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Both of these mugs were soaked 15 minutes at cone 6 (2200F), then cooled at 100F per hour to 2100F and soaked for 30 minutes and then cooled at 200F/hour to 1500F. This firing schedule was done to eliminate glaze defects like pinholes and blisters. Normally the GA6-A glaze crystallizes (devitrifies) heavily with this type of firing, but an addition of 1% tin oxide to the one on the left has prevented this behavior.
This is GA6-A Alberta Slip base glaze (80 Alberta Slip:20 Frit 3134) fired using the C6DHSC firing schedule (on Plainsman M390 iron red clay). If this is cooled rapidly or 1% tin oxide inhibitor is added, it fires to a glossy clear amber glass with no crystals. This base is needed to make GA6-C rutile blue, otherwise we recommend the GA6-B base (which uses frit 3195 instead), it has a lower thermal expansion and guarantees gloss regardless of cooling speed. If, on the other hand, you want to enhance this effect, just add a little more frit (to gloss it more) and more iron oxide (that should enable matting even at faster cooling speeds.
Materials |
Tin Oxide
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Glossary |
Crystallization
Ceramic glazes form crystals on cooling if the chemistry is right and the rate of cool is slow enough to permit molecular movement to the preferred orientation. |
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