These were applied to the bisque as a slurry (suspended by gelling with Epsom salts). They were fired at cone 10R. The left one is a Grolleg porcelain (P700) with Custer feldspar. On the right is whiteware (P600) with Covia Nepheline Syenite. Feldspars and nepheline syenite are high in K2O and Na2O. These oxides have by far the highest thermal expansions (COE), so when used pure like this, they will always craze. Glaze recipes having a high percentage of either are in danger of crazing also.

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This reduction celadon is crazing. Why? High feldspar. Feldspar supplies the oxides K2O and Na2O, they contribute the brilliant gloss and great color but the price is very high thermal expansion. Scores of recipes being traded online are high-feldspar, some more than 50%! There are ways to tolerate the high expansion of KNaO, but the vast majority are crazing on all but high quartz bodies. Crazing is a plague for potters. Ware strength suffers dramatically, pieces leak, the glaze can harbor bacteria and customers return pieces. The simplest fix is to transplant the color and opacity mechanism into a better transparent, one that fits your ware (in this glaze, for example, the mechanism is simply an iron addition). Fixing the recipe may also be practical. A 2:1 mix of silica:kaolin has the same Si:Al ratio as most glossy glazes, this glaze could possibly tolerate 10% of that. That would reduce running, improve fit and increase durability. Failing that, the next step is to substitute some of the high-expansion KNaO, the flux, for the low-expansion MgO, that requires doing some glaze chemistry.

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This glaze, "Bamboo Cone 10%", contains 50% potash feldspar, so it certainly qualifies as a high-feldspar glaze. K2O and Na2O are thus oversupplied. They have the highest thermal expansions of all oxides, by far. These are needed and valuable, but when grossly oversupplied the result is crazing. This glaze used to work on this body, H550. The previous version of H550 was firing near the bloating point of the body, about 1% porosity, so the recipe had to be changed to provide more margin for error. The new recipe has a more practical 2.0-2.5% porosity; it has no danger of bloating or warping and still has excellent maturity and strength. This glaze was crazing before, and pieces did not leak because the body was dense enough - so they were still watertight. But now it does not work. The solution is to do something that should have been done before: Use a silky matte base recipe that does not craze. I recommend the G2571A base (below right) - the Zircopax, rutile, and iron oxide in the original can be added to it instead.
| Materials |
Feldspar
In ceramics, feldspars are used in glazes and clay bodies. They vitrify stonewares and porcelains. They supply KNaO flux to glazes to help them melt. |
| Materials |
Nepheline Syenite
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| Troubles |
Glaze Crazing
Ask the right questions to analyse the real cause of glaze crazing. Do not just treat the symptoms, the real cause is thermal expansion mismatch with the body. |
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