A glaze is a thin layer of glass fused to the surface of pottery in a kiln. It is applied as a liquid mix of powdered minerals and water, and melts in the firing to make the piece colourful, smooth and, when made for it, watertight.
What Is Glaze?
Every glaze needs three things: silica to form the glass, a flux to make the silica melt at a reachable temperature, and alumina to stiffen the melt so it stays on the pot. Colourants such as iron, cobalt, copper and commercial stains give the colour, and opacifiers make it opaque.
Glazes are made for a particular temperature range. A low-fire glaze fired to stoneware temperatures will run off the pot, and a high-fire glaze fired low will be dry and rough. Commercial glazes list their cone on the label, and studio recipes say which cone they were developed for.
Glazes can be dipped, poured, brushed or sprayed onto bisqueware, and the thickness matters a lot to the result. Thick glaze can run or crawl, while thin glaze may look dry or show the clay through. Testing on tiles, on your own clay and in your own kiln, is the only reliable way to know what a glaze will do.
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.