Glaze

Composite body, painted, and glazed bottle. Dated 16th century. From Iran. New York Metropolitan Museum of Art.

Herakles resting on a couch, black-figure side of a vase by the Andokides Painter, circa 520/510 BC. Munich: Staatliche Antikensammlungen

Glaze is a layer or coating used on pottery or ceramics. It may be called vitreous enamel or porcelain enamel. Powdered glass is fused onto ceramics by firing to between 750 and 850 °C (1,380 and 1,560 °F). The powder melts, flows, and then hardens to a smooth, lasting vitreous coating on metal, or on glass or ceramics. This is done in a kiln.

There are many different types of glaze, some are used for decoration and some are used to make pottery watertight so it can hold liquids. Glaze serves to colour, decorate, strengthen or waterproof an item.

Glazing is important for earthenware vessels as otherwise they would leak water. Glaze is also used on stoneware and porcelain. In addition to the functional aspect of glazes, they can form a variety of surface finishes, including degrees of gloss and matte and colour. Glazes may also enhance an underlying design or texture.

Glaze is used on building materials. The Iron Pagoda, built in 1049 in Kaifeng, China, of glazed bricks is an example.

Ceramic glaze raw materials generally include silica, which forms glass when fired. Metal oxides, such as sodium, potassium and calcium, act as a "flux" to lower the melting temperature. Alumina, often derived from clay, stiffens the molten glaze to prevent it from running off the piece. Colour comes from iron oxide, copper carbonate or cobalt carbonate. Tin oxide or zirconium oxide make the glaze opaque.

References

Red-figure side of the vase illustrated above, same subject