Commercial Underglazes
Beth E Peterson
Commercial underglazes used to be formulated to be basically highly pigmented colored slips: raw pigment, plus clay, plus water. Like all slips, they were made to be applied to the wet or leather-hard clay before it was bisqued.
Today, most underglazes on the market are formulated more like engobes. They are created using fritted material, which reduces shrinkage and allows them to be applied to bisqueware. (In some cases, they can be applied to both greenware and bisqueware.)
Besides liquid underglazes, there are increasing options available for underglaze application. These include underglaze pencils, crayons, chalk, and semi-moist pads which can be used like watercolors.