Underglaze is a coloured decorating material, a clay-based slip with stains, that is painted onto greenware or bisque and usually covered with clear glaze. It doesn’t run or move in the firing, so painted details stay crisp.
What Is Underglaze?
Underglazes are made from clay, colour stains and a small amount of flux, so they behave more like coloured clay than like glaze. That stability is their great advantage: designs painted with a fine brush look much the same after firing, which is why they are used for illustration, lettering and paint-your-own pottery.
They can be applied to leather-hard, bone-dry or bisque-fired clay, brushed, sponged, stamped or used with stencils and transfers. Two or three coats usually give solid colour. Most are covered with a clear glaze, which deepens the colour and seals the surface.
Many underglazes are rated from low fire to mid fire, but some colours, especially reds, pinks and purples, can fade at higher temperatures, so check the label and test.
Underglaze pencils and chalks work the same way for drawing. Because underglazes can be fired without a glaze too, some potters use them alone for a matte, velvety surface on decorative pieces.
This pottery glossary entry from Le Studio Art Space is general educational content. Always follow the clay, glaze and kiln makers’ data for your own materials and equipment.