Having Trouble with Crawling Glazes? Here’s a Way to Fix Them! Sue McLeod

The Solution to Crawling Glazes

The solution to crawling glazes is to replace some of the clay with calcined clay.

Kaolin — Al₂O₃·2SiO₂·2H₂O —> Al₂O₃·2SiO₂ — Calcined Kaolin

This pre-shrinking of the clay reduces the shrinkage of the glaze as it dries and as it’s fired. Say you have a recipe with over 20% EPK and your glaze is cracking/crawling. You can replace some of the raw EPK with calcined EPK. You don’t want to replace all of it. We require un-calcined clay in our glazes because clay is what keeps all the other glaze materials suspended in water. Once clay is calcined, it loses its suspending properties. I try to keep my raw kaolin percentage in a glaze recipe between 10% and 15% and then calcine the rest.

How to Calcine Your Own Kaolin

Kaolin can be purchased pre-calcined and is sometimes sold under the name Glomax. Or, you can calcine your own. It is much cheaper and very easy to calcine your own. You can calcine your own EPK by putting some through a bisque firing. Fill a bisque-fired bowl or lidded vessel (to reduce dust) with EPK and put it in your next bisque firing.* When it comes out, all the chemically bound water will be driven off. The fired kaolin will be very light and fluffy, use caution when using. Wear a respirator when you take it out of the kiln and immediately put it into a sealed container. Label it as Calcined EPK and use it for substituting EPK in recipes.

Using UMF to Substitute EPK for Calcined EPK

When replacing EPK with calcined EPK in a glaze recipe, we can’t do a 1:1 substitution, gram for gram. This is because EPK loses some of its mass (H2O) during the firing. We have to account for the chemically bound water that was driven off (called loss on ignition, or LOI).

Let’s see what happens chemically if we do a straight trade, grams for grams. I created this glaze on the site Glazy.org with 25% EPK. Note the analysis, specifically the SiO₂ and Al₂O₃ levels.

Now see what happens if we subtract 10% EPK and add 10% calcined kaolin.

The SiO₂ and Al₂O₃ levels go up because when we remove the H₂O, we have a higher concentration of everything else (SiO₂ and Al₂O₃). We have to reduce the number of grams of calcined EPK to get the equivalent chemistry as with the raw EPK. By reducing the calcined EPK to 8% instead of 10%, my analysis now matches the original.

Note: Glazy.org uses a generic chemical analysis for calcined kaolin, rather than the analysis for actual calcined EPK. Actual EPK has a LOI (loss on ignition) of 14.81% so it would be more accurate to use 85.19% calcined EPK to replace uncalcined EPK, or in this example, replacing 10% EPK with about 8.5% calcined EPK.

Conclusion

So, in the end, we removed 10% EPK from the total recipe and replaced it with 8.5% calcined EPK. This can also be worded as, for every 10 grams of EPK that you remove from a recipe you can replace it with 8.5 grams of calcined EPK. The resulting recipe will have the same chemistry as the original, but will behave better before and during the firing.

*You do not have to reach bisque temperatures to calcine EPK. As long as it is fired above 1000ºF (537.8°C), it will be calcined. The reason for firing it to bisque temperatures is because that’s the most convenient method if you don’t have a small test kiln.

the author Sue McLeod has been a studio potter since 2010 and worked as the ceramics studio technician at a community studio in Victoria, British Columbia, Canada, for six years. She now works full time, researching glazes in her home studio and teaching three online glaze courses: Glaze Mixing Essentials, Mastering Glaze Consistency, and The Art of Glaze Chemistry. She also runs a large, free, glaze-support community on Facebook titled “Understanding Glazes with Sue.” Read more ceramics articles and register for her online glaze courses at suemcleodceramics.com.