Cornish Stone

Cornish stone substitute is a dry blend of feldspar, quartz and kaolin. In a glaze it works as a secondary flux and a feldspar alternative, but it is not a one for one swap for feldspar. It contains more silica and less flux, so it melts at a higher temperature, and a recipe that trades feldspar for it straight across comes out stiffer, and a glossy surface can shift towards satin or matte. In return it gives a gradual melt and lower surface tension through cone 6 to 9, so it is common in recipes that crawl or craze. In an engobe it helps the slip bond to the body, and in a clay body it supplies fluxing oxides and fired strength. It is low in iron and fires white.