Techno File: Glaze Blisters Jeff Zamek

Appears in the February 2024 issue of Ceramics Monthly.

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

• Overfiring can result when any glaze is taken past its maturation temperature and lowermelting-point oxides within the glaze volatilize. Correction: Fire the glaze one or two cones lower to bring it into its maturing range.
• Excessively long firing in the glaze maturing range can cause volatilization of oxides, resulting in blisters. A longer time to temperature imparts additional heat work in the glaze even if it is taken to its correct maturating temperature. Correction: Shorten the firing cycle while still firing the glaze to its maturing range.
• An excessively long cooling cycle in the glaze kiln contributes more heat work when the glaze is in the molten state, causing oxides to boil in the liquid glaze. Similar results can occur in over-insulated kilns that allow the glaze to remain in its maturing range for extreme periods of time. Correction: Upon reaching temperature, pull the damper out and unblock the secondary burner ports for a short time to cool the kiln faster.
• Down firing the kiln, or leaving burners or electric elements on after the glaze has reached maturity exposes it to excessive heat work when molten. Correction: In most instances, it is not necessary to down fire a kiln to achieve a stable glaze.
• Fast firing leaves blisters in the glaze. Some glazes go through a heating period when they boil and blister on their way to maturity. If this interval is too short, blisters are frozen in place and do not heal. Fast firing can also trap mechanical and chemical water locked in the glaze materials, which are not completely driven off until above 932°F (500°C). Correction: Extend the length of time to reach the end-point temperature.
• Firing the glaze below its maturation range can leave a dry, pale color, or blistering in the glaze surface. Correction: Fire the glaze to its correct maturing range.
• Fast firing of the bisque kiln can trap organic materials in the clay, which can then volatilize during the glaze firing. The gas exits through the stiff liquid glaze causing a blister. Correction: A longer bisque-firing cycle will enable organic material to escape.
• Non-oxidation bisque firing can trap organic material in the clay, which exits at higher temperatures as a gas through the molten glaze as a blister. Correction: In hydrocarbon-fueled kilns, always use more air than fuel to create an oxidation atmosphere. In electric kilns, use a venting system to remove organic matter from the kiln atmosphere.
• Direct flame impingement an result in an over-fired and/or over-reduced area on a glaze causing a blister. Correction: Move pieces away from the heat source to stop over reduction and over-fired areas on the glaze.
• Early and/or too heavy reduction in the glaze kiln can trap organic material in the clay or add carbon through excessive fuel introduction. Carbon trapped in the clay body can release at higher temperatures as a gas through the molten glaze causing a blister. Correction: Use an excess of air-to-fuel ratio in the burners until 1860°F (1916°C) to remove organic matter from the clay body, then use a slightly reducing atmosphere until the end-point temperature is reached.
• A loosely stacked glaze kiln reduces thermal mass and subsequent radiant heat in the transmission to the ware. Correction: Densely stack the kiln to produce slower increases and decreases of temperature while radiating more heat between pieces, kiln shelves, and posts.
• A kiln atmosphere with no movement (most prevalent in electric kilns) can allow a saturation of volatile glaze materials resulting in blisters. Correction: Use a venting system in electric kilns to circulate the kiln atmosphere. In hydrocarbon-fueled kilns, change damper settings and primary and secondary air intake at the burner ports to increase the kiln atmosphere movement.

Clay Body Conditions

• Higher than normal levels of organic material not removed from the clay during bisque firing. Periodically, some clays (notably fireclays) can contain high percentages of organic material. In such instances, a typical bisque-firing cycle will not remove all the organic material from the clay. During the subsequent glaze firing, organic material carbonizes and releases as a gas through the clay body into the molten glaze causing a blister. Correction: A clean oxidization atmosphere in the bisque kiln, fired to the correct temperature in enough time, will release organic material from the clay.
• Clay bodies containing high percentages of plastic clays, when raw glazed, can trap organic material. As the covering glaze vitrifies during firing, the resulting carbonaceous gas in the clay exits through the glaze causing a blister. Correction: Substitute coarser for finer particle clays to open the clay body and help release trapped organic material.
• Grog exposed in the clay body during the trimming process can cause glaze contraction around the particles leaving air pockets and eventual blisters in the glaze. Correction: Less surface area will be exposed when using a finer-mesh grog. The grog can also be pushed down into the clay during the trimming process.
• Clay slip or engobe applied to once-fired or bisque-fired pieces can release mechanical and chemical water, which can turn into a gas exiting through the covering glaze layer. Correction: The amount of water used in mixing can be reduced by the addition of small percentages of a deflocculant. Also, slowing down the rate of heat increase until 1112°F (600°C) is reached will allow mechanical and chemical water to escape through the glaze layer.
• Raw glazing an unfired clay body can drastically increase its absorbency. When glaze is applied it can be drawn into the clay body too rapidly, causing bubbles and air pockets as the glaze dries. During firing the bubbles migrate to the surface causing a blister. Correction: The use of gums such as CMC, Vee Gum CER, or other binders (0.125–2% added to the dry weight of the glaze) can slow down the drying rate of the glaze, preventing fast absorption.
• Raw glazing can trap organic material and/or moisture in the clay body or engobe, which at higher temperatures exits as a gas through the glaze layer. Correction: Slowing down the rate of heat increase in the 572–1292°F (300–700°C) range can safely release volatile organic materials and moisture from the clay body.
• Soluble salts in the clay body can migrate to the surface as the clay dries, leaving a disruptive layer of sulfates releasing gas into the covering molten glaze. Correction: The addition of barium carbonate (0.25– 2% based on the dry weight of the clay body) can neutralize soluble salt migration.
• Thin-walled ware saturated by water during spraying, dipping, or painting during glaze application. Trapped moisture on the clay surface can be released as a vapor during glaze firing causing a blister. Correction: Use less water in the glaze batch, and wait until the first glaze layer dries before applying another.
• Low bisque firing can yield extremely absorbent ware that sucks in wet glaze. If the glaze is highly viscous, air pockets formed in the application process can migrate to the surface leaving blisters in the stiff glaze. Correction: Increasing the bisque firing by one or two cones will decrease the absorbency of the pottery.
• Warm glaze on a cold bisque pot can trap air in the glaze layer causing a blister during firing. Correction: Warm the bisque pot before applying the glaze.

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.

• Bubbles forming in leadless glazes are common, with some breaking the surface and remaining 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 creating a smooth, blemish-free surface. Correction: Since lead is not a recommended glaze material, greater care must be taken in glaze formulation 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.
• A metallic coloring oxide such as manganese used in an underglaze wash or engobe can break down, releasing oxygen bubbles into the covering glaze and cause blisters. Correction: Slowing down the rate of heat increase during the 1112–1976°F (600–1080°C) 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 reduction atmosphere, loses oxygen at 1652°F (900°C), which can migrate through the glaze layer causing a blister. Correction: Slowing down the rate of heat increase until 1652°F (900°C) allows 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: Decrease the percentage of metallic coloring oxide and/or decrease the amount of reduction atmosphere in the kiln.
• Contamination of the glaze with materials such as silicon carbide, wood, rust, or salt can cause blisters. Correction: Carefully clean and maintain the pottery shop, tools, equipment, and supplies. Always sieve the wet glaze before application, as this will remove any unwanted particles.
• A delay in the second glaze application can result in insufficient bonding of glaze layers, resulting in 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: Test every overlap glaze combination on vertical test tiles to determine compatibility.
• Glaze spayed 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.
• Extremely thick glaze application can result in a blister. In thicker glazes, any bubbles that form take longer to reach the surface. Correction: Try successively thinner glaze applications.
• 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.
• An excessive amount of medium used in underglazes, engobes, or over glazes, such as oil, organic gum binders, gum arabic, glue, CMC, or Vee Gum CER, can ferment, causing 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 forming 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.
• Excessively thick glaze applications can delay the time for bubbles to reach the glaze surface. Once bubbles are at the surface the firing cycle can already be completed leaving a blister. Correction: Test the application of glazes more thinly to obtain the desired surface texture, opacity, and color.

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.