Ceramic Glazes
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Ceramic Glazes Even ancient potters knew that after their earthenware vessels had been fired, the clay walls remained slightly porous, allowing stored liquids to percolate slowly to the outside. Peoples on the island of Fiji attempted to deal with this problem by applying vegetable varnishes to the inner surface of their pots. A later technique for sealing earthenware involved the application of a glaze—a finely ground glass powder usually suspended in water. The glaze was painted, dipped, or sprayed on a fired pot, both inside and out. When the pot was fired a second time, the glaze would melt and form a fused, amorphous layer of glass, sealing the clay pores. In ancient Greece around the 4th century B.C., the art of pottery reached a high point with two distinctive forms of glaze—black figure, in which a design of shiny black glaze was painted on a background of red clay, and red figure, where the entire background was painted black except for the design, which was left as plain red clay. The red and black colors were the natural results of the same Attic clay under different conditions. The iron oxide in the clay turned red when it was fired in an oxidizing atmosphere (in a well-ventilated kiln) and black in a reducing atmosphere (in a smoky, air-restricted kiln, usually caused by throwing wet wood into the fire). To obtain both colors on the same vase, the object was first fired in a well-ventilated kiln to produce the usual reddish color of terra cotta, then it was painted with another coating of the glaze and fired again in a reducing kiln, which turned the fresh layer black. Examples of Egyptian pottery from the third millennium B.C. have a similar two-toned effect, which was perhaps created by half-burying a vase in sand while it was being fired, leaving the top to remain red while the bottom turned black. The technique of glazing red and black vases spread from Rome to Gaul and across Europe to Roman Britain by the second century A.D. As glazes became more sophisticated, a flux was added to the ground-glass and water mixture to lower the melting temperature of the glass. The flux also affected the properties and appearance of the glaze.
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An alkaline glaze made with a flux of ash (containing soda ash from burned marine plants or potash from the ash of forest plants) is transparent, as is a glaze fluxed with lead (usually "red" lead or lead tetroxide). Glazes fluxed with tin oxide, however, are opaque and white and are similar to a layer of paint. Glazes can be colored via the addition of metallic oxides: cobalt produces grayish to bright blue; manganese, bright redpurple to purplish brown; antimony, yellow; copper, a blue, green, or bluish red; and iron, from pale yellow, to orangered, to black. Uranium has been used to give brilliant shades of orange and yellow. The composition and uses of new kinds of glazes excited keen interest among potters. In the Near East, many potters kept and exchanged meticulous records of their experiments with different additives. In China, the art of glazing took a differe
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