The Collaborative International Dictionary
Underglaze \Un"der*glaze`\, a. Applied under the glaze, that is, before the glaze, that is, before the glaze is put on; fitted to be so applied; -- said of colors in porcelain painting.
Wikipedia
Underglaze is a method of decorating ceramic articles in which the decoration is applied to the surface before it is glazed. Because the glaze will subsequently cover it, such decoration is completely durable, and it also allows the production of pottery with a surface that has a uniform sheen. Underglaze decoration uses pigments derived from oxides which fuse with the glaze when the piece is fired in a kiln. However, because the glost firing is at a higher temperature than used in on-glaze decoration, the range of available colours is more limited. Examples of oxides that do not lose their colour during a glost firing are the cobalt blue made famous by Chinese Ming dynasty blue and white porcelain and the cobalt and turquoise blues, pale purple, sage green, and bole red characteristic of İznik pottery.
Usage examples of "underglaze".
Myriad white beeswax candles in branched candelabra reflected in fanciful epergnes of crystal or silvered basketwork, golden salvers lifted on pedestals and filled with sweetmeats or condiments, sets of silver spice-casters elaborately gadrooned, their fretted lids decorated with intricately pierced patterns, crystal cruets of herbal vinegars and oils, porcelain mustard pots with a blue underglaze motif of starfish, oval dish-supports with heating-lamps underneath, mirrored plateaux and low clusters of realistic flowers and leaves made from silk.
It was during the Yuan dynasty that white translucent porcelain with the blue underglaze was first used.