The term glazing can be used as a noun to refer to anything that is transparent. In architecture it can refer to windows, or in cooking to a sugar-based syrup that coats baked goods. The term is most often used in the arts and crafts world, where glazing is a verb describing the process that adds a shiny, protective topcoat to pottery or ceramics.
In ceramics, glazing adds a vitreous, or impervious, surface to the otherwise porous material. Without a coat of glaze, most earthenware products would not be able to hold water, and would absorb it, making them easy to damage. The glaze also adds an element of decorative appeal to the item and can enhance the patterns beneath.
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The exact origin and history of glazing is sketchy. Glazed pots from Japan dated to about 8000 BC have been recovered during archaeological digs, while those unearthed in China and Iran date from about 5500 BC. Due to this, Japan is often credited as the birthplace of glazing, although this cannot be confirmed absolutely.
Glaze is typically made of a crystallized, glassy material, such as silica, mixed with iron oxides. The material melts when heated, but maintains enough hardness to keep from sliding off of the pottery. The length of the process varies slightly, depending on which type of glaze is used — as does the curing time. Glaze can be found in both liquid and powder form.
The glazing process is fairly simple in steps, but requires some practice to obtain the correct thickness and smoothness. Glaze is spread evenly over the item using a brush. If it will be used for holding liquids, both the interior and exterior of the piece must be coated. The item is then placed into a kiln and fired at high temperatures for a given period of time. The pottery is then left to cool, cure, and settle before handling.
Patterns and decorations can be placed both under and over the glaze. Some pieces are fired briefly before painting, then glazed. Other patterns are painted onto fresh pieces, called greenware, then fired and glazed. Overglaze is a pattern that is painted on after the initial coat of glaze is applied. It can be painted with enamels or other non-glaze paints, and fired once again. This firing helps the new paint to lay flat against the glaze. Sometimes an additional coat of glaze is placed over top, sandwiching the design between two layers of glaze.