Technicians at factories cannot do this test, but potters can. It is easy to slurry up and dewater a couple of pounds of two samples of kaolin powder (a benchmark and the current shipment). There is no quicker way to compare their plasticities; just throw equal weights as high and thin as possible. This shipment of light EPK (there were two versions, light and dark) was just barely plastic enough to throw this cylinder with a slight belly. Notice the splits on the rim; these are indicative of the shortness. EPK has a cohesiveness (evident by the stickiness of the wet material), that is also notable when using it as a pottery body. This property is unusual and part of the reason it worked so well in glaze slurries. It imparted a toughness, enabling it survive the stresses of throwing even though it was so non-plastic.

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These are SHAB test bars prepared from two different shipments of the same commercial kaolin (EPK is commonly used in porcelain body production). They have just been rolled and marked. The darker set of bars is markedly more plastic.
This underscores the need for body manufacturers to characterize the materials they use in production and maintain an ongoing testing program. This difference flagged an adjustment to the production recipes: A reduction in the percentage of bentonite in the bodies.

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#6 Tile kaolin is a "designer kaolin"; they add bentonite to the material during manufacture to increase the plasticity far beyond what Mother Nature can make. This 10-inch vase is made from the pure kaolin, it is very thin-walled and light. This would be impossible with almost any other kaolin. The fired colour is darker than some other North American kaolins, but less of this (and less added bentonite) is required in a recipe to get the same plasticity. Thus, in the end, a plastic porcelain body can often fire just as white using this as other whiter-burning, but less plastic kaolins.
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Tile #6 Kaolin
The most plastic kaolin in North America, most suitable for making plastic porcelains but increasingly being used in glazes to replace EPK. |
| Materials |
EP Kaolin
A kaolin that gels slurries (good for suspending glazes). It has two variations: a low and medium plasticity. It fires white for use in porcelain bodies. |
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Testing a New Load of EP Kaolin
Kaolins can vary in many ways. Yet they are the foundation of all porcelain and light stoneware clay body recipes. It is vital to test each incoming shipment of material. Problematic brands can even require some testing on every pallet. |
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