Dave Finkelnburg on tue 28 dec 04
Laurie,
Please do not take the following comments as criticism. You have asked a really valid question. The answer, though, isn't so simple.
Crazing, as most potters realize, is just a mismatch between clay and glaze. The glaze shrinks more than the clay and crazes, just like when my pants are too tight and the seam rips out in the...well you know...the back... However, you can't say that glaze recipe A will always have exactly X amount of fit or lack of fit on clay recipe B. Firing has to be considered, and your particular glaze ingredients.
In oversimplified terms, at high fire porcelains are held together by a glass made from melting together ingredients from silica and clay and fluxes and feldspar. The higher the firing temperature the more silica from the body goes into the glass. In general, silica by itself has a higher coefficient of shrinkage than the glass, but once melted into the glass, the silica lowers the shrinkage of the glass. So, the higher you fire your clay, the lower expansion glaze you will need. How, exactly, does this work for stoneware. I'm not sure, yet.
The very best way to get a glaze to fit your clay body as you fire it is to take a recipe you like and adjust it to fit. Any decent glaze calculation program (there are at least a dozen on the market) can get you started, but you still need to fire test tiles because the clay body and how you glaze (whether you raw glaze, glaze thickness, etc) affect the glaze.
If one could look over Clayart posts over the past decade, questions asking for a glaze that fits a specific clay body are routine. The helpful answers are a good start, but may not give you exactly what you seek for the reasons above. You still need to allow for your materials and methods.
I am writing this partly because I have come to realize very recently how much I have underestimated the importance of good glaze fit. I also know from experience that just because a recipe fits a particular clay in someone else's studio and kiln doesn't guarantee it will perform exactly the same in mine.
Good potting and Happy New Year!
Dave Finkelnburg
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