Underglaze is a method of decorating 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...

 articles, the decoration is applied to the surface before it is glazed. Because the glaze will subsequently cover it such decoration is completely durable, but because the subsequent glost firing is at a higher temperature than used in on-glaze decoration
On-glaze decoration
On-glaze is a method of decorating ceramic articles, where the decoration is applied after it has been glazed. When the ware is fired, or re-fired in the case of twice-fired ware, the colours fuse into the glaze and so the decoration becomes durable...

 the range of available colours is more limited. It uses pigments derived from oxides which fuse with the glaze
Ceramic glaze
Glaze is a layer or coating of a vitreous substance which has been fired to fuse to a ceramic object to color, decorate, strengthen or waterproof it.-Use:...

 when the piece is fired in a kiln
A kiln is a thermally insulated chamber, or oven, in which a controlled temperature regime is produced. Uses include the hardening, burning or drying of materials...

. Examples include Blue and white porcelain and İznik pottery
Iznik pottery
İznik pottery, named after the town in western Anatolia where it was made, is highly decorated ceramics that was produced between the late 15th and 17th centuries....

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