|Monthly Tech-Tip |
From these SHAB test bars EP kaolin appears to have a much higher fired shrinkage. But half of that happened during drying. Still, EPK shrinks 4% more during firing. Yet Grolleg produces more vitrified porcelains. The EPK bar also appears to be whiter. Yet in a porcelain body, Grolleg fires whiter. That higher drying shrinkage proves that EPK is much more plastic, right? Not really. Throwing porcelains containing either require plasticity augmentation using similar percentages of bentonite. What do we learn? To compare materials like this we need to see them "playing on the team", in a recipe working with other materials. Don't rely on material data sheets, do the testing in your recipes.
In ceramics, EPK, or EP Kaolin, is used in clay bodies and glazes. Top right: Pure EPK is fairly plastic on wedging. But during the throwing process, it splits at the rim like this. The forming properties that it does have thus seem to be as much related to its stickiness and cohesion as plasticity. Without the help of bentonite or ball clay this would not be able to host the addition of feldspar and silica to make a usable porcelain or stoneware. But EPK shines in what it does as a slurry. Left: This slurry has been mixed to only 1.15 specific gravity and forms a thixotropic gel that clings to the spatula in an even layer. Bottom: This low specific gravity gel is thick enough that it will hold this heavy spatula vertical! Considering that our typical glazes have a specific gravity of 1.45 yet are still quite fluid it is evident just how much EPK can gel (and suspend a suspension). It is thus no surprise that it is the kaolin of choice in ceramic glazes. Are there other sticky kaolins that gel like this? Yes. Grolleg and New Zealand kaolin.
These have just been thrown on the wheel, which we find to be a foolproof method of comparing the plasticity of two clays. They were slurried up and dewatered to about the same moisture content and the same amount was thrown on a potter's wheel to compare the plasticity. While the Grolleg is stickier and dewaters a little slower, it is not nearly as plastic as EPK (which itself is not that plastic either). Curiously, New Zealand Halloysite is quite a bit less plastic than the Grolleg but it responds to plasticity augmentation (in porcelain recipes) just as well as Grolleg (similar amounts of bentonite producing similar plasticities). And, bodies containing EPK need about the same amount of bentonite to produce plasticity suitable for throwing large forms. So, the plasticity a porcelain appears to have by itself is not completely indicative of what it will contribute to a body. The EPK used here is the darker and more plastic of the two varieties we receive.
A white burning kaolin from the UK, commonly used in porcelain bodies and as a glaze suspender. Sticky when wet, low plasticity.
A kaolin that gels slurries (thus handy to suspend ceramic glazes). It is plastic and fires white enough that it is also valuable in porcelain bodies.