did work. Mostly. The glaze is a bit grayer than I wanted, so I went back and wrote another version, which I adjusted so it would be a bit darker and a bit more purple.
The new glazes do indeed seem to fit both Miki Willis and Kenzan, and the colors they appear to make are ones I like. (The tile on the left in the photo bears the same glaze that's on the teamug; the tile on the right has one that I configured to be slightly darker and more purple; it seems to have worked. They are slightly sandy-looking here because they are slightly underfired. I actually like that appearance.)
Inasmuch as I have been messing around with this glaze family since late 1996, and have never previously succeeded in making a version that was a clean match to any of the bodies I use, I regard the current results as a modest triumph. They don't quite match the richness of color of the glaze that they are an echo of (I'll put up a picture of that glaze as well, at some point), but I've just written a version that may come closer than these do, and these are very happy glazes in their own right.
Of course, it remains to be seen whether I can
get them to work when I build my own kiln. But
that's another story, for another time. I gotta
get back to packing, else I'm not going to get
to build a kiln any time soon.
Pseudo-mailto: jon [at] bazilians [put it here] org
Last modified: Mon Jan 29 20:08:29 PST 2001
