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These melted-down-ten-gram GBMF test balls of glaze demonstrate the different ways in which tiny bubbles disrupt transparent glazes. These bubbles are generated during firing as particles in the body and glaze decompose. This test is a good way to compare bubble sizes and populations, they are a product of melt viscosity and surface tension. The glaze on the top left is the clearest but has the largest bubbles, these are the type that are most likely to leave surface defects (you can see dimples). At the same time its lack of micro-bubbles will make it the most transparent in thinner layers. The one on the bottom right has so many tiny bubbles that it has turned white. Even though it is not flowing as much it will have less surface defects. The one on the top right has both large bubbles and tinier ones but no clouds of micro-bubbles.
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This is a buff stoneware body, Plainsman M340. A L3954F black engobe was applied inside and upper outside at leather hard. The piece was fired at cone 6 using the PLC6DS schedule. The inside, totally clouded glaze, is G2926B. Outside is GA6-B Alberta Slip amber transparent. Normally, this inside glaze is crystal-clear on other bodies (and on this one without the black engobe). Clearly, the black stain in the engobe is generating tiny gas bubbles at the exact wrong time during the firing and the melt is unable to pass them. The outside glaze is on the same engobe, but the GA6-B glaze is demonstrating its ability to clear the micro-bubble clouding. It contains a lot of Alberta Slip, a material that is not finely ground like others. Particles across the range from 60-200 mesh are present, some of them appear to be acting as a fining agent to clear the bubbles.
Tests |
Glaze Melt Fluidity - Ball Test
A test where a 10-gram ball of dried glaze is fired on a porcelain tile to study its melt flow, surface character, bubble retention and surface tension. |
Troubles |
Clouding in Ceramic Glazes
There a many factors to deal with in your ceramic process to achieve transparent glazes that actually fire to a crystal-clear glass |
Glossary |
Melt Fluidity
Ceramic glazes melt and flow according to their chemistry, particle size and mineralogy. Observing and measuring the nature and amount of flow is important in understanding them. |
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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