Old and New Cornwall Stone

A material commonly used by potters, Cornwall Stone was first commercially produced for the pottery industry in 1807 in the region of St. Stephen, St. Austell, Cornwall, England. It has many names: English Stone, English Cornwall Stone, Cornish Stone, D.F. Stone, China Stone, and Carolina Stone. In a way, the variety of names and chemical analyses is appropriate since there have been several variations in its chemical composition depending on how it was processed and/or the site at which it was mined. Over the years production has ranged from a high of 70,000 tons before the first World War to 28,000 tons on average per/yr. from 1999 to 2003, which were mined for domestic and foreign use by potters and industry.1.

Cornwall Stone contains feldspar, quartz, kaolinite, mica, and trace amounts of calcium fluoride. It is a partially decomposed granite feldspathoid having a greater diversity of alkaloids causing it to go into a gradual melt with less surface tension than true feldspars.2. When Cornwall Stone is used in a glaze this characteristic can result in the elimination of crawling (the fired glaze surface rolls back on itself resulting in exposed clay). This is due in part to the diverse composition of granite in various stages of decomposition and alteration from which it was formed.3. Cornwall Stone contains an equal molecular equivalence of calcium, potassium and sodium along with alumina and silica, making it a unique feldspar-like material. However, it is more refractory and goes into a melt at a higher temperature than either sodium or potassium feldspars due to its higher silica content. Additionally, at cone 6 (2232 0 F.) it is more refractory and melts less than other feldspars at this temperature range due to its higher silica content. The high silica content of Cornwall Stone when used in a clay body or glaze can also be a factor in reducing or eliminating crazing (a network of fine lines in the fired glaze surface).

Cornwall Stone is not one mineral but a complex group of minerals that at different times in its processing history have produced varied chemical analyses depending on the deposit being excavated and processed, resulting in no distinct chemical formula. In this respect it is similar to another common flux material used in glaze and clay body formulas, nepheline syenite; both are combinations of minerals. Cornwall Stone has properties similar to feldspars in that it can be used as a flux above cone 6 in clay body and glaze formulas. However, it differs from true feldspars because it is partly kaolinized and responds well to flocculation and deflocculation adjustments in the glaze batch, resulting in increased wet glaze handling properties.4.

Processing History

The processing history has played a major part in the chemical composition of Cornwall Stone, which in turn altered the results it produced in glazes over its production run. From 1960 to 1973 fluorite and mica were removed from Cornwall Stone, resulting in a designation of “D.F.” Stone, which was defluorinated. The processed Cornwall Stone also contains quartz and feldspar. The “D.F.” Stone had reduced fluorine, potash, calcia, iron, alumina and increased silica. This was an improvement as calcium fluoride in a glaze has the potential to exit as a gas, causing blisters in the fired glaze surface.

In 1973 the “D.F.” refining of Cornwall Stone came to an end due to increased production costs. After that the material that was not defluorinated was distinguished by a vegetable dye that burns off in the kiln without a trace; the color is not due to flourspar in the material. The raw color of Cornwall Stone can change, perhaps indicating different batches from the mine.5. After the “D.F.” was discontinued Purple Stone was produced which contained soda and potash feldspar, quartz, mica, and fluorite. A second type known as Hard White, with some of the soda feldspar being replaced by kaolinite, was also available. Hard White contains less fluorine compared to Purple Stone. A blend of both Purple and White Stone was also was sold. Since the chemical composition of Cornwall Stone has changed the potter should be aware of which variation they will be using. Once known, it is best to obtain its chemical analysis sheet before using the material in a glaze formula6..

However, all variations of the material have low iron content. Cornwall Stone is commonly used in cone 6 (2232 0 F.) to cone 9 (2300 0 F.) glazes as a flux, helping to bring other materials into a melt. It can also be used in slips as it has excellent adhesion properties to bind it to the underlying clay body. The raw color can be white, purple, or green; however, Cornwall Stone melts to a white opaque glass between 2102 0 F. and 2372 0 F 7. Because Cornwall Stone is imported from England it can cost more than domestically available feldspars. However, the cost of any raw material used in the production of pottery is marginal compared to the time and labor required to make the pots.

As has happened with many other raw materials, the mine producing Cornwall Stone has closed. Its demise represents an ongoing ceramics “fact of life” that at some point a favorite material will become unavailable. The reasons for the loss of any given raw material are almost always economic, as many are still geologically available. However, the market demand by larger industries has shifted. In many instances a raw material will go out of production because the volume users drop demand and potters as marginal customers are left to seek another material. What can potters do to correct this disadvantage? The direct answer is nothing, as the larger players determine the demand and continuance of any material. Potters can educate themselves about the raw materials in their clay body and glaze formulas and then have the tools to make an appropriate substitution when and not if a material has gone the way of Albany Slip, Barnard/Black Bird, Oxford feldspar, P.B.X fireclay, Ocmulgee, A-3 feldspar, Kingman feldspar, Avery kaolin, NYTAL HR 100 talc. The complete list of materials no longer available to potters would fill many books

Hammill & Gillespie, a supplier of specialty clays and minerals, has found a substitute named   H&G Cornwall Stone which can be used as a replacement in most glaze formulas. However, it is always best to test any new material before committing to large scale production.