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Rationalizing Conflicting Opinions About Plasticity

Section: Clay Bodies, Subsection: General


How can two potters have completely different opinions about the plasticity and workability characteristics of the same clay body

Article Text

Those of us that have done pottery for a long time have pretty firm ideas about what works and does not in glazes and clay bodies. But it always amazes me how two experienced people can have opposite philosophies and methods and yet both make them work. I find trying to explain 'the whys' of specific instances of this very educational.

At Plainsman Clays (I work there part time) they employ a kaolinite/halloysite material from Troy, Idaho in a number of our clay bodies. It is inherently very fine so requires almost no grinding, however it has very high fired shrinkage so they don't use more than about 20%. Its plasticity is quite low and even a body containing 70% would still lack plasticity. However this same material is successfully processed and promoted by Michael Wendt as Helmer clay at www.wendtpottery.com. He uses only 40% Helmer as the exclusive source of clay in his buff stoneware body and claims that while it is very plastic he has zero drying loss. Strangely his total shrinkage figures matched what I would consider normal. This seemed impossible. I mixed test batches of his clay body and we emailed back and forth without being able to resolve the difference in opinions. Finally he sent me a sample that he had mixed using his wet process. It threw a little better but I still found plasticity much lower than I am used to. And no wonder, its drying shrinkage was only 4% (compared to a typical of 6.5% in bodies made from our raw materials). This is a huge difference (highly plastic clays employing ball clay and bentonite have high drying shrinkages while low plasticity kaolin-based bodies have low shrinkages). In harmony with this I found that the clay tended to split during wedging, generated a lot of slip during throwing and had a lower dry strength.

How do we resolve this? First, Michael's total shrinkage measurements are comparable to what we consider normal because its drying shrinkage is much lower and compensates for the higher fired shrinkage. We all tend to evaluate bodies in terms of the ones we were 'brought up' with and almost unknowingly we develop ways of working that mesh with these bodies. I think Michael's and my priorities and therefore observations are different for this reason. Our opinions about plasticity obviously conflict, neither of us are necessarily right. Plasticity is a hard thing to quantify. I am inclined to define plasticity as 'a readiness of soft clay to assume a new or return to an old shape while maintaining strength during the move'. Others think of plasticity as the ability to hold a shape once it is assumed but they are less inclined to talk about its strength during the move (well traveled potters can tell stories about bodies in Japan or Europe that they found impossible to throw yet the natives handle them easily)?

Life is tradeoffs. To enjoy the advantages of this type of body Mr. Wendt has developed techniques that address its disadvantages. Michael would point out that he cannot tolerate the disadvantages of the high-drying-shrinkage kinds of clays available in our area. Low firing shrinkages would not be as important to him as low drying shrinkage. He would be hesitant to employ ball clay and cut its plasticity with sandy or siltly materials and would say that the dry strength of his body is fine.

This is fascinating. The range of plasticity possibilities is amazing. Some people use 70%+ ball clay bodies (i.e. Fairey F97) on one end of the spectrum, whereas others employ kaolin-only bodies on the other. Fara Shimbo, the author of the book Crystal Glazes, is another example. She employs a porcelain body containing 31% Grolleg and 15% EPK kaolin as the only sources of clay. Neither of these are plastic kaolins in the same league as #6 Tile kaolin, and yet I regard a body of 50% #6 Tile as much too short! It seems that people have found ways to make both plasticity extremes work well and will defend them and articulate their merits tirelessly. When it comes to using bodies that we have developed ourselves we may be tempted to claim that a particular one breaks all the rules (e.g. it is both plastic and perfect drying, vitreous yet does not warp, low in silica yet does not craze glazes). These statements defy the logic of clay body physics, on further investigation there turns out to be a lot more to the story. It is usually a question of the potter having adapted to the material.

Out Bound Links

By Tony Hansen

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