Two transparents having opposite melt fluidity/surface tension balances

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.

Related Pictures

High and low surface tension Frits

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Fired at 1850F. Frit F-524 has run off the tester since 1850F. Frit 3602 is lead bisilicate.

Surface tension differences between two glazes

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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.

The perfect storm of high surface tension and high LOI: Blisters.

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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.

Videos

Links

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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