Glaze is used on earthenware pottery to waterproof it and on porcelain and stoneware pottery to decorate it. Glaze is also used on construction materials such as bricks and tiles. Glaze can be applied as a dry mixture dusted on the surface of a clay object, and also by placing soda or salt in high-temperature kilns to make sodium vapor, which interacts with the silica and aluminum oxide in the amaco clay to deposit a glass coating on the object. However, the most common type of glazes are liquid glazes which are suspensions of metal oxides and powdered minerals. They are applied by pouring the glaze over the object, by dipping the objects into the glaze, by spraying the object with an airbrush, or by painting the glaze on the object. Often all or part of the bottom of an object is left unglazed, or placed on stilts or kiln spurs, so that the object doesn't stick to the kiln.
Glazes can range from complete transparency to complete opacity. Opaque glazes are caused by tiny air bubbles or particles in the glaze suspension; many glazes which appear white are actually opaque rather than containing a white pigment. Decorations applied to the clay underneath the glaze is termed underglaze, which can be applied either to raw, unfired or to bisque-fired (already fired) pottery. Transparent wet glaze is applied over the underglaze, and the pigments in the underglaze fuse with the glaze. The well-known blue and white clay pottery art of Holland, England, Japan and China is an example of the underglaze technique. The characteristic blue color comes from the cobalt oxide or carbonate in the glaze formulation. When decorations are applied over a layer of glaze, they are termed overglaze. Overglazes are fired at low temperatures and give a glassy appearance. The color of a glaze is affected not only by its chemical composition but also by the atmosphere in the firing kiln. A kiln with a high level of oxygen produces an oxidation firing while a kiln low in oxygen produces a reduction firing. An oxidation firing of copper carbonate glaze produces a turquoise color, but a reduction firing of the same glaze produces a bright red.
Choosing the correct glaze is a science unto itself. Factors which need to be taken into account include the firing range, the formulation, and the use to which the object will be put (for example, dinnerware objects require non-toxic glazes). The firing range is determined by the type of clay which is used. Mid range stonewear clays require mid range glazes and low range earthenware clays require low fire glazes. Although many professional ceramicists formulate their own glazes to achieve specific effects, beginners are advised to use commercial formulations such as amaco glaze whose properties are well-known and tested. Commercial suppliers can provide sample tiles which indicate properties such as surface texture, color, transparency, and food-safety.
The knowledge of glazes in clay pottery art is a science unto itself. In addition to taking the type of AMACO clay which is used into account, selecting the correct AMACO glaze requires consideration of what artistic effect is desired; and to what use (e.g. dinnerware) the object will be put.
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