All Glossary

Overglaze

A method of applying decoration over the glaze surface of ceramics. It can be done before or after the glaze firing.

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Details

'Onglaze' decoration can refer to two very different processes. The first involves the application of liquids applied onto the fired glaze surface. These include china paints, lusters, gold, and other metallics. They are fired on at very low temperatures, compared to normal glaze-melting temperatures of cone 06+ (1800F+), these fire at cone 018 (about 1300-1350F). Obviously, if the existing glaze softens underneath at this temperature its surface character and the integrity of the overglaze will be detrimentally affected. To develop a glassy durable surface at this very low temperature the ceramic pigments employed must be blended with a very low melting glass medium. Common frits do not melt anywhere near this low of a temperature thus much more expensive bismuth-based frits must be employed. To apply and dry on an already-fired glaze surface the stain-glass mixture must be suspended in petroleum carriers and solvents (this accounts for the strong fumes these products normally have).

'Overglaze' also refers to the process of painting gummed suspensions of metallic oxides or stains (compounded with a glaze-like medium) over a raw yet-unfired glaze. This is done for standard cone 06-04 bisque stoneware and earthenware. Stains have high melting points, they will not give a good surface if painted pure over a glaze. Each stain class is unique and needs to be blended with a medium, which is a blend of frit, clay and other materials that provides a sympathetic host for development of its color and melts it to the degree necessary (enough to be a durable glass but not so much that it bleeds into the cover glaze at the edges of its brush strokes).

Related Information

Why pure stain powders make poor inks, slips, underglazes and overglazes

This picture has its own page with more detail, click here to see it.

On the left are pure blue stain brush strokes, on the right are green ones (both painted over a glaze). Clearly, the green is refractory, stiffening the glaze enough to trap bubbles and sit on the surface as a dry, unmelted layer. The blue is the opposite, melting and bleeding profusely into the glaze. Under the glaze, these problems would be magnified (the blue bleeding more, the green causing crawling and blistering). Stains are not ceramic, they are ceramic additives. Stains are not safe for direct food contact. Stains are expensive. Stains don't suspend in water, paint poorly and dry as a lose powder. These stains each need to be added, as a minor percentage, to a ceramic painting medium (one with CMC gum and a mix of ceramic materials tailored to melt to the desired degree and have a compatible chemistry for develop the color (as per manufacturer guidelines).

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