My attitude toward glazes is typically sappy. (What can I say? I'm a sappy Hungarian with ADHD; it comes with the territory.) Rocks and ashes and clay are chrysalises, from which potters must coax their eventual butterflies. Here is a Basho poem ('scuse me: the final "o" in that name and the final "o" in the poem should both be long -- "o" with a bar over it), translated by Lafcadio Hearn in his book Kwaidan. If I knew how, I'd give the actual Japanese along with the transcription into Roman characters.
(Wake up! wake up! -- I will make thee my comrade, thou sleeping butterfly!)
Ahem. Enough sap for now. Back to harsh reality.
The latest glaze test (probably around August third) came out of the kiln very thin, very dry, and somewhat harsh. At this stage, I'm entirely at ease with that: I was looking for something with very fine grain that doesn't crawl, and that is exactly what I got. In fact, if it were slightly shinier, it would be a very pleasant substitute for a high-temperature version of terra sigillata. I may play with that later, but right now it isn't what I'm going after.
Another reason why I'm not displeased is that I already have a version that doesn't crawl and is not quite dry enough. All other things being equal (famous last words), I should be able to combine them to get what I want, right?
(Here we will have a moment of respectful silence for all those who have torn out their hair from trying things that would have been easy if all other things actually had been equal. In general, there's no way in hell that all other things could even remotely conceivably be equal.)
Well, truthfully, I am half-seriously hoping that this will just quietly work out. I've set another crucible of sieved kaolin out to be bisqued (remember, this sorta pre-shrinks it to help prevent crawling), and I've come up with four recipes, any of which could be satisfactory. It was two recipes, but then I remembered that the original satin matte formulation contained petalite, where the recent ones have used feldspar, so I have two with feldspar and two with both.
That original glaze was about as soft as a baby's butt, so I called it "BabyFace". (Hey, whaddya want? I should call such a sweet glaze "BabyButt"? Hrumpfh. Don't think I didn't consider it; but even I have my limits.)
(Note added later) -- One reason why the glaze was dry and
harsh was the body I put it on. This is yet another reason
why potters tear their hair out. Hamada was probably right:
it's a good idea to use your local materials to make what
you can, and to move to a place with good local materials
if you need to. In this day and age that's not fully viable
for someone like me, and I will almost certainly never
attain a full expression of even an American version of the
folk tradition that Hamada came out of, much as I might
like to. (Mind you, that tradition was partly observable,
and partly a concoction created by Yanagi, Hamada, Leach,
and a few others... but that's neither here nor there.)
I keep thinking about doing parodies of the trademarks of famous potteries. I'll try to put drawings or scans of some originals and my little Bazilian parody versions up here at some point. Some of them are easy. Instead of the "RP" logo of Rookwood, for example, with its tall reversed "R" and the thirteen flames pointing outward, a reversed "B" and thirteen flames pointing inward. Perhaps a dead crow under the logo instead of a Roman numeral, just to be absolutely sure that nobody thinks it's a fake intended to fool people. I say that because there are people making such fakes, more's the pity.
Actually, this satin matte glaze was inspired by the glazes on some of the American Art Pottery pieces I've encountered. It was the first glaze I tried to make, as I think I've mentioned here, and I'm still working on it; these things aren't easy or obvious. It is no surprise to me that the finish on some makes of pottery was perhaps the biggest selling point.
If you want to see some other glazes that I think are particularly excellent, have a look at some of the Web pages of other potters out there: people like Lasse Östmann, for example, or Richard and Carol Selfridge, or Wang Chun Wen. There are lots of other superb people as well, some with Web pages and some without. One thing I particularly like about Lasse Östmann is that he gives recipes for a lot of his glazes. These are not, of course, to be followed blindly, but they provide a lot of food for thought. (I'm going to give a recipe or two of my own, later on, and you'll see that these things are tailored to specific requirements.)
Here is a superb example of an ancient glaze that is extremely difficult to reproduce. There are only four such bowls known, of which this (the photo on the right in the URL) is the best. All of them are National Treasures of Japan, and are in museums there. It turns out that even though the old potters of China and Japan had superb control over kiln atmospheres, there were still lots of interesting accidents (and lots of failures, too, as the mountains of shards and wasters attest). I would love to work up a glaze that did this sort of thing from time to time. (No, I don't expect to make any National Treasures like this tea bowl, but it would certainly be fun to make something that was indisputably special.)
But I was talking, before I interrupted myself, about the search for a nice satin matte glaze, which...
It is August 11th. I calcined more kaolin and reformulated
the glaze; last Saturday I put two glaze tests and a small
teacup into the kiln, and they emerged this morning. I'm
intrigued by the results, and will explain after I give
the formulation of the glaze as it now stands:
| G-200 Feldspar | 26 |
| Petalite | 10 |
| Wollastonite | 30 |
| Calcined Kaolin | 20 |
| Raw Kaolin | 10 |
| Alumina Hydrate | 4 |
(Thanks to Kathy Li for that table, btw: I complained about the fact that tabs didn't work, so she reminded me, by sending this one as an example, that tables do.)
Ahem. More details: I milled it, as a moderately thick slip (about right for dipping bisqueware in), for three and a half hours, and then passed it through a 120-mesh sieve. (It was a bit too thick to pass really easily, but it did go through. I'm seriously thinking about milling the next batch for 6 hours.)
On a regular commercial throwing porcelain (MiKi Willis), it is too dry, and I'm thinking about modifications to it. On my own translucent porcelain mix, however, it is like silk. If I can, I'll take a stereo photo of a sample, in decent light, and put it up here so you can at least see it. Seeing it, let me assure you, is not quite the same thing as drinking from a teacup that is covered with it, but I can't give you that here.
(Added, 20 August)
I have now tried, on a MiKi Willis test tile, a version of this glaze with about 2% cobalt carbonate (and an extra 2% calcined kaolin). It was a bit too thin, and very dry; but where I double-dipped the tile, it looks rather like some of the American Art Pottery matte glazes, particularly the smoother greens from places like Grueby, except that it is a rich blue. I am both pleased (that's a look I want to be able to produce) and annoyed (that's not the look I was trying to produce, and it's not the look I want right now). I may try adding some alumina hydrate to a second test, and I may try a test on a tile made of my translucent porcelain. ...And if anyone is fool enough to ask, I'll try to put a photo of the first test up here.
(Added, 8 September)
I did also try adding cobalt aluminate to the standard version (see recipe, above), at 2% and 5%, and putting that on my translucent porcelain. While it is somewhat nicer than the MiKi Willis test from August 20th, it still (ahem) sucks rocks, IMHO. So much for my little conjecture that perhaps the cobalt was sucking alumina out of the mix. Well, perhaps I overstate; maybe it was, and maybe the aluminate doesn't do that but does flux the glaze to some extent. I don't know.
In fact, at this point I do not understand how cobalt
ruins the surface texture so thoroughly, and I'm thinking
about it. I may try additions of various things to
determine the truth of this matter by trial & error.
As I've mentioned, one of the projects I've been pursuing involves glazes that fluoresce under UV light. So far, I have good results with Dysprosium, Terbium, and Thulium, and I'm trying to acquire samples of Europium and Samarium. (Colors: Dy gives me something that is mostly apricot, with a bit of salmon in it. Tb is a yellowish green, actually rather bright. Tm is a lovely deep indigo. I expect a slightly orangy red from Eu, and I'm not sure what Sm will do.)
I'm adding the oxides of these elements to a clear
base glaze, which has been misbehaving in fairly
predictable ways. For one thing, I haven't been
using my original translucent body for the test tiles,
so I have been obliged to mix the glaze to have a
different coefficient of thermal expansion. This has
been (how can I put this delicately?) less than
entirely successful. On the other hand, it continues
to help me learn about formulating glazes. My current
clear, which is so new that I haven't even tested it
yet, is about as follows:
| Petalite | 65 |
| G-200 Feldspar | 21 |
| Wollastonite | 8 |
| French Chalk | 2 |
| Kaolin | 4 |
This glaze has a calculated (Richard Burkett's HyperGlaze program) Coefficient of Thermal Expansion of about 62 (that's parts in 10 million per degree celsius), and a silica to alumina ratio of 7.6:1. Because it contains lithium, however, the calculated CTE is highly suspect. Just as a comparison, my original clear, which is suitable only for my original translucent whiteware body, has a calculated CTE of just over 55 and three quarters, and is 7.9:1. It is quite amazing, the change in CTE that you get when you tweak the body composition. My original body is 40% kaolin, 40% feldspar, and 20% silica, with an extra 1% of magnesium carbonate added to enhance the translucency. The newer body is a bit softer: 35% kaolin, 40% feldspar, and 25% silica, again with the added 1% of magnesium carbonate.
...But enough technoid stuff, at least for now.
If you really want to see more disgusting detail
here, send me email and ask for it.
Pseudo-mailto: jon [at] bazilians [put it here] org
Last modified: Thu Nov 15 00:01:41 PST 2001