Q&A: Diagnosing Glaze Blisters.
POSSIBLE CAUSES OF GLAZE BLISTERS
KILN FIRING CONDITIONS
Over-firing can result when any glaze is taken past its maturation temperature, and lower melting point oxides within the glaze volatize. The effect is similar to water taken past its boiling point. Correction: Firing the glaze one or two cones lower will 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 which allow the glaze to remain in its maturing range for extreme periods of time. Correction: Long cooling cycles are more prevalent in hydrocarbon fueled kilns (natural gas, propane, wood, oil, sawdust) which tend to be better insulated and larger in size, having more thermal mass than electric kilns. Upon reaching temperature, pulling the damper out and unblocking the secondary burner ports for a short time will 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. However, if a particular glaze requires down firing, progressively shortening the down firing interval will decrease its time in the maturing range.
Fast-firing leaves blisters in the glaze that would have healed in a longer firing. 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[degrees]F. 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 volatize during the glaze firing. The gas exits through the stiffliquid 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 causing a blister. Large platters stacked together or tiles placed atop one another do not allow for combustion and removal of organic material due to their relatively large surface areas touching. Correction: In hydrocarbon fueled kilns always use more air than fuel to create an oxidation atmosphere. In electric kilns, the use of an active venting system removes organic matter from the kiln atmosphere.
Direct flame impingement can result in an over-fired and/or over-reduced area on a glaze causing a blister. Correction: Moving pottery away from the heat source will 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[o]F as it will 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 pottery. Correction: A densely stacked kiln can produce slower increases and decreases of temperature while radiating more heat between pottery, kiln shelves and posts. A densely packed kiln is a factor which can apply more heat work to the glaze, which liberates gases trapped in the glaze.
A kiln atmosphere with no movement -most prevalent in electric kilns-can allow a saturation of volatile glaze materials resulting in blisters. Correction: In electric kilns an active venting system can circulate the kiln atmosphere. In hydrocarbon fueled kilns changing damper settings and primary and secondary air intake at the burner ports will 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 fireclay -can contain abnormally high percentages of organic material. In such instances a 'normal' 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. During the firing, when the covering glaze vitrifies the resulting carbonaceous gas in the clay exits through the glaze causing a blister. Correction: Substituting coarser for finer particle clays will 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 (engobe) applied to once-fired or bisque pottery 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 an engobe can be reduced by the addition of small percentages of a deflocculant (such as sodium silicate, Darvan #7 or Darvan 811). Also, slowing down the rate of heat increase until 1112[degrees]F is reached will allow mechanical and chemical water to escape through the glaze layer.
Raw glazing an unf ired 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 (carbonxymethylcellulose), Veegum[R] CER or other binders (1/8% to 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[degrees]F to 1292[degrees]F 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 (1/4% to 2% based on the dry weight of the clay body) can neutralize soluble salt migration.
Thin-walled pottery 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: Less water used in the glaze batch, and waiting until the first glaze layer dries before applying another will prevent blisters.
Granular manganese added to the clay and naturally occurring nodules of manganese decompose at 1112[o]F and liberate oxygen at 1976[o]F which can exit as a gas through the molten glaze causing a blister. Correction: Decreasing the rate of heat increase in the 1112[degrees]F to the 1976[degrees]F temperature range can allow the slow release of oxygen through the glaze layer.
Low bisque firing can yield extremely absorbent ware that 'sucks' in the 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. Also, gums such as CMC (carbonxymethylcellulose), Veegum9 CER or other binders added to the glaze (1/8% to 2% based on the dry weight of the glaze) can slow down the drying rate of the glaze.
Warm glaze on a cold bisque pot can trap air in the glaze layer causing a blister during firing. Correction: Match the temperature of the glaze and pot before applying the glaze.
Contamination of the bisque clay body from the breakdown and disintegration of organic materials such as sponges used to clean the bisque before glazing. Correction: Use a clean source of water, tools and sponges.
Contamination in the clay from plaster molds or deteriorating wedging boards can impart plaster chips into the moist clay which, upon heating, release gas and/or water vapor in the covering glaze layer. Correction: Covering the wedging board with canvas will prevent chips from entering the clay. Mixing plaster with the correct ratio of water will ensure maximum set strength. Discarding molds that show signs of wear can prevent plaster contamination in the moist clay.
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 and application and 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: Lowering the percentage of zirconium in the glaze or substituting other opacifters 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 1112[degrees]F and liberates oxygen at 1976[degrees]F, can exit as a gas through the molten glaze causing a blister. Correction: Slowing down the rate of heat increase in the 1112[degrees]F to 1976[degrees]F 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 releasing oxygen bubbles into the covering glaze and cause blisters. Correction: Slowing down the rate of heat increase during the 1112[degrees]F to 1976[degrees]F temperature range will allow the liberation of oxygenfrom 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 1652[degrees]F which can migrate through the glaze layer causing a blister. Correction: Slowing down the rate of heat increase until 1652[degrees]Fallows 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 such as 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, 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.
Over-lapping 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 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.
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.
Chemical water in glaze materials driven off between 842[degrees]F and 932[degrees]F, and the decomposition of clays and organic materials between 1044[degrees]F and 1652[degrees]F, 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, as is the case with some frits which travel through the molten glaze causing blisters. For example, whiting (calcium carbonate) is a widely used glaze material but it loses over 40% of its weight above 1650[degrees]F, releasing carbon dioxide gas into the melting glaze.
Feldspars, when heated, 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 which 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 he highly viscous and stiff when mature, resulting in large bubbles which are trapped on the glaze surface.
Additionally, 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 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 the ceramic raw materials listed have been used in clay body and glaze formulas provided the appropriate heating and cooling cycle is employed in the kiln.
An excessive amount of medium used in underglazes, engobes, glazes or overglazes, such as oil, organic gum binders, gum arabic, glue, CMC or Veegum[R] 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. Also, increasing the flux content of the glaze will allow it to flow when mature.
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: Many glazes can be applied more thinly resulting in acceptable surface texture, opacity and color.
DIAGNOSING THE POTENTIAL CAUSES OF GLAZE BLISTERING
Asking Questions Can Yield Answers
When confronted with any kind of defect, it is important to determine the point of origin then apply the appropriate adjustment(s). Obviously, if the wrong correction is enacted-aside from being ineffective-there is a loss of time and labor, so it is imperative to diagnose the defect correctly. 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. The answers will offer guidelines to determine the appropriate correction.
Questions to Ask
Does the blister glaze heal when fired again? A general rule that can apply to any glaze defect: if the glaze can be re-fired successfully, it should have been fired longer during the first glaze firing. The second firing supplies more heat work to the glaze which can bring it into a defect-free configuration.
Are different glaze formulas in the same kiln blistered? The problem probably originates in the firing procedures, glaze mixing errors, or 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 with resulting 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 shelves. If the glaze is not forrnulated or fired correctly, the radiant heat from kiln shelves upon 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 occurring only in one kiln and not in Others? This could be an indication of an error in kiln firing.
Are blisters occurring in only one part of the kiln? Check for direct heat source impingement or kiln atmosphere irregularities.
Are blisters presenting on one clay body and not another? Check the level of organic material in the clay body causing the blisters. Has the clay body been bisque fired long enough in an oxidation kiln atmosphere? If the clay body contains high levels of iron-bearing clays or iron oxide, it can be more reactive to extreme reduction in the glaze firing which can cause glaze blistering.
Are blisters presenting only on underglaze, engobe or overglaze areas? Check levels of gums and percentages of metallic coloring oxides used in the underglaze, engobe or overglaze formulas.
Does the glaze have high percentages of whiting or other raw material gas producing components? Statistically, whiting, calcium carbonate (CaC[O.sub.3]) is one of the leading causes of glaze blistering. Wollastonite, calcium silicate (CaO.Si[O.sub.2]) which dissolves mon readily in the molten glaze, can be substituted with an adjustment to the silica content of the glaze.
Are blisters presenting only on one color glaze and not on other color glazes that use the same base glaze formula? Some glazes have an excessive percentage of metallic coloring oxides and/or has the kiln atmosphere been too heavily reduced?
Are blisters presenting only after a new batch of glaze is used? It is best to carefully weigh a new batch of glaze and note any new raw materials used in the problem batch.
While the defect itself is clear (a sharp crater like void in the fired glaze surface) glaze blistering represents one of the most difficult problems to diagnose correctly due to its many possible causes. A little time spent investigating the cause will save a lot of time in resolving this frustrating glaze disruption.
Images courtesy of the author.
Written by Jeff Zamek
About the Author
Jeff Zamek walked into a pottery studio in 1967 and started his career as an amateur potter. After completing a degree in business from Monmouth University. W. Long Branch, NJ he obtained B.F.A./M.F.A. degrees in ceramics from Alfred University, College of Ceramics, NY. While there he developed the soda firing system at the college and went on to teach at Simon's Rock College and Keane College. During this time he earned his living as a professional potter. In 1980 he started Ceramics Consulting Services a ceramics-consulting firm developing clay body and glaze formulas for ceramics supply companies throughout the United States. He works with individual potters, ceramics companies, and industry offering technical advice on clays, glazes, kilns, raw materials, ceramic toxicology, and product development. He is a regular contributor to several ceramics magazines and technical journals. Jeff's books What Every Potter Should Know and Safety in the Ceramics Studio, featuring the safe handling of ceramic materials, and The Potters Health & Safety Questionnaire are available from Jeff Zamek/Ceramics Consulting Services. His latest book, The Potter's Studio Clay & Glaze Handbook, was published in June, 2009.
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|Date:||Jan 1, 2018|
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