Diagnosing Glaze Blisters, Part 4: Glaze Conditions

In addition to kiln firing and clay body factors, conditions in the glaze itself can lead to blisters. Bubbles forming in leadless glazes are common. Some of the bubbles break the surface of the glaze and remain unhealed as blisters. When lead was used in glazes, it caused a strong reactive effect with other oxides and increased the release of glaze bubbles to create a smooth, blemish-free surface. Since lead is not a recommended glaze material, greater care must be taken in glaze formulation and application, as well as in kiln firing, to ensure a defect-free glaze surface.

High surface tension, high viscosity glazes that contain zirconium can trap escaping gases from other glaze materials, metallic coloring oxides, stains, gums and binders. This type of “stiff” glaze is less likely to heal itself of surface irregularities due to its inability to flow when molten.
Correction: Lower the percentage of zirconium in the glaze or substitute other opacifiers, such as titanium dioxide or tin oxide.

Any operation that violently agitates the wet glaze can introduce bubbles during the application, resulting in blistering as the glaze matures.
Correction: Mix the wet glaze carefully to prevent bubbles from forming. Rock the glaze bucket slightly until any remaining bubbles come to the surface, and then skim them off.

Glazes containing manganese dioxide, which decomposes at 1112ºF and liberates oxygen at 1976ºF, can exit as a gas through the molten glaze and cause a blister.
Correction: Slowing down the rate of heat increase in the 1112 to 1976ºF temperature range will allow the liberation of oxygen from the manganese.

A metallic coloring oxide, such as manganese, used in an underglaze wash or engobe can break down and release oxygen bubbles into the covering glaze to cause blisters.
Correction: Slowing down the rate of heat increase during the 1112 to 1976ºF temperature range will allow the liberation of oxygen from the manganese in the glaze.

Cobalt oxide in an underglaze or glaze, along with copper oxide and iron oxide in a reduction atmosphere, loses oxygen at 1652ºF, which can migrate through the glaze layer and cause a blister.
Correction: Slow down the rate of heat increase until 1652ºF to allow oxygen in the underglaze to dissipate.

Glazes containing an overload of metallic coloring oxides in reduction kiln atmospheres can cause blisters due to excessive fluxing of the glaze.
Correction: Decreasing the percentage of metallic coloring oxide and/or decreasing the amount of reduction atmosphere in the kiln will eliminate blistering.

Contamination of the glaze with materials like silicon carbide, wood, rust, salt or other pottery shop materials can cause blisters.
Correction: Carefully clean and maintain the pottery shop, tools, equipment and supplies. Always sieve the wet glaze before application to remove any unwanted particles.

A delay in the second glaze application can result in insufficient bonding of glaze layers, causing blisters.
Correction: Try applying the second glaze application while the first layer is slightly damp.

Overlapping glazes can have a eutectic effect where a combination of oxides increases the melting action of both glazes.
Correction: Before committing to production, test every overlap glaze combination on vertical test tiles to determine compatibility.

Glaze sprayed with excessive pressure or spraying wet glaze on wet glaze can break the glaze bond with the clay body or other glaze layers, causing blisters in the fired ware.
Correction: Spray the glaze with less pressure and/or move the spray gun back from the pottery surface. Spray the glaze only when the surface is slightly damp or dry. Never spray the glaze on a wet surface.

Excessively thick glaze applications can delay the time for bubbles to reach the glaze surface. Once bubbles are at the surface, the firing cycle might already be completed, leaving a blister.
Correction: Many glazes can be applied more thinly, resulting in acceptable surface texture, opacity and color.

Extremely fine raw materials in a glaze and/or over-ball-milling of the glaze increase soluble salts found in some frits. Over-grinding of frits can cause hydration and subsequent water release during glaze maturation, resulting in blistering.
Correction: Reduce ball milling time and coarser grind raw materials in the glaze batch.

Over-fluxed glazes and/or low-temperature fluxes in high-temperature glazes can blister due to excessive melting or the lower melting oxides “boiling” off when the glaze matures.
Correction: Reduce the percentage of flux in the glaze and use the appropriate flux for the glaze temperature.

Incompatible glazes placed too close together can release fumes during firing, causing the glazes to blister.
Correction: Increase the separation of incompatible glazes in the kiln.

Lead in glazes or frits in reduction kiln atmospheres can blister due to the removal of the oxygen component in the lead.
Correction: Remove lead and lead frits from the glaze, and use a non-lead frit or other appropriate fluxes.

An excessive amount of medium used in underglazes, engobes, glazes or overglazes, such as oil, organic gum binders, gum arabic, glue, CMCor Vee Gum CER, can ferment and cause gas bubbles exiting as blistering in the glaze layer. The rate of fermentation, if any, is in part determined by the wet storage life of the materials, storage temperature, water pH and organic materials in the mixture.
Correction: Use less medium and keep wet mixtures in cooler storage areas.

The glaze viscosity in the fluid state can promote blisters. High-viscosity glazes (stiff glazes) can trap bubbles, which break at the surface and form blisters.
Correction: Lowering the viscosity by increasing the time to maturity or firing the glaze to a higher temperature will increase the flowing characteristics, allowing for any bubbles to rise to the surface, break and heal. Also, increasing the flux content of the glaze will allow it to flow when mature.

Other Considerations
Chemical water in glaze materials driven off between 842 and 932ºF, and the decomposition of clays and organic materials between 1044 and 1652ºF, can release gases into the forming glaze. Other commonly used glaze materials, such as barium carbonate, strontium, carbonate, talc, zinc oxide, manganese dioxide, manganese carbonate, nickel oxide, nickel carbonate, cobalt oxide, cobalt carbonate, rutile, iron oxide, dolomite, crocus martis, Cornwall stone, fluorspar and whiting, are also capable of releasing gases or chemically combined water. This is the case with some frits that travel through the molten glaze and cause blisters. For example, whiting (calcium carbonate) is a widely used glaze material, but it loses over 40% of its weight above 1650ºF, releasing carbon dioxide gas into the melting glaze.

When heated, feldspars release gases, most likely generated by the decomposition of impurities within the material. Soda feldspars such as Minspar 200, Kona F-4, NC-4 and closely associated nepheline syenite, release small bubbles that can be trapped in the glaze. These bubbles often exit on the surface, sometimes as a blister. Potash feldspars such as Custer feldspar, G-200 and Primas P, can release larger bubbles into the glaze.2 Alkali and zirconium-based glazes can be highly viscous and stiff when mature, resulting in large bubbles that are trapped on the glaze surface.

Additionally, a rapid heat increase during the molten glaze period can dissociate gases that form a blister or many small clumped blisters. Glaze can go through a transition period when gases are released, causing bubbles in the glaze and blisters on the glaze surface.

A slower firing cycle will allow blisters to flatten and dissolve. Each glaze has appropriate time-to-temperature parameters that will produce a non-blistered glaze surface. A general rule is that ceramic materials offer better results when heated and cooled slowly, allowing mechanical and chemical water to leave the clay without excessive pressure and without cracking the ware.

Always allow the glaze to dry before placing it in the kiln. Slow increases in heat also allow for gases in raw materials to safely dissipate through the glaze layer. All of the ceramic raw materials listed have been successfully used in clay body and glaze formulas, provided the appropriate heating and cooling cycle is employed in the kiln.

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