TECHNO TIP Conflicting Opinions about Plasticity These of us who have done pottery for a long time have pretty firm ideas about what docs and what does not work in glazes and clay bodies. But it always amazes me how two experienced people can have opposite philosophies and meth- ods ard yet both make them work. | find trying to explain ‘the whys’ of specific instances. of this very educational. At Plainsman we employ a kaolinite! halloysite material, from Troy Idaho, in a number of our clay bodves, [ts inherently very fine so requires almost no grinding, however ithas very high fired shrinkage so we don't use more than about 20%. Its plasticity is quite low amd even a body containing 70% would still lack plasncity. However this same material is success- fully processed and promoted by Michacl Wendt as Helmer clay = al wea wendipottery.com. He uses only 40% Helmer as the exclusive source of clay in his bulf stoneware body and claims thal while it is very plastic he has zero drying loss. Strangely his total shrinkage figures matched what | would consider normal. This seemed impossible. | mixed test hatches of his clay body and we emailed back and forth without being able te re- solve the difference in opinions. Finally he sent ine a sample that he had mixed using his wet process, It threw a little: beter but I still found plasticity much lower than 1 am used to. And no wonder, its drying shninkage was only 4% (compared to a typical of 6.5% in bodies made from our raw mitterials), 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, | found that the clay tended to split during wedging, penerated a lot of slip during throwing ond had a lower dry strength. How do we resolve this? First, Michael's total shrinkage measurements are compra rable to what we consider normal because its drying shrinkage is much lower and compensates for the higher fired shrink- age. 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 wrth these bodies. I think Michael's and my priorities and therefore observations are different for this reason. Gur opinions about plastic- ity obviowsly conflict, neither of us is necessarily nght. Plasticity is a hard thing to quantify. | am inclined to define plastic- ity as ‘a readiness of soft clay io assume a new or return to an old shape while main- taining strength during the move.” Others think of plasticity as the ability to hold a shape once lis assumed, but they are less inclined to talk about its strength during the move. Well travelled potters can tell lores about bexties in Japan or Europe that they found impossible to throw yerthe natives de it easily, Life is a senes of 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 disadvantage of the kinds of clays available in our area, Low firing shnnkages 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 silty materials and would say that the dry strength of his body is fine. This is fascinating. The range of plasticity possibilities ts amazing. Some people use Ti+ ball clay bodies (i.c. Fairey F9T) on one end of the spectrum, whereas thers employ kaolin only bodies on the other (although I know of mone as low as 40%). People have found ways lo make both of these extremes work well, will defend them to the limit and even dowht that the other extreme is possible to work with. We may be tempted to claim that a particular clay bxxly breaks all the rules (e.g. itis both plastic and perfect drying, vitreous yet does not warp, low in silica yet docs not craze glazes). These statements defy the logic of clay body physics, and on further investigation you will always find that there is alot more to the story. [tis usually a queshon of the potter adapting to the material, Tony Hansen Ina June‘July issue of London City Life magazine, Joa Chalke, a well known Al- berta potter and ceramist says thar ~... InpIWiting such a daunting project was refreshing .... (lonathon's] enthusiasm does not cease... This will be the first major show that Canada fas haed in a long New,” INTERNATIONAL CALL FOR SUBMISSIONS Cheongju, Korea 2™ Cheongyu International Craft Biennale October 5-2] Deadline July 2 Call for works expressing the theme breath of nature: the struggle in nalure, ms har- mony with the five traditional elements and its comtinuing transformation. The judges will look for works ihuminating nature’s values and presentenyvironmental problems arising from technobogical and industrial society. Prizes range from S20.000 to $1,000 US. Information and registration: Cheongju International Craft Biennale, 755 Sagik-Idong, Heundeok-gu, Cheongyu-city, Chungcheongbuk-do, 361- 10] Korea, 82.43.220.6275 or fax 62.49.220,.6280 or email See www cheongjuliennale.orkr,