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Glaze blisters are a surface defect in fired ceramic glazes. They have caused every potter and company grief at one time or another. The problem is often erratic in nature and counter intuitive measures may be needed to resolve it (we have a page in the trouble-shooting section dedicated to this defect). The blisters trace their origins to the generation of gases as particles in the body and glaze itself decompose during firing (loosing H2O, CO, CO2, SO2, etc). Many materials and mineral particles gas and they each have their own thermal history as they break down in the firing. Complicating the matter further, the body and glaze are composites of these. If the glaze has already begun melting while these gases are still being generated, bubbles grow within the glaze matrix. Depending on the viscosity and surface tension of the glaze melt and its thickness of application populations of small or large bubbles will increase within the glaze matrix and later begin breaking at the surface. Since bodies do not melt they can generate gases over an extended temperature range that extends all the way to the top of the firing temperature curve.

Strategies to deal with the problem can involve adjusting the chemistry of the glaze recipe to favor materials of lower LOI, favor chemistries that increase or decrease melting temperature, viscosity or surface tension or employ fluxes that melt later in the firing. Glaze thickness can be reduced. Body recipes can the changed to employ lower LOI materials, those having lower concentrations of gas-producing particles (like coal) or ones using materials of finer particle size. Firing curves can be adjusted to slow the rate of rise as the top of the curve approaches (even soaking at temperatures on the up ramp to give gases more time to escape), soak at the top, drop more slowly or even soak at a place on the down ramp (to give blisters a chance to heal where the glaze has a high enough viscosity that the surface tension holding the bubbles can be overcome).

Rutile blue glazes: Love the look, hate the trouble to make it

Rutile blue glazes: Love the look, hate the trouble to make it

A closeup of a cone 10R rutile blue (it is highlighted in the video: A Broken Glaze Meets Insight-Live and a Magic Material). Beautiful glazes like this, especially rutile blues, often have serious issues (like blistering, crazing), but they can be fixed.

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

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

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% CaO added (there is no blistering without the CaO). 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.

Serious blistering at low fire: How to fix it

Serious blistering at low fire: How to fix it

An extreme example of blistering in a piece fired at cone 03. The glaze is Ferro Frits 3195 and 3110 with 15% ball clay applied to a bisque piece. Is LOI the issue? No, this glaze has a low LOI. Low bisque? No, it was bisqued at cone 04. Thick glaze layer? Yes, partly. Holding the firing longer at temperature? No, I could hold this all night and the glaze would just percolate the whole time. Slow cooling? Close, but not quite. The secret I found to fix this was to apply the glaze in a thinner layer and drop-and-hold the temperature for 30 minutes at 100F below cone 03. Doing that increased the viscosity of the glaze melt to the point that it could break the blisters (held by surface tension) while still being fluid enough to smooth out the surface.

What can you do using glaze chemistry?

What can you do using glaze chemistry?

There is a direct relationship between the way ceramic glazes fire and their chemistry. Wrapping your mind around that and overcome your aversion to chemistry is a key to getting control of your glazes. You can fix problems like crazing, blistering, pinholing, settling, gelling, clouding, leaching, crawling, marking, scratching, powdering. Substitute frits or incorporate better, cheaper materials, replace no-longer-available ones (all while maintaining the same chemistry). Adjust melting temperature, gloss, surface character, color. Identify weaknesses in glazes to avoid problems. Create and optimize base glazes to work with difficult colors or stains and for special effects dependent on opacification, crystallization or variegation. Create glazes from scratch and use your own native materials in the highest possible percentage.

The difference: Firing schedule!

The difference: Firing schedule!

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.

Blistering in a cone 6 white variegated glaze. Why?

Blistering in a cone 6 white variegated glaze. Why?

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!

Out Bound Links

  • (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

  • (Glossary) Surface Tension

    Surface tension is of concern in ceramics because the behavior of a molten glaze is affected by this phenomenon. Glazes with low surface tension spread over the body surface and shed bubbles well. Glazes with high surface tension resist spreading out, resist releasing bubbles and can crawl. Surface ...

  • (Glossary) LOI

    Simplistically, LOI is the percentage of weight a material loses on firing. Assuming firing to a typical stoneware temperature of 1200C, the amount of weight loss can be surprising. Kaolins, for example, lose around 12% (mainly crystal-bound water). Ball clays lose about half of that (a combination ...

  • (Glossary) Decomposition

    Decomposition is the breaking of inter-molecule bonds during melting in the kiln. To understand it we need to understand elements, oxides, compounds, solutions and mixtures (from the chemistry jargon point-of-view). "Elements" are one kind of atom which cannot be broken down any further (except b...

  • (Glossary) Glaze Bubbles

    As glazes melt, gases from decomposition of organics, carbonates, sulphates and hydrates are generated (if the body was glazed green, or unbisqued, many more of these gases will be present). If glazes are already melting while the gases are being generated, bubbles form and suspend in the glass melt...

In Bound Links

By Tony Hansen

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