Glaze    page 1 of 3   next

Glazes are a specialized type of glass used to waterproof low temperature ceramics and to provide a decorative surface for all types. Glass is a solid that has many of the characteristics of a liquid, including; transparency and a glossy surface free of the crystals that characterize most of the materials we identify as a solid. In a conventional solid the molecules are aligned in a three-dimensional pattern that prevents them from moving in relation to each other. In a glass there is no crystalline structure. If cooled quickly enough, many materials will assume a non-crystalline solid structure. In practice silica is one of the few materials that forms a glass at conventional rates of cooling.

Glaze Ingredients

Silica (SiO2) - Glass former. An acid chemically, Silica is the basic material of all glass. Silica is an extremely common material in nature ( beach sand, for instance, is pure silica (an oxide of silicon). Pure silica melts at about 3100o F. Silica glass is very hard and durable. it also has a low coefficient of thermal expansion which makes it resistant to thermal shock. Because of its high melting point pure silica glass is difficult to make and expensive.

Flux - Chemically alkaline, fluxes cause silica to melt at a lower temperature than it will by itself. Fluxes are usually refractory by themselves and melt only when mixed with silica.
Fluxes also contribute different optical effects to a glaze. Some form very small crystals when cooling that cause the surface of the glaze to appear mat. Fluxes also react with coloring oxides for different colors.

Some commonly used fluxes in ceramics
Feldspar - Mineral containing silica and potassium (K2O) or lithium (Li2O) or sodium (Na2O). The metal component acts as the flux.
Whiting ­ Calcium Carbonate (CaCO3)
Dolomite and Talc - Both minerals contain Magnesium (MgO),which forms very small crystals during cooling after firing. Most glazes made with these minerals will have a mat surface.
Barium - (BaO) Must be used with care as it is somewhat poisonous in its raw state. Commonly used in small quantities along with other fluxes; tends to brighten colors.
Lead - (PbO) Also poisonous and seldom used. In the USA, lead has been made illegal for any ceramics used for containing food. Lead is a strong low temperature flux still commonly used in third world countries that are dependent on low temperature ceramics for cheap containers.
Colemanite - A low temperature flux containing boron (B2O3). This flux has been used to replace lead in the first world countries.

Stabilizer - Always alumina Al2O3 (an oxide of aluminum) A neutral material that is refractory and does not participate in the eutectic reaction between the flux and silica. During melting, the glass formed by silica and fluxes have a low viscosity (It's very runny), and would tend to run off the pot. The stabilizer (alumina) thickens the glaze melt in the same way that adding flour to water will thicken it. Alumina in the glaze also lends hardness to the glass and makes it less transparent.Colorant - A class of metallic oxides that in small quantities (usually .5-6%) give the glaze its color and opacity.

Coloring oxides - a family of metallic oxides that are added to glazes in small quantities (usually from 1 to 6 %) to effect the color of the glaze.

Commonly used coloring oxides
Iron oxide - Fe2O3 Iron is the most useful. It appears as many different colors including dull and metallic red, pale green, yellow and many shades of brown depending on its reaction to the fluxes being used and its concentration in the glaze.

Cobalt Carbonate - CoCa3 Cobalt is probably the second most common colorant giving a strong blue in most glaze formula. This chemical is usually very expensive as it is rare and prices are sensitive to African politics where it is mined. Its coloring power is so strong however, that only very small percentages are necessary (usually less than 1%)

Copper Oxide - (CuO) Copper is very sensitive to the gasses in the kiln during firing. Colors are bright red, blue green, blue.

Rutile - a combination of titanium and iron. Usually gives a dull yellow color; sometimes light blue depending on the fluxes present.

Opacifiers - a class of minerals that like alumina do not dissolve in the glaze melt. Small crystals remain suspended in the glaze making the glass appear white. Usually a type of zirconium silicate purchased under the commercial names such as ultrox or zircopax. Tin oxide, although expensive, is also used.

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