High Fire Glazes

Grid sets that contain glazes suitable for high fire.

These sets contain glazes that are suitable for around 1300 deg C. or around cone 10... plus or minus 50 deg C at least, or a couple of cones. Many of the glazes on these tiles will be better at higher or lower firings, to be determined by the experimenter. Although there is usually a best firing range for a particular glaze, there is no best temperature for any grid tile; any grid may be fired over a range of at least 100 deg. C, and at different temperatures, different glazes will reach their best result.

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This set shows some typical features of high calcium glazes. In the bottom left quarter (high in fluxes) the glazes are quite runny in this stoneware reduction firing. They exhibit many of the characteristics of wood-ash glazes (also high-calcium), including the rivulets of glaze on a vertical surface called webbing or stringing. The top left quarter (high alumina) glazes show a tendency towards colour-break, breaking to the brownish colour where the glazes are applied thinly. This type of high calcium alumina matt has been exploited for decades in a range of colour-break glazes, especially with colourants iron (earthy red breaking to yellow) and cobalt (blue breaking to yellow). The bottom right quarter (high silica) shows some typical high silica effects: opaque glazes and opalescent blue glazes. The crystals in glaze number 34 seem to be typical of some high calcium glazes; the effect requires low alumina and the right amount of silica.

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Showing the effect of a high percentage of light magnesium carbonate in the grid. One can use 40 or 50 % of light magnesium carbonate in the C-corner, with the balance of the fluxes being frit for mid-fire or nepheline syenite for high-fire. Add other fluxes and additives like colouring oxides and rutile if you wish.