Cornish Stone

Source: http://ctmpotterssupplies.co.uk/ceramicmaterialsglossary.html

K2O 3.8%, Na2O 4%, Al2O3 15.3%, SiO2 69.5% typical. Secondary flux in earthenware glazes, and an alternative to feldspar at higher temperatures, giving greater fired strength. Low iron and high silica content promotes whiteness and transparency so particularly useful in porcelain and white bodies.