Solving a glaze fault can be not only frustrating and time consuming, but also very rewarding when you discover the answer. Here is one fairly simple solution to a common glaze fault.
Defining the Terms
Crawling: When a glaze pulls away from the clay body due to a combination of shrinkage, poor adhesion, and high surface tension.
Calcining: Heating a raw material to a temperature hot enough to remove the chemically bound water molecules from the clay particles.
Edgar Plastic Kaolin (EPK): A common raw clay material used in clay body and glaze recipes.
Loss on Ignition (LOI): The amount of weight the material loses as it decomposes to release water vapor and various gases during firing.
Unity Molecular Formula (UMF): Also known as a Seger Formula, this is a method used in ceramics to represent glaze components in terms of the number of molecules. It’s a glaze recipe format that normalizes the total flux oxides to unity (1.0) instead of using weights of raw materials. The formula shows the ratio between fluxes, as well as the ratio of combined fluxes to the silica and alumina.
Correcting a Glaze Fault
If you have too much clay in a glaze recipe, you might have issues with your glaze crawling during the firing. Because clay shrinks as it dries, a high percentage of clay in your glaze recipe will cause the glaze to shrink. If it shrinks too much, it will crack. This loosens the glaze from the surface of the clay body. If the cracking is severe, bits of the glaze will fall off the pot before the firing. If you notice your glaze cracking at all, your best bet is to rinse it off, let your bisqueware dry, and glaze again with a thinner application. Check your glaze’s specific gravity before adding water. If your specific gravity is already below 1.5, then your glaze probably doesn’t need more water and may need to be deflocculated instead. Deflocculating will lower a glaze’s viscosity without adding extra water, which leads to excessive shrinkage. When we fire these cracked glazes, the glaze continues to shrink. If the surface tension of the glaze is stronger than the adhesion of the glaze to the clay body, the glaze contracts and pulls away, exposing the clay body underneath. This is called crawling and usually happens with over 20% clay in a glaze recipe, common with matte glazes. Since clay shrinks when it dries, glazes with high clay content will also shrink when they dry. If we replace some of the clay with calcined clay, we are essentially pre-shrinking some of the clay so it won’t shrink when it dries.
In addition to correcting crawling glazes, another excellent use of calcined kaolin is in your kiln wash recipe. If your kiln wash flakes off really easily, it’s probably due to the clay content in the recipe. Again, the clay shrinks as it dries and as it’s fired and cracks to relieve the stress. Replacing some or even all of the clay in your kiln wash with calcined clay will solve that issue. Experiment with different amounts to see what works best for you.
The Solution
The solution is to replace some of the clay with calcined clay.
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.
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Solving a glaze fault can be not only frustrating and time consuming, but also very rewarding when you discover the answer. Here is one fairly simple solution to a common glaze fault.
Defining the Terms
Crawling: When a glaze pulls away from the clay body due to a combination of shrinkage, poor adhesion, and high surface tension.
Calcining: Heating a raw material to a temperature hot enough to remove the chemically bound water molecules from the clay particles.
Edgar Plastic Kaolin (EPK): A common raw clay material used in clay body and glaze recipes.
Loss on Ignition (LOI): The amount of weight the material loses as it decomposes to release water vapor and various gases during firing.
Unity Molecular Formula (UMF): Also known as a Seger Formula, this is a method used in ceramics to represent glaze components in terms of the number of molecules. It’s a glaze recipe format that normalizes the total flux oxides to unity (1.0) instead of using weights of raw materials. The formula shows the ratio between fluxes, as well as the ratio of combined fluxes to the silica and alumina.
Correcting a Glaze Fault
If you have too much clay in a glaze recipe, you might have issues with your glaze crawling during the firing. Because clay shrinks as it dries, a high percentage of clay in your glaze recipe will cause the glaze to shrink. If it shrinks too much, it will crack. This loosens the glaze from the surface of the clay body. If the cracking is severe, bits of the glaze will fall off the pot before the firing. If you notice your glaze cracking at all, your best bet is to rinse it off, let your bisqueware dry, and glaze again with a thinner application. Check your glaze’s specific gravity before adding water. If your specific gravity is already below 1.5, then your glaze probably doesn’t need more water and may need to be deflocculated instead. Deflocculating will lower a glaze’s viscosity without adding extra water, which leads to excessive shrinkage. When we fire these cracked glazes, the glaze continues to shrink. If the surface tension of the glaze is stronger than the adhesion of the glaze to the clay body, the glaze contracts and pulls away, exposing the clay body underneath. This is called crawling and usually happens with over 20% clay in a glaze recipe, common with matte glazes. Since clay shrinks when it dries, glazes with high clay content will also shrink when they dry. If we replace some of the clay with calcined clay, we are essentially pre-shrinking some of the clay so it won’t shrink when it dries.
In addition to correcting crawling glazes, another excellent use of calcined kaolin is in your kiln wash recipe. If your kiln wash flakes off really easily, it’s probably due to the clay content in the recipe. Again, the clay shrinks as it dries and as it’s fired and cracks to relieve the stress. Replacing some or even all of the clay in your kiln wash with calcined clay will solve that issue. Experiment with different amounts to see what works best for you.
The Solution
The solution 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.
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