glaze(redirected from unglazed)
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glaze,in meteorology: see sleetsleet,
precipitation of small, partially melted grains of ice. As raindrops fall from clouds, they pass through layers of air at different temperatures. If they pass through a layer with a temperature below the freezing point, they turn into sleet.
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glaze,translucent layer that coats pottery to give the surface a finish or afford a ground for decorative painting. Glazes—transparent, white, or colored—are fired on the clay. Of the various artificial mixtures used for glazes, that for whiteware contains borax and lead, whereas a salt glaze is used for stoneware. No lead is used for porcelain. The coloring agents are oxides of different metals. In the 16th and 17th cent. glazes were also used in painting to enhance the luminosity of oil or tempera colors. Titian and Rembrandt were especially adept at glazing techniques.
a 0.15-0.3 mm thick vitreous coating on ceramics, fixed by firing.
Chemically, glazes are alkaline, alkaline-earth, and other aluminosilicate and aluminoborosilicate glasses. Glaze protects ceramics from dirt, acids, and alkalies; it also waterproofs them and provides them with decorative qualities, appropriate to architectural and artistic needs.
Glazed ceramic artifacts have a history that goes back 1,000 years. In ancient Egypt, Babylonia, and Assyria, glazes were used on ornaments (such as beads and amulets), housewares (such as pottery), and construction materials (such as tiles and bricks). Glaze was produced in greenish and turquoise tones in imitation of the malachite and lazurite items which were in vogue during the period; later (c. 2550 B.C.), dark blue glazes, close to the color of indigo, appeared. Glazed objects were widely used in the tenth-13th centuries in Middle Asia, such as the azure glazed bricks used to decorate the domes of mausoleums, madrasas, and mosques. A mat-white glaze made opaque with tin oxide and known in Iran in the 12th and 13th centuries was first created in Europe in 1438 by the Italian ceramist Luca della Robbia. The so-called lusters, or glazes with metallic brilliance, were developed on a base of copper and iron oxides.
In France, B. Palissy (1510-89) produced articles with both white and colored translucent glazes. In Russia in the 16th and 17th centuries tiles with a polychrome coating were widely used. This sort of glaze was called poliva (enamel); a green poliva was known as murava.
Glazes are subdivided on the basis of sintering temperatures as refractory (1100°-1350° C) or fusible (900°-1100° C). The fusibility of a glaze depends on the nature and composition of the materials of which it is made. The glazing temperature for porcelain is 1132°-1420° C; for semiporcelain, 1250°-1280° C; for faience, 1100°-1180° C; and for majolica 940°-1040° C. Other fine coatings, such as a white or colored argillaceous slip (engobe), may also be applied to ceramic objects. Opaque white or colored glazes, used primarily in coating metallic objects, are called enamels.
The chemical composition of the refractory glazes, which withstand high firing-temperatures and are generally spread “raw” over the objects, that is, without preliminary melting (fritting), is enriched with silicon dioxide and aluminum oxide through the use of alkaline-earth oxides. The composition of these glazes includes quartz, kaolin, clay, and the natural carbonates of bivalent metals, such as dolomite and marble; most glazes also contain feldspar.
The chemical composition of the fusible glazes, which require lower firing temperatures and which usually undergo fritting, is enriched with alkaline and alkaline-earth oxides and boric anhydride. The materials used in the melting of these glazes include quartz, feldspar, borax or boric acid, strontium carbonate, magnesite, and dolomite.
Raw glazes are prepared by pulverizing the initial substances in a ball mill, with the addition of plastic clay to maintain the glass particles in a suspended state. Glazes are applied to the articles in the form of a homogeneous suspension. They are colored by mixing with pigments (for opaque coatings) and by fusion with dyeing oxides (for transparent coatings). Compounds of tin, zirconium, titanium, and other elements are used in clouding the mixtures. There are many ways of obtaining decorative coatings, including crackleware (a network of cracks), “snakeskin” (creased glaze), and “lace” (raised glaze); it is also possible to paint one glaze over another or to decorate underglazes and overglazes with paints. Glazes are applied to dried articles and then fired once, or they are applied to previously fired articles (bisque) and then glaze fired.
REFERENCESOrlov, E. I. Glazuri, emali, keramicheskie kraski i massy, 3rd ed., parts 1-2. Moscow-Leningrad, 1937-38.
Barzakovskii, V. P., and S. K. Dubrovo. Fiziko-khimicheskie svoistva glazurei vysokovol’tnogo farfora. Moscow-Leningrad, 1953.
Glazuri, ikh proizvodstvo i primenenie (collection of articles). Riga, 1964.
Nosova, Z. A. Tsirkonievye glazuri. Moscow, 1965.
Shteinberg, Iu. G. Strontsievye glazuri, 2nd ed. Leningrad-Moscow,1967.
N. IA. GOSIN
a thin transparent or translucent layer of color applied over a dry or drying underlayer of paint to change, intensify, or soften the hues; to enrich the range of colors; or to achieve a unifying effect. The application of glazes was usually the last stage in the process of painting a picture in the 16th to 19th centuries.