In-glaze or Inglaze colours have been formulated for the decoration of already glazed ceramic

Ceramic

A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...

ware. They can be applied by brush, roller, decal or even spray. Following colour application the pieces are usually allowed to dry before firing. The firing at a high temperature allows the colour to sink into the glaze to give addition durability.
The Potters Friend: Inglaze, onglaze and underglaze colours
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