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This cone 04 flow tester compares two commercial low-fire transparent glazes. Their different approaches to the chemistry are revealed by these melt flows. While 3825B appears to have a higher melt fluidity, its higher surface tension is the real story. This is demonstrated by how the flow meets the runway at a perpendicular angle. Notice that A, by contrast, meanders down the runway in a broad, flat and relatively bubble-free river. Low-fire glazes must pass many more bubbles than their high-temperature counterparts, the low surface tension of A aids in that. A is Amaco LG-10. B is Crysanthos SG213 (Spectrum 700 behaves similarly, although flowing less). Both have advantages and disadvantages and are worth testing in your application.
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Fired at 1850F. Frit F-524 has run off the tester since 1850F. Frit 3602 is lead bisilicate.
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Both are low fire transparents. In a melt fluidity test they flow in a similar fashion. But here, where a 10 gram ball has melted down onto the tile, differences in surface tension are clearly evident by the angle at which the edge of the glaze meets the tile.
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An example of how calcium carbonate can cause blistering as it decomposes during firing. This is a cone 6 Ferro Frit 3249 based transparent (G2867) with 15% calcium carbonate added (there is no blistering without it). Calcium carbonate has a very high loss on ignition (LOI) and for this glaze, the gases of its decomposition are coming out at the wrong time. While there likely exists a firing schedule that takes this into account and could mature it to a perfect surface, the glaze is high in MgO, it has a high surface tension. That is likely enabling bubbles to form and hold better.
Glossary |
Surface Tension
In ceramics, surface tension is discussed in two contexts: The glaze melt and the glaze suspension. In both, the quality of the glaze surface is impacted. |
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 |
Terra Cotta
A type of red firing pottery. Terra cotta clay is available almost everywhere, it is fired at low temperatures. But quality is deceptively difficult to achieve. |
Glossary |
Transparent Glazes
Every glossy ceramic glaze is actually a base transparent with added opacifiers and colorants. So understand how to make a good transparent, then build other glazes on it. |
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 |
Troubles |
Glaze Blisters
Questions and suggestions to help you reason out the real cause of ceramic glaze blistering and bubbling problems and work out a solution |
Materials |
Ulexite
A natural source of boron, it melts at a very low temperature to a clear glass. |
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