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.

Related Pictures

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.

High feldspar glazes craze. Don't put up with it.

This glaze, "Bamboo Cone 10%", contains 50% potash feldspar so or certainly qualified as a high feldspar glaze. K₂O and Na₂O are this over supplied. They have the highest thermal expansions of all oxides, by far. These are needed and valuable - but when grossly over supplied 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 water tight. 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. We recommend our G2571A base (below right) - the Zircopax, rutile and iron oxide in the original can be added to it instead.

Glazes of the same chemistry: The fritted one melts better

It seems logical (and convenient) to just say that the kiln does not care what materials source the oxides in a glaze melt. Li₂O, CaO, Al₂O₃, SiO₂ are oxides (there are about ten common ones). The kiln just melts everything and constructs the glaze from the ones available. Right? Wrong! Things get more complicated when frits are introduced. Frits are man-made glasses, they melt much more readily than raw materials like feldspar. Raw materials are often crystalline. Crystals put up a fuss when asked to melt, often holding on as long as they can and then suddenly melting. Frits soften over a range and they start melting early. To illustrate: These two glazes have the same chemistry. But the one on the left sources sodium and alumina (Na₂O₃, Al₂O₃) from the 48% feldspar present. The other sources these from a frit (only 30% is needed for the same amount of Na₂O₃). The remainder of the recipe has been juggled to match the other oxides. The frit version is crystallizing on cooling (further testament to how fluid the melt is). What has happened here is great. Why? First, the chemistry has not changed (fewer firing differences). The frit has no Al₂O₃, it is being sourced from kaolin instead, now the slurry does not settle like a rock. Even better, silica can be added until the melt flow matches (might be up to 20%). That will drop the thermal expansion and reduce crazing. The added SiO₂ will add resistance leaching and add durability. Frits are great! But you need to know how to incorporate them into a recipe using a little glaze chemistry.

Click here for case-studies of Insight-Live fixing problems

You will see examples of replacing unavailable materials (especially frits), fixing various issues (e.g. running, crazing, settling), making them melt more, adjusting matteness, etc. Insight-Live has an extensive help system (the round blue icon on the left) that also deals with fixing real-world problems and understanding glazes and clay bodies.

Links

Materials	Ferro Frit 3110 High sodium, high thermal expansion low boron frit. A super-feldspar in clay bodies. Melts a very low temperatures.
Oxides	SiO2 - Silicon Dioxide, Silica
Oxides	KNaO - Potassium/Sodium Oxides
Glossary	Leaching Ceramic glazes can leach heavy metals into food and drink. This subject is not complex, there are many things anyone can do to deal with this issue
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.
Glossary	Glaze Chemistry Glaze chemistry is the study of how the oxide chemistry of glazes relate to the way they fire. It accounts for color, surface, hardness, texture, melting temperature, thermal expansion, etc.
Glossary	Frit Frits are used in ceramic glazes for a wide range of reasons. They are man-made glass powders of controlled chemistry with many advantages over raw materials.
Troubles	Runny Ceramic Glazes Glazes of high melt fluidity are likely to run if applied to thickly or have not catcher glaze
Typecodes	Glaze Chemistry Case studies where glaze chemistry was used to solve a problem.
Articles	Where do I start in understanding glazes? Break your addiction to online recipes that don't work or bottled expensive glazes that you could DIY. Learn why glazes fire as they do. Why each material is used. How to create perfect dipping and brushing properties. Even some chemistry.

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