Cornwall Stone, SG919 (50# Bag)

Safety Data Sheet

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Cornwall stone was a low iron feldspar material used primarily as a flux in clay bodies and glazes. Is was mined in the Cornwall area of the UK. It melted 1150-1300C. It had a more diversified selection of fluxes than other feldspars but also had one of the highest silica contents. By itself it did not melt as well as feldspars (melt flow began around cone 11 oxidation). It was popular in engobes for its adhesive power during and after firing and in glazes for its low shrinkage and purity.

Long-time users often commented on the difference in color between different batches. The parent ores are more complex than other feldspars, and tend to be a mix of varying types of igneous rock in different stages of decomposition. Earlier stages of the ore materials are bluer (from fluorine) and contain more fluxes than newer rocks (some of the alkalis and fluorine have been leached and washed away creating a softer material). Cornish stones tended to be classified into major types according to the amount of flux present. Although Cornish stone was quite variable in composition, its low iron made it an attractive material. Documentation on this material provided a wide range of chemistries over the years, so no one really knew what a truly representative analysis should have looked like (the one provided here is an average of many that we collected).

Its diversity of oxides made it similar in composition to common stone, thus its name. Now it is common to see synthetic substitutes for this material (since it is easy to blend other feldspars to approximate the analysis). These substitutes have the advantage of having no fluorine (which is suspected in various glaze faults).