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I poured 4 teaspoons of two glazes onto a non-absorbent butcher’s board and let them sit for a minute, then inclined the board. The one on the right employs Gleason Ball clay, the left one has Old Hickory #5 ball clay. Neither has any rheology modifier additions. The one on the right has settled and, on incline of the board, the watery upper is running off. The other has gelled and the whole thing is running downward slowly. Below that you can see where I have begun to sponge them off, the one on the right is sticky. The most amazing thing about this: This difference appears despite the fact that there is only 7% ball clay in this heavily fritted recipe.

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In ceramics, EPK, or EP Kaolin, is used in clay bodies and glazes. EPK feels fairly plastic on wedging. But it splits like this during throwing (top right). It needs the help of bentonite or ball clay to be usable in porcelain or stoneware bodies. Tile #6 kaolin is thus a much better choice for plasticity (although higher in iron and titania).
But EPK shines in glazes. A slurry of pure EPK (bottom), mixed to only 1.15 specific gravity, holds this spatula upright! Glazes employing as little as 20% EPK, without added electrolytes, are naturally thixotropic. And its sticky nature helps with adherence and dry hardness. Consider the engobe employing it (top left). It gels and clings to this rubber spatula in an even layer, with no drips. If leather-hard ware is carefully dipped and extracted, it stays put (even if it takes hours to dry). The performance of EPK suspending glazes for use on bisque is even better.
Are there other sticky kaolins that gel? Yes. Grolleg and New Zealand kaolin. What if you cannot get either? Tile #6 is a good choice because it dries hard. But, it may need the help of Epsom Salts to produce a slurry having thixotropy like EPK.
| Glossary |
Suspension
In ceramics, glazes are slurries. They consist of water and undissolved powders kept in suspension by clay particles. You have much more control over the properties than you might think. |
| Materials |
No. 5 Ball Clay
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| Materials |
No. 1 Glaze Ball Clay
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| Materials |
Ball Clay
A fine particled highly plastic secondary clay used mainly to impart plasticity to clay and porcelain bodies and to suspend glaze, slips and engobe slurries. |
| Troubles |
Glaze Slurry is Difficult to Use or Settling
Understanding glaze slurry rheology is the key to solving problems and creating a suspension that does not settle out, applies well, dries crack free. |
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