It is axiomatic among potters that a glaze applied to unfired clay must contain an appreciable amount of clay itself. It is proverbial that the glaze should be applied before the piece has dried.
In a test-in-progress I have here two free-standing test tiles (cut from a cylinder form) which have been glazed, while bone dry, with a glaze containing no clay whatsoever. The glaze surface has dried with no cracking and the tiles are very much intact.
To expand on Britt's explanation from my own experience, one of the keys to addressing once-fired glazing is to begin, not with the nature of the glaze, but with the nature of the clay body in use.
If the clay is composed primarily of fireclay and is relatively non-plastic in working properties, then it is likely very fragile when bone dry and is most safely glazed when leather-hard -- the standard practice. However, if the clay runs high on ball clay and is quite plastic, it tends to have a much higher dry strength. This type of clay may well respond better to bone-dry glazing.
So for once-firing the first question to address is the nature of the clay. The second is to match the glaze to the selected glazing sequence.
The requirements for the glaze recipe itself, therefore, flow naturally from the clay in use. The necessary clay content in the glaze will depend on whether one is applying it at the leather-hard stage or after the piece has fully dried. In this case, the key issue is that of drying shrinkage. High clay content in the glaze (assuming one is using a slop glaze) makes for significant shrinkage as the glaze dries. This is desirable when the glaze is being applied to a piece that still has considerable moisture in it as it will allow both clay and glaze to shrink together. The important thing is to remember that the shrinkage rate of clay and glaze must be compatible -- whatever that rate may be.
Glazing a bone-dry piece, despite the fact it inevitably absorbs moisture and expands during the process, requires a glaze shrinkage closer to that normal for bisqued ware. Glazing a leather-hard piece will require considerably greater shrinkage in the glaze to avoid cracking during drying.
As I make not only my own glaze recipes, but also my own clay recipes, it has been very helpful to understand these details. My stoneware clay is highly plastic, with around 11% OM4 ball clay. It has phenomenal dry strength, of the sort that almost allows one to drop it on the floor without damage! Strength at leather-hard stage, however, is not high at all. For these reasons it makes much better sense to glaze it when fully dried, and I have been using a simple Tenmoku glaze (about 7 % kaolin) that would probably work well on bisque also.
The test tiles shown above are a more extreme test, revisiting my favorite white glaze. This is a high-feldspar type, containing plenty of Custer Feldspar as well as spodumene (to minimize crazing). At .52 by formula, it is already very high in alumina from the feldspars alone, and there is no reason to add any clay! I had always bisque fired before using it, but decided this time to test it on raw clay. As you can see, there was no problem applying it to my bone dry stoneware; no cracking has developed at all.
These tiles are part of a larger test to check behavior of my glazes in a slow cooling cycle following firing. I must go glaze the rest of the test pieces now...
|This pot is glazed in my white feldspathic glaze using a normal cooling cycle.|