Underglaze

Underglaze is a pigmented, clay based paint used to add color and detail to pottery before the final glaze goes on. It can be brushed, sponged, sprayed, or applied with a slip trailing bottle for fine lines, and unlike many glazes it fires with very little movement or running, which makes it the standard choice for anything needing crisp detail, lettering, illustrations, or controlled patterns rather than an all over field of color.

Underglaze can be applied at two different stages, onto still damp greenware before the first firing, or onto absorbent bisque ware after it, and each timing has trade offs; greenware application blends more into the surface while bisque application tends to give slightly crisper, more controllable lines since the piece is already firm. Either way, underglaze almost always needs a clear or lightly tinted glaze fired over it afterward to protect the color and give the surface its final sheen, since bare fired underglaze is typically dry and unsealed.

When buying underglaze, check whether a given color is rated food safe if the piece will hold food or drink, since not every underglaze color is formulated that way, and check the recommended firing range matches the clay body and glaze being used over it. A frequent beginner mistake is applying underglaze too thin, letting the clay color show through unevenly, or skipping a test tile entirely and discovering only after the glaze firing that a favorite color fired duller or more muted than expected under a particular clear glaze.