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This is a Gerstley Borate based recipe (45%) melted in crucibles at increasing temperatures. Although the recipe is well melted at cone 2, it is still not fluid enough to enable their migration in the time available. By contrast, the melt at the upper temperature is much less viscous, enabling all bubbles to completely clear on the thinner sections. If this glaze were applied to ware it would be in a thin layer and the bubbles would likely clear at cone 6. Not to be ignored is the degree to which the thousands of bubbles passing upward through the melt have helped to mix the melt and remove discontinuities in the cone 7 and 8 specimens.
This is a combination dolomite/barium matte. It has been fired at cone 10 reduction. It contains 17% barium carbonate and 17% dolomite (in a nepheline syenite base). Most carbonates decompose and gas off the CO2 well before the glaze melts, but not barium carbonate. It can turn the glaze matrix into an "aero chocolate bar" of bubbles. The glaze melt viscosity of some glazes, like this one, makes them vulnerable to preserving the bubbles as dimples or sharp-edged holes.
Glossary |
Glaze Bubbles
Suspended micro-bubbles in ceramic glazes affect their transparency and depth. Sometimes they add to to aesthetics. Often not. What causes them and what to do to remove them. |
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