ABOUT CLAY AND GLAZE FIT
By Martin Butt of Coyote Glazes
Not all glazes will fit well on all clay bodies (nor do all clay bodies fit well with all glazes), so it is always a good idea to test a glaze on the clay you are using to insure they are compatible. Most of the Coyote glazes have fairly average expansion rates, and are likely to fit most average clay bodies. We have one series of low expansion glazes that can shiver if used on a high expansion clay body, so care should be taken to make sure these glazes fit the clay you are using. They are: Archie's Base, Blue Purple, Eggplant, Gun Metal Green, Ice Blue, Opal, Red Gold and Rhubarb. We have had reports of this series shivering on the following clay bodies: Standard #112, #201 and #245: Highwater Loafer's Glory and Brownstone: Axner's Mike's Stoneware: New Mexico Clay's WH8: Alligator Clay's MC360 Lovestone high fire: Continental's Mid-Range White and Mid-Range Oxidation Body. If you know of any others please let us know. Also if you would tell us the bodies you use that work well with these glazes we can start a list of those too. If the clay you are using doesn't fit with these glazes but you want to keep using them (they are beautiful), the only solution is to try a different clay body. These glazes work fine with most clays, only bodies with unusually high expansion cause a problem.
More about Clay and Glaze Compatibility
Each clay and glaze have their own expansion (and contraction) rate, and if they are too different problems can result. At about 1000 degrees F. the glaze solidifies, and the clay and the glaze undergo contraction side by side.
The most common fit problem is crazing, caused by the glaze contracting more than the clay body on cooling. This means the glaze is stretched over the clay, resulting in a crazing or crackle pattern. Although there is some evidence that crazed glazes may result in a weaker finished pot, the main concern is aesthetic, and many people just ignore it.
Of much more concern is the opposite problem of shivering. In this case the glaze contracts less on cooling than the clay body, putting the glaze under compression. Some compression can be a good thing, resulting in a stronger pot, but too much can cause the glaze to flake off the pot (shivering). In extreme cases this condition can cause the pot to break (shattering). It is important to realize that this might not happen for days or even weeks after work comes out of the kiln, so do your testing early.
Expansion/contraction is often confused with firing shrinkage, which is irreversible. Expansion is temperature dependant and occurs with each heating and cooling. Shrinkage doesn't tell you anything about the expansion rate. Some high shrinkage clays have a low expansion rate and vice versa.
Brant says..... Exactly! Don't use that clay with those glazes or Laguna's beautiful crackle glazes. And while you are at it don't use WH8 for crystal glazes as it inhibits the crystal growth. And don't use a Geil Brand kiln on high iron bodies, just asking for trouble.