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.

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These two transparent glazes are opposites:

In melt fluidity and surface tension

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This cone 04 flow tester compares two commercial low-fire transparent glazes. Their different chemistry strategies are revealed by the shape of these melt flows. While 3825B appears to have the higher melt fluidity, it also has much higher melt surface tension. This is evident in the narrow, rope-like stream and the way the flow meets the runway at a high angle before pulling into a rounded bead. A, by contrast, spreads and wets the runway, meandering downward in a broad, flat and relatively bubble-free river.

This difference is important in low-fire ware because these glazes must pass far more gases and bubbles than high-temperature glazes. The lower surface tension of A aids bubble release and healing after bubbles break. A is Amaco LG-10. B is Crysanthos SG213 (Spectrum 700 behaves similarly, although flowing less). Both approaches have advantages and disadvantages and are worth testing in your application.

Surface tension differences between two glazes:

This simple test shows them best

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Both are low-fire transparents. In a melt fluidity (GBMF test) they flow similarly. But here, where the 10 gram ball of the dry glaze is simply melted down onto a square tile (a GBMF test), differences in surface tension are more 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 a fast firing in our electric kiln. This is a cone 6 borosilicate glaze with 15% calcium carbonate added (there is no blistering without it). Calcium carbonate has a very high loss on ignition (LOI), and for this early-melting glaze, the gases of its decomposition are still escaping after melting begins. Another factor is also involved: Although the glaze has good melt fluidity, bubbles survived till near the end of the firing, resisting rupture (likely because of the high surface tension of the melt). When the bubbles finally did burst, there was inadequate time for healing to occur.

The difference that caused blistering: Firing schedule!

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These are the same glaze, same thickness, Ulexite-based G2931B glaze, fired to cone 03 on a terra cotta body. The one on the right was fired from 1850F to 1950F at 100F/hr, then soaked 15 minutes and shut off. The problem is surface tension. Like soapy water, when this glaze reaches cone 03 the melt is quite fluid. Since there is decomposition happening within the body, gases being generated vent out through surface pores and blow bubbles. I could soak at cone 03 as long as I wanted and the bubbles would just sit there. The one on the left was fired to 100F below cone 03, soaked half an hour (to clear micro-bubble clouds), then at 108F/hr to cone 03 and soaked 30 minutes, then control-cooled at 108F/hr to 1500F. During this cool, at some point well below cone 03, the increasing viscosity of the melt becomes sufficient to overcome the surface tension and break the bubbles. If that point is not traversed too quickly, the glaze has a chance to smooth out (using whatever remaining fluidity the melt has). Ideally I should identify exactly where that is and soak there for a while.

Carbonate gassing can cause glaze blisters

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An example of how a carbonate can cause blistering. Carbonates produce gases during decomposition. This glaze (G2415B) contains 10% lithium carbonate, which likely pushes the initial melting temperature down toward the most active decomposition temperatures.

Blistering in a cone 6 white variegated glaze. Why?

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This glaze creates the opaque-with-clear effect shown (at cone 7R) because it has a highly fluid melt that thins it on contours. It is over fired. On purpose. That comes with consequences. Look at the recipe, it has no clay at all! Clay supplies Al₂O₃ to glaze melts, it stabilizes it against running off the ware (this glaze is sourcing some Al₂O₃ from the feldspar, but not enough). That is why 99% of studio glazes contain clay (both to suspend the slurry and stabilize the melt). Clay could likely be added to this to increase the Al₂O₃ enough so the blisters would be less likely (it would be at the cost of some aesthetics, but likely a compromise is possible). There is another solution: A drop-and-soak firing. See the link below to learn more. One more observation: Look how high the LOI is. Couple that with the high boron, which melts it early, and you have a fluid glaze melt resembling an Aero chocolate bar!

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High and low melt surface tension: As demonstrated by two pure frits

Why do gummed dipping glazes do this as they dry? How to fix it.

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