What Affects a Glaze Surface?

Q: What affects a glaze surface?

A: A glaze surface may be glossy, satiny, or rough (dry matt); the actual result will revolve around the silicon oxide and the alumina oxide contents, both in relative and absolute terms. A glass that contains 60% silicon oxide (silica, flint, quartz), plus or minus 5%, will usually make a good glaze. However, if the basic oxide mix (the “Seger” Formula) shows less than 55% SiO2 then the glaze is not “balanced” and will likely not form a “coherent” glassy material and therefore will have a non-uniform surface. Yet, because a glaze-mix coats the surface of a claybody that contains a lot of SiO2, most often a silica-deficient glaze will take up some silica from the body to form a glaze closer to a good glass, i.e., a balanced glaze. If the silicon oxide content is 70%+ SiO2, the glaze becomes high-melting and may not form glass at the expected cone/temperature. Then, its surface could exhibit some unwanted effects, eg, crawling (the glaze clumps in pools).

A good long-lasting glaze surface contains sufficient alumina (Al2O3) to make the glaze melt stay put (non-runny) and to form an alumino-silicate polymer that is strong and resists scratching. The ratio of silica molecules to alumina molecules gives an indication of how the new glaze will behave: at a ratio of 10, a uniform glass is formed; it will have a glossy surface. Between 5 and 10, the surface will go from dry matt to glossy, the actual transition point being quite variable and dependent on the precise mix of materials, and body/glaze interaction. Above 10 the glaze will be glossy and perhaps runny, again body interaction being the deciding factor.

by Tom Buck