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The two cone 04 glazes on the right have the same chemistry but the center one sources it's CaO from 12% calcium carbonate and ulexite (the other from Gerstley Borate). The glaze on the far left? It is almost bubble free yet it has 27% calcium carbonate. Why? It is fired to cone 6. At lower temperatures carbonates and hydrates (in body and glaze) are more likely to form gas bubbles because that is where they are decomposing (into the oxides that stay around and build the glass and the ones that are escaping as a gas). By cone 6 the bubbles have had lots of time to clear.
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 |
Decomposition
In ceramic manufacture, knowing about the how and when materials decompose during firing is important in production troubleshooting and optimization |
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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. |
Materials |
Calcium Carbonate
In ceramics, calcium carbonate is primarily a source of CaO in raw stoneware and porcelain glazes. |
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