'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).
Why you should not paint pure stain powders over glaze
On the left is a pure blue stain, on the right a green one. Obviously, the green is much more refractory. On the other hand, the green just sits on the surface as a dry, unmelted layer. For this type of work, stains need to be mixed into a glaze-like recipe of compatible chemistry (a medium) to create a good, paintable color. The blue is powerful, it would only need to comprise 5-10% of the recipe total. Its medium would need to have a stiffer melt (so the cobalt fluxes it to the desired degree of melt fluidity). A higher percentage of the green stain is needed, perhaps double. It's medium needs much more melt fluidity since the stain is refractory. Of course, only repeated testing would get them just right. Guidelines of the stain manufacturer for chemistry compatibility need to be consulted also (as certain stains will not develop their color unless their glaze medium host has a compatible chemistry). And, to be as paintable as possible, use use a gum-solution/water mix (e.g. 2 parts water to one part gum solution).
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