Glaze Chemistry

A ceramic glaze is a thin layer of glass. It is fused onto the surface of a fired clay object. This seals the clay and gives it a smooth, often shiny finish. Making this glass layer requires two different ingredients. The first is silica, which forms the actual glass. The second is a flux, a substance that lowers silica's melting point. Silica alone melts at a temperature much higher than any normal kiln can reach. Without a flux, no glass would form during a typical firing. Luckily, potters do not need to search far for flux. One mineral, feldspar, supplies both silica and flux oxides, such as potassium, sodium, and calcium, in a single material. This is part of why feldspar has long been called "the potter's friend." As the kiln heats up, the flux lets the silica soften and flow. This process is called vitrification. During vitrification, the powdery glaze ingredients melt together into one smooth layer of glass. Once the glaze cools, this glass layer makes the ceramic watertight and resistant to stains. Unglazed earthenware, in contrast, has a porous surface that slowly absorbs liquid.