How to Convert Kaolin to Calcined Kaolin
(For the purpose of this article, I’m going to use the words clay, kaolin and EPK[1] synonymously.)
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The Issue
If you have too much clay in a glaze recipe, you might have issues with your glaze crawling during the firing. Crawling is where the glaze pulls away from the clay body due to a combination of shrinkage, poor adhesion and high surface tension.
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.
Examples of glaze cracking as it dries due to shrinkage. This can happen with high percentages of clay in a recipe and also from thick glaze application.
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 bisque dry and glaze again with a thinner application. Check your glaze’s specific gravity before adding water. It may need to be deflocculated.
When we fire these cracked glazes, the glaze continues to shrink. 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 usually happens with over 20% clay in a glaze recipe, common with matte glazes.
Examples of crawling glazes, from minor to severe.
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 kiln wash 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. Calcining is where the kaolin is fired to a temperature hot enough to remove the chemically bound water molecules from the clay particles
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. The current Greenbarn price for a 50 lb bag of Calcined Kaolin is $106. Regular EPK is $35 for the same amount.
You can calcine your own EPK by putting some through a bisque firing[2].
Just 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 kaolin will be very light and fluffy, use caution. 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 LOI – loss on ignition).
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.
25% EPK: 2.34 SiO2, 0.54 Al2O3
15% EPK, 10% Calcined: 2.40 SiO2, 0.56 Al2O3
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.
Edit: 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
15% EPK, 8% Calcined: 2.34 SiO2, 0.54 Al2O3
I tend to leave it with the new reduced total as a reminder to myself that I made a change to it.
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 10g of EPK that you remove from a recipe you can replace it with 8.5g of Calcined EPK. The resulting recipe will have the same chemistry as the original one, but will behave better before and during the firing.
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Very well done!
Thank you!
Thanks ? this was great
If you’re calcining why leave any EPK? Why not sub out all of the EPK with 80% calcined EPK?
Hi John,
The reason we don’t calcine all of the EPK is because we need some regular, un-calcined clay in our glazes to keep all the other materials in suspension. Once clay is calcined, it loses its suspending properties and will sink and settle like a rock in the bottom of the bucket. It’s the clay that keeps everything floating freely in the water. I like to keep my regular EPK between 10-15% of the recipe. That is enough to keep the glaze suspended. If you have little to no clay in your glaze recipe, you can add 1-3% bentonite, which is a very good suspending agent. A little goes a long way.
Hi, is this calcined epk the same as mollochite 200? Or is that one calcined at a higher temperature?
Hi Carmen,
Molochite is fired to a much higher temp than regular calcined kaolin. I’ve never worked with it, but I’d guess it would stay unmelted in a glaze due to all the chemical conversions that have already taken place. A very good question! I’d be curious to know if anyone has tried adding it to a glaze.
Sue
Hi, please which reagent is use in calcination of kaolin?
I just put the kaolin in the bisque firing. Nothing is added to it.
Thank you!!! I’m a free-form sculptor (beginner) who uses a cement-based medium and one of the key ingredients in the recipe is metakaolin (aka calcined kaolin). It is a pozzolan which makes the resulting concrete much stronger and clay-like in its ability to hold fine detail, even vertically. The problem is I can’t find a supplier of metakaolin in New Zealand but we have plenty of kaolin. Thanks to your great instructions I now have hope I’ll be able to sculpt the pieces I want.
Do you have an online course that explains how to calculate the values and percentages of each component (Al2O3, B2O3, SiO2, and others) that you mention in the table?
or unit formula ?
Hi Maria,
Yes, I do teach those subjects and I’m working on an online course to be ready this spring. Make sure you’re on my email list and you’ll be notified as soon as it’s ready!
Sue
Beautifully said.
——-
When I was young and naive, I was mixing up glaze formulas from books I had. One of the recipes called for “calcined borax”.
I had just loaded a perfect, tightly packed bisque kiln. Everything in the kiln was boxed, rim to rim, and foot to foot. There wasn’t a single shelf in the kiln.
Except, there was room for one more pot in the very center of the kiln right on top. So… I took a serving bowl that just barely fit, loaded it to the brim with borax to calcine it in the firing.
I stood back to admire the beauty of that tightly packed bisque load. It was a thing of beauty.
After firing the kiln, the next day, I cracked the kiln a bit to expedite the cooling.
GASP!!!
The borax had bubbled rather violently at bisque temperature. Enough so as to split the serving bowl in half. This allowed all that borax to cascade down the pots in the center of the kiln, fusing many of the pots in the kiln together.
Evidently borax melts below 1850 ℉. Far below.
Who knew? I could only scratch my head and think, “Oh, so THAT’S fused borax!”
GASP is right!! Thanks for sharing your story. Some things we learn the hard way. Hopefully only once 😉
Sue
Well those who forge weld iron knows that Borax liquefies at about 1369*F and is used as a flux
I was wondering if any other clays (like ball clay) are ever calcined and at what temperature, if so? Or in the case of recipes that are largely clay – slip glazes, would one do something like this? Appreciate your detailed tests in these videos.
Hi Autumn,
Yes, Ball clay and other clays can also be calcined. I don’t know the exact temperature they need to reach, I just always put them in my bisque firing and that does the job. In high clay slips and glazes, I would replace some of the clay with calcined clay if I was having issues with the slip cracking, or the glaze shrinking too much. I would never replace all the clay, just enough to fix the problem I’m trying to solve.
Sue
Hi Sue, thanks for this very informative article,
What is the difference between chemically bound water and regular water content in the clay.
Thank you!!!
Hi sue
You place your EPK in a pre-bisqued pot and then into the kiln. Can you place it in an unbisqued pot in a bisque load? That is, can you bisque the pot and the EPK at the same time?
Thank you!
Hi Linda,
I’ve never done it this way but I don’t see why not. Just make sure the bowl is completely dry. I generally don’t use a bowl that I’m going to eventually glaze as I’m not sure how residual powder could affect the glazing process.
Hi Sue, and thank you for this quick tutorial. I bisque to ^04, is this temperature going to be too high for calcining?
No, cone 04 will be fine.
Sue