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Some glaze defects can be very difficult to diagnose. Often a potter thinks they have corrected the fault only to discover it continues to surface in subsequent firings. One such defect is glaze blistering.
Defining the Terms
Chemical Water: Water that is chemically bound to raw materials used in the clay body or glaze. For example, the chemical formula for clay is 1 Al203•2 Si02•2 H20. The H20 is chemically bound water.
End Point Temperature: The highest temperature reached in a kiln firing.
Glaze Blister or Boil: Sharp round crater defects found in a fired glaze.
Hydrocarbon-Fueled Kilns: A kiln fired by combustible fuel such as wood, coal, propane, or natural gas.
Mechanical Water: Water required for lubrication in a forming process such as wheel throwing, handbuilding, or jiggering.
Diagnosing Glaze Blisters
A blister appears as a pronounced, sharp-edged, burst bubble similar to a crater on the fired glaze surface, often revealing the underlying clay body. The reason behind glaze blistering can really tax a potter’s investigative abilities. Any exploration into this common defect will require an analysis of kiln firing, clay body, and glaze conditions. In most instances, several possible causes have to be examined and eventually eliminated to arrive at the exact point of origin. The priority is to accurately diagnose the problem and then determine what incident or series of events caused it. Only then will it be possible to enact the appropriate correction.
Possible Causes for Glazes Blisters
Kiln Firing Conditions
Clay Body Conditions
Glaze Conditions
Chemical water in glaze materials driven off between 842–932°F (450–500°C), and the decomposition of clays and organic materials between 1044–1652°F (562–900°C), 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, which travel through the molten glaze causing blisters.
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, often exiting 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.
Alkali- and zirconium-based glazes can be highly viscous and stiff when mature, resulting in large bubbles that are trapped on the glaze surface.
A rapid heat increase during the molten glaze period can dissociate gases, which 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. Correction: A slower firing cycle allows blisters to flatten and dissolve. Each glaze has appropriate time-to-temperature parameters that will produce a non-blistered glaze surface. Slow increases in heat also allow for gases in raw materials to safely dissipate through the glaze layer.
Asking Questions Can Yield Answers
When confronted with any kind of defect, it is important to determine the point of origin and then apply the appropriate adjustment(s). It is essential to have a systemic approach to isolate the actual factor(s) causing blistering. Specifically, there are several questions the potter can ask to isolate glaze blistering:
Does the blister glaze heal when fired again? Generally, if the glaze can be re-fired successfully, it should have been fired longer (more heat work) during the first glaze firing.
Are different glaze formulas in the same kiln blistered? The problem probably originates in the firing procedures, glaze-mixing errors, or from a common raw glaze material.
Are the blisters only on one side of the pot? If so, direct flame impingement might cause an over-fired area and/or an over-reduced area in hydrocarbon-fueled kilns.
Are the blisters only on overlapping glaze surfaces? Incompatible glazes when overlapped can have a eutectic effect, which can result in over-fluxed areas and blisters.
Are the blisters only on horizontal surfaces? High surface tension glazes with high viscosity are stiff and do not move when molten. Gravity on the vertical molten glaze pulls down causing the formed blister to heal. Another possible cause occurs when flat pots are placed directly on the kiln shelf. If the glaze is not formulated or fired correctly the radiant heat from the shelf during cooling can cause it to remain in its maturity range longer causing a blister.
Are the blisters only on the edges or high areas of the pots? Fast cooling of the kiln and/or pottery loosely stacked can freeze the glaze in its maturation process.
Are blisters present only in one kiln and not in others? This could be an indication of an error in kiln firing.
Are blisters present in only one part of the kiln? Check for direct heat source impingement or kiln atmosphere irregularities.
Are blisters present on one clay body and not another? Check the level of organic material in the clay body causing the blisters.
Footnotes:
1. Cullen W. Parmelee, Ceramic Glazes, third edition, (Boston: Cahners Books, 1973), 580.
2. Parmelee, 580.
3. W.G. Lawrence, Ceramic Science for the Potter, (New York: Chilton Book Company, 1972), 116.
4. Frank and Janet Hamer, The Potter’s Dictionary of Materials and Techniques, Fourth Edition,
(London: A&C Black/ University of Pennsylvania Press,1997), 27.
5. Richard A. Eppler, Understanding Glazes, (Westerville, Ohio: The American Ceramics Society,
2005), 250.
6. Eppler, 250.
7. Lawrence, 114.
the author Jeff Zamek started his career 48 years ago. He obtained BFA/MFA degrees in ceramics from Alfred University, College of Ceramics, New York. In 1980, he started Ceramics Consulting Services, a ceramics-consulting firm developing clay body and glaze formulas for ceramics supply companies throughout the US. His books, The Potter’s Studio Clay & Glaze Handbook, What Every Potter Should Know, Safety in the Ceramics Studio, and The Potters Health & Safety Questionnaire are available from Jeff Zamek/Ceramics Consulting Services. For technical information, visit www.jeffzamek.com.
Unfamiliar with any terms in this article? Browse our glossary of pottery terms!
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