A down side of high feldspar glazes: Crazing!

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.

Related Pictures

A high feldspar glaze is settling, running and crazing. What to do?

The original cone 6 recipe, WCB, fires to a beautiful brilliant deep blue green (shown in column 2 of this Insight-live screen-shot). But it is crazing and settling badly in the bucket. The crazing is because of high KNaO (potassium and sodium from the high feldspar). The settling is because there is almost no clay in the recipe. Adjustment 1 (column 3 in the picture) eliminates the feldspar and sources Al₂O₃ from kaolin and KNaO from Frit 3110 (preserving the glaze's chemistry). To make that happen the amounts of other materials had to be juggled. But the fired test revealed that this one, although very similar, is melting more (because the frit releases its oxides more readily than feldspar). Adjustment 2 (column 4) proposes a 10-part silica addition. SiO₂ is the glass former, the more a glaze will accept without losing the intended visual character, the better. The result is less running and more durability and resistance to leaching.

Cone 10 mug is crazing after a year. That's OK because it's high-fire, right?

Wrong. This cone 10R grey stoneware, H550, is OK if the glaze fits. But this high feldspar glaze on the inside has begun to craze after a year. The greyer coloration around the craze lines indicates that water is soaking into the slightly porous body. And, this mug has lost the ring it had out of the kiln. Imagine your customers returning these pieces! Could they be refired to be as good as new? Perhaps. But they would return to this condition. The practical solution is to reformulate this glaze to reduce its thermal expansion.

ChatGPT is completely wrong about the cause of glaze crazing!

A ChatGPT answer to a question about crazing

Today, ChatGPT is parroting common wrong suggestions about the cause and solution of the serious issue of crazing. Yet it trained on thousands of internet pages about the subject! Crazed functional ware is defective, and customers will return it. So fixing the problem is serious business, we need correct answers. Consider ChatGPT's suggestions: #1 is wrong. There is no such thing as an "incompatible mix" of ceramic materials. Crazing is an incompatibility in thermal expansions of glaze and body, almost always a result of excessive levels of high-expansion K₂O and Na₂O in the chemistry of the glaze. The solution is reducing them in favor of other fluxes (the amount per the degree of COE mismatch). #2 is wrong, firing changes don't fix the incompatibility of thermal expansions. #3 is wrong, refiring makes the crazing go away but not the stress of the mismatch, it will for sure return. #4 is completely wrong. Firing higher takes more quartz grains into solution in the melt and should reduce the COE (and mature the body more which often improves fit). And melt fluidity has nothing to do with crazing. Furthermore, if a glaze does not run off the ware, it is not overfired.

Links

Glossary	Celadon Glaze A type of stoneware glaze normally fired in a high temperature reduction atmosphere kiln. It is transparent and stained green or blue by the presence of iron oxide.
Glossary	Calculated Thermal Expansion The thermal expansion of a glaze can be predicted (relatively) and adjusted using simple glaze chemistry. Body expansion cannot be calculated.
Glossary	Limit Recipe This term refers to sanity-checking ceramic glaze recipes by noting whether materials present or their relative percentages fall outside typical norms for functional ware.
Oxides	Na2O - Sodium Oxide, Soda
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.
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.

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