Glaze firing

Glaze firing is the firing most people picture when they think of finished pottery, the one that turns a chalky bisque piece coated in dull, dusty glaze into something glossy or matte, colorful, and waterproof. The kiln is brought up to whatever temperature the glaze and clay body are formulated to mature at, commonly cone 06 to cone 04 for low fire earthenware glazes or cone 5 to cone 10 for stoneware and porcelain, and held there long enough for the glaze to fully melt and flow into a continuous glassy layer.

Because the glaze becomes liquid at peak temperature, glaze loaded shelves need real spacing, pieces must never touch each other or the kiln walls, and any glaze on the foot of a pot will fuse it permanently to the shelf. This is also where kiln wash on the shelves and stilts under some pieces earn their keep, catching drips before they bond glass to brick. The firing schedule itself, how fast the kiln climbs and how it cools, affects the final glaze surface as much as the recipe does, with slow cooling in particular helping some glazes develop matte or crystalline effects instead of a plain glossy finish.

The most common glaze firing frustrations, crazing, where a network of fine cracks appears in the glaze, and glaze crawling or pulling away from bare patches of clay, both usually trace back to a mismatch between the glaze's expansion and the clay body's, or an application problem like glaze applied too thick, too thin, or over dust and grease. Testing a new glaze on a small tile of the exact clay body it will be used on, fired on the exact same schedule, catches most of these problems before they show up on a favorite pot.