Why do some pots have white crystals on their surface? If you are in ceramics long enough, eventually the effects of soluble salts in clay bodies and glazes will be observed.
Defining the Terms
Bloating: The fired clay body has internal bubbles, which can erupt on the surface. During the firing, organic matter forms, releasing gas that is trapped in the clay as it vitrifies.
Deflocculation: Adding dispersion agents (soluble alkalis, sodium silicate, soda ash), which cause clay platelets to repel each other, ensuring fluidity. The resulting slip mixture becomes fluid using more clay and less water.
Gel: A network of particles forming a thick semi-solid structure, causing glaze materials to remain in suspension due in part to the boron component in Gerstley borate.
Hygroscopic: A substance’s ability to attract and absorb water molecules from the surrounding environment, usually air, at room temperature. This reaction depends in part on several factors such as the degree of hygroscopy in a material, the amount of moisture in the atmosphere where stored, and the type of barrier sealing the material. When moisture is present in hygroscopic materials, they can clump together during storage.
Microcracks: Sub-visible cracks in a clay body or glaze, weakening the ceramic structure.
Solubility chemistry is often similar in both clays and glazes, but can also behave differently in each. Soluble salts are composed of many materials, each one having a different melting point, from 212°F (100°C) to 1950°F (1065°C).1 Often the defects they produce are difficult to identify and are sometimes uncorrectable. The variable nature of soluble materials and their seemingly random occurrence is a significant factor in resolving defects.
Appearance of Soluble Salts
In clay bodies, scumming or efflorescence presents itself as the evaporation of salts, resulting in dark halos, with white or gray crystal powder forming on the surface of the clay. Crystals can occur in the drying or bisque-firing stages. Iron-bearing clays can produce iron salts, leaving brown or black staining on the drying clay’s surface. Low-fired red clays and common red building bricks are highly susceptible to growing crystals. Such defects are most often found on edges or high points of the pottery or the exposed surface of bricks, which dry faster than other parts. The wicking action of evaporation deposits higher levels of soluble materials in these areas, disrupting the clay’s surface subsequent to firing.
Soluble Materials in Mining Operations
Water-soluble salts can be found in the mining and processing of clays. Different clay pits yield varying fluctuations of soluble materials. Clays can contain sulfates of calcium, magnesium, or sodium, along with chlorides such as potassium and sodium carbonates, all of which can eventually travel in the clay body water system. Hard water (dissolved calcium or magnesium from the water source) in the clay mining operation will also increase soluble salt migration. While mines try to minimize any variation in their product, the nature of the material sometimes overrides any quality control procedures. Bacterial growth during the clay mining and storage process can activate the movement of soluble salts in moist clays.
Soluble Materials in Clay Bodies
Water used in the clay mixing process dissolves soluble salts, which then progress to the clay body’s surface. During evaporation, crystals are formed. Surface crystals can also be an indication of material still contained within the clay body. Any movement of soluble materials happens before the clay is fired, but can also be visible after firing.
Additives such as soda ash, sodium silicate, Darvan #7, or #811 used for deflocculation in slip-casting bodies can introduce soluble materials due to their sodium components.2 Bacterial action in aging moist clay can also increase soluble salt amounts and subsequent migration. All of these factors can be a delayed response at first, but eventually, a build-up of soluble materials is noticeable.
Soluble salts can act as a strong flux or melting agent in localized areas of the drying clay body, causing matte areas or glossy spots. Microcracks due to crystal growth cause internal pressure, weakening glaze adhesion. Additionally, crystal growth can yield different rates of shrinkage in the clay. The differential shrinkage rate between crystal and non-crystal areas can cause cracking or warping on the edges of functional pottery or sculpture.
Any disruption of the clay-body surface can produce glaze defects due to underlying soluble-salt deposits. Defects include:
crawling: the fired glaze pulls back from the clay body, much like water on a glass surface
pinholing: the fired glaze has a rounded edge to a conical hole in the clay body
glaze flaking: the fired glaze peels off like a paint chip
blistering: the fired glaze is over-fluxed, resulting in a sharp-edged crater
In fired ware, any soluble material functioning as a flux has already melted, leaving “burnt” areas, surface discoloration, or blisters. Other defects are poor glaze adhesion, drying cracks, and glaze color shifts. Often noted are glaze surface blisters and weak rims on cups or bowls. Such defects are caused by the migration and high concentration of soluble materials drawn to these areas. If any soluble material disrupts the interface layer between the clay and the glaze, it is a potential cause of glaze defects.
Trapped Soluble Materials
Faster drying times can draw greater concentrations of soluble materials to the clay-body surface. Conversely, in some instances, slow, even drying can lessen surface migration. The actual amounts of soluble material can increase or decrease depending on drying cycles. However, soluble materials can still be present within the clay body, acting as a flux when fired to higher temperatures. The localized decomposition of soluble materials can result in bloating and black coring.
Effects of Soluble Salts in Clay Bodies
In the bone-dry or bisque state, fingerprints can disturb fragile soluble-salt deposits on the clay surface, causing a noticeable flashing or discoloration on the fired clay. High concentrations of soluble salts on bisque clay surfaces can result in fused areas, which stunt absorption of glaze and result in uneven glaze deposits. It can also create a mechanical disruption of the clay body/glaze interface, which can result in the glaze crawling due to the fluxing action of the soluble salt on the clay body. The relatively low melting point of soluble salts can obstruct the release of organic material in the clay, resulting in carbon trapping at higher temperatures. The melting action of soluble salts on the clay surface can also fuse pottery to kiln shelves.
Any clay body, whether the potter mixes their own clay or buys commercially mixed clays, can produce a defect-free batch of clay many times, but not the next. This is due to the specific level of soluble salts found in one or more of the clays used in the body. In most instances, soluble salt deposits on the clay body surface do not come from water used in the clay mixing process—provided the water does not originate from a static source, which, through evaporation, can concentrate salts found in the water. Ceramics suppliers or companies that mix clay are often not always aware or cannot test for clays having soluble salt contamination.
Clays with Soluble Salts
Bentonite clays can have high levels of soluble salts. Their use in clay-body formulas to increase plasticity should be limited to 2% or less.
Most ball clays and kaolins have relatively low concentrations of soluble salts; however, depending on the specific levels of soluble material in the clay deposit, they can exhibit various shades of discoloration in drying or bisque firing.
High-iron-content clays, which contain lime, such as Cedar Heights Redart, can periodically leave soluble deposits on the surface of the clay. This effect is often observed with slightly porous high-iron-content red building bricks containing sulfates of sodium, potassium, or calcium. After a rain, the deposits reveal white powdery patches on the brick’s surface. The rainwater dissolves the soluble material inside the brick, and evaporation’s wicking action brings it to the surface. However, if salts form underneath the brick’s surface, flaking or spalling can damage the structure of the brick.
Other high-iron, organic-bearing clays such as C-Red and Lizella tend to be higher in sulfate content and seem to generate intermittent excess soluble salts when used in clay body formulas. Iron sulphates of marcasite (a brittle, pale bronze-yellow iron sulfide mineral) and pyrite can also be found, along with lime, in some high-iron-content clays. The disassociation of iron sulfides gives rise to sulfate in free radical form, which can cause salt migration to the clay surface.
Eliminating Soluble Salts in Clay Bodies
There are several ways to eliminate soluble salt migration.
Mix clay bodies or select pre-moist clays low in iron content and/or low in soluble-salt content. Often this information can be obtained from ceramics suppliers who mix clay and have a record of current problems.
Test for hard water (dissolved calcium and magnesium ions) or soft water (low mineral content) when mixing your own clay bodies. Testing for high sulfates or chlorides can indicate soluble salt migration in clay body formulas. A practical low-cost method is to make several flat test bars, leave them out to dry, and note any white discoloration or crystal-like growth on the clay’s surface. Distilled water, which does not contain soluble salts, can be used in a test batch as a control to see if the water source is the problem. Note: The recycling of clay scraps can amplify soluble-salt levels in clays.
Controlled drying of clay will not remove soluble salt migration, but can reduce its influence as it slows the movement of water and wicking action in the clay. Rewetting the clay can start soluble salt migration again.
The most widely used correction for soluble salts is the addition of barium carbonate. Add 1⁄16–2% barium carbonate based on the dry weight of the total clay body formula. When thoroughly mixed with water into the clay body, barium carbonate will precipitate soluble salts from solution, yielding barium sulphate, which is insoluble. Additions of barium carbonate should be used even in random high-soluble salt conditions. Caution: Barium carbonate is a poison and should not be ingested; however, it can be used in a clay-body formula.
Another alternative for eradicating soluble salts is the use of Additive A in clay bodies. Additive A (Type 1 and Type 3) is a blend of lignosulphonates and barium carbonate produced by Borregard - Ligno Tech, U.S.A. Aside from the benefits of stopping soluble salts, it also increases green strength and plasticity in the moist clay. Add 1⁄10–1⁄2% based on the clay body’s dry weight.
Soluble Materials in Glazes
Whenever possible it is best to use insoluble materials in a glaze as they are relatively inert in the glaze water. Soluble materials dissolve in the glaze water to varying degrees depending on the material, the pH (acidic or basic) balance of the water, and the wet-glaze storage conditions relative to heat and time in the liquid state. Simply stated, some soluble materials will break down into the water system of a glaze faster and to a greater concentration than others.
Why use any soluble materials in a glaze? They can contribute higher levels of sodium or potassium and other oxides as compared to natural materials that are insoluble. Some glaze formulas contain soluble materials, and the potter does not know how to substitute insoluble materials. At that point, they are committed to using the original formula. In many instances, a soluble material such as Gerstley borate or Gillespie borate in a glaze formula produces a unique variegated glaze, which is difficult to obtain with insoluble materials.
Soluble materials are dispersed in the glaze water and are an actual part of the glaze formula. The formula can change as water is poured off or added during glaze mixing procedures. If glaze water evaporation occurs in the bucket, the glaze formula becomes more concentrated. In the glaze application process, water in the glaze (containing soluble material) is wicked into the bisque-fired pot. As evaporation occurs, water moves to higher areas of the pot in ridges and handles, causing a different glaze formula as compared to other areas of the pot. Soluble migration results in blisters, discoloration, and excessive disruption of the fired glaze surface. Potters may experience no problems in one glaze batch and encounter defects in the next.
Most Common Soluble Glaze Materials
Gerstley borate (see sidebar on page 59)
Gillespie borate
Soda ash
Colemanite (calcium borate)
Pearl ash (potassium carbonate)
Borax
Lithium carbonate
Gerstley Borate
When we encounter glaze problems with soluble materials they often originate with Gerstley borate. This naturally occurring, evaporate mineral deposit suffers from the sin of complexity, meaning there are many forces acting before, during, and after when it is used in a glaze formula. Gerstley borate is composed of ulexite (Na2O 2CaO 5B2O3 16H2O), with small amounts of colemanite (2CaO 3B2O3 5H2O) and probertite (Na2O) 2CaO 5B2O3 10H2O), resulting in low levels of pore water and much higher levels of chemically bound water. Due to its erratic chemical composition it is roughly described as a hydrated sodium calcium borate, making it difficult to substitute with frits.
Gerstley borate is a low-grade, inconsistent mineral that was popular with potters from the 1940s until its recent demise. Gerstley borate found its way into many glaze formulas over the years and it produced variegated effects. Today, many published glaze formulas still list it as an ingredient and it can still be found in many potters’ raw material bins.
When used in a glaze it produces phase separation, a unique quality when two or more glasses are formed at slightly different temperatures within the molten state, producing different viscosities and surface tensions. As the glaze cools, random instability occurs, producing the unique variegated quality. This reaction produces light areas of opalescence and color shifts in the glaze.
Gerstley borate has always been subject to variations in particle size, chemical composition, moisture content, and gangue (pronounced “gang”) a tramp material that encompasses bentonitic impurities. Specifically, the small black specks in the material, which can be iron-bearing minerals, manganese, particles of clay and shale, and/or carbonaceous materials not removed in the mining process, all of which can alter the fired glaze result.
Gerstley borate is a strong flux in cone 06 to cone 6 glazes, but is unstable in its diverse chemical composition. Potters were looking for a capricious glaze effect as created in hydrocarbon-fueled kilns instead of flat colors most often but not exclusively produced in electric kilns. When used as part of a glaze formula, it can gel in the glaze water inconsistently from one batch to the next, resulting in application problems.
Gerstley borate used in a glaze can yield excessive fired shrinkage, causing crawling. Additionally, when a kiln is fired too fast in its initial stages, the glaze can pop off the clay body surface resulting in the sudden release of vaporized water. This can yield areas devoid of glaze on the pot and a “halo” of glaze debris on the kiln shelf caused by the high chemical water in the mineral being rapidly released in the first part of the firing.
Gerstley borate is partially soluble and hygroscopic, resulting in the possibility of inaccurate amounts being placed into a glaze formula. Due to its solubility, some of the borate can dissolve into the glaze water, causing an alteration of the glaze as it dries on the pot. The water wicks into the interior of the bisque-fired pot during application. During drying, the water in the glaze containing soluble material migrates to the ridges or high areas of the form causing blistering and dry glaze areas.
The recommended substitute for the no-longer-mined Gerstley borate is Gillespie borate, produced by Hammill & Gillespie, is carried by many ceramics supply companies. It is a blend of Ulexite, colemanite, and clay-like materials acting as a boron flux. It has a consistent particle size with fewer impurities, producing a brighter color response when used as part of a glaze formula. As with any substitute material always test before committing to a large glaze batch.3
Soluble Materials in Glaze Formulas
Soluble materials can be used in glaze formulas, provided several precautions are taken.
Always place soluble materials in a sealed plastic bag, as they are hygroscopic and can accrue moisture in storage and weigh differently depending on their water content.
Always cover the glaze bucket with a tight lid to prevent water evaporation.
Always mix the glaze to the correct water/dry material ratio.
Do not pour off or add water to the glaze batch.
Whenever possible try to mix only enough glaze batch for one session. Think of your glaze containing soluble materials as a “soup” that is always cooking and changing. How much change is difficult to predict, but certain glaze formulas have a better track record of storage stability than others. It is not unusual to mix a new batch of glaze with good results, and then the same batch of glaze weeks or months later produces defects caused by the breakdown of soluble materials in the liquid glaze.
In instances where a raw material failed 100% of the time, the simple solution would be not to use it. However, the intermittent nature of defects caused by soluble glaze materials sometimes makes their faults difficult to diagnose.
Gaining knowledge of materials and how to store and use them in glazes will aid in preventing or identifying defects as they occur.
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 Potter’s Health & Safety Questionnaire are available from Jeff Zamek/Ceramics Consulting Services. For technical information, visit www.jeffzamek.com.
We understand your email address is private. You will receive emails and newsletters from Ceramic Arts Network. We will never share your information except as outlined in our privacy policy. You can unsubscribe at any time.
Please enjoy this complimentary article for the month.
For unlimited access to Ceramics Monthly premium content, please subscribe.
We understand your email address is private. You will receive emails and newsletters from Ceramic Arts Network. We will never share your information except as outlined in our privacy policy. You can unsubscribe at any time.
Subscribe to Ceramics Monthly
Why do some pots have white crystals on their surface? If you are in ceramics long enough, eventually the effects of soluble salts in clay bodies and glazes will be observed.
Defining the Terms
Bloating: The fired clay body has internal bubbles, which can erupt on the surface. During the firing, organic matter forms, releasing gas that is trapped in the clay as it vitrifies.
Deflocculation: Adding dispersion agents (soluble alkalis, sodium silicate, soda ash), which cause clay platelets to repel each other, ensuring fluidity. The resulting slip mixture becomes fluid using more clay and less water.
Gel: A network of particles forming a thick semi-solid structure, causing glaze materials to remain in suspension due in part to the boron component in Gerstley borate.
Hygroscopic: A substance’s ability to attract and absorb water molecules from the surrounding environment, usually air, at room temperature. This reaction depends in part on several factors such as the degree of hygroscopy in a material, the amount of moisture in the atmosphere where stored, and the type of barrier sealing the material. When moisture is present in hygroscopic materials, they can clump together during storage.
Microcracks: Sub-visible cracks in a clay body or glaze, weakening the ceramic structure.
Solubility chemistry is often similar in both clays and glazes, but can also behave differently in each. Soluble salts are composed of many materials, each one having a different melting point, from 212°F (100°C) to 1950°F (1065°C).1 Often the defects they produce are difficult to identify and are sometimes uncorrectable. The variable nature of soluble materials and their seemingly random occurrence is a significant factor in resolving defects.
Appearance of Soluble Salts
In clay bodies, scumming or efflorescence presents itself as the evaporation of salts, resulting in dark halos, with white or gray crystal powder forming on the surface of the clay. Crystals can occur in the drying or bisque-firing stages. Iron-bearing clays can produce iron salts, leaving brown or black staining on the drying clay’s surface. Low-fired red clays and common red building bricks are highly susceptible to growing crystals. Such defects are most often found on edges or high points of the pottery or the exposed surface of bricks, which dry faster than other parts. The wicking action of evaporation deposits higher levels of soluble materials in these areas, disrupting the clay’s surface subsequent to firing.
Soluble Materials in Mining Operations
Water-soluble salts can be found in the mining and processing of clays. Different clay pits yield varying fluctuations of soluble materials. Clays can contain sulfates of calcium, magnesium, or sodium, along with chlorides such as potassium and sodium carbonates, all of which can eventually travel in the clay body water system. Hard water (dissolved calcium or magnesium from the water source) in the clay mining operation will also increase soluble salt migration. While mines try to minimize any variation in their product, the nature of the material sometimes overrides any quality control procedures. Bacterial growth during the clay mining and storage process can activate the movement of soluble salts in moist clays.
Soluble Materials in Clay Bodies
Water used in the clay mixing process dissolves soluble salts, which then progress to the clay body’s surface. During evaporation, crystals are formed. Surface crystals can also be an indication of material still contained within the clay body. Any movement of soluble materials happens before the clay is fired, but can also be visible after firing.
Additives such as soda ash, sodium silicate, Darvan #7, or #811 used for deflocculation in slip-casting bodies can introduce soluble materials due to their sodium components.2 Bacterial action in aging moist clay can also increase soluble salt amounts and subsequent migration. All of these factors can be a delayed response at first, but eventually, a build-up of soluble materials is noticeable.
Soluble salts can act as a strong flux or melting agent in localized areas of the drying clay body, causing matte areas or glossy spots. Microcracks due to crystal growth cause internal pressure, weakening glaze adhesion. Additionally, crystal growth can yield different rates of shrinkage in the clay. The differential shrinkage rate between crystal and non-crystal areas can cause cracking or warping on the edges of functional pottery or sculpture.
Any disruption of the clay-body surface can produce glaze defects due to underlying soluble-salt deposits. Defects include:
In fired ware, any soluble material functioning as a flux has already melted, leaving “burnt” areas, surface discoloration, or blisters. Other defects are poor glaze adhesion, drying cracks, and glaze color shifts. Often noted are glaze surface blisters and weak rims on cups or bowls. Such defects are caused by the migration and high concentration of soluble materials drawn to these areas. If any soluble material disrupts the interface layer between the clay and the glaze, it is a potential cause of glaze defects.
Trapped Soluble Materials
Faster drying times can draw greater concentrations of soluble materials to the clay-body surface. Conversely, in some instances, slow, even drying can lessen surface migration. The actual amounts of soluble material can increase or decrease depending on drying cycles. However, soluble materials can still be present within the clay body, acting as a flux when fired to higher temperatures. The localized decomposition of soluble materials can result in bloating and black coring.
Effects of Soluble Salts in Clay Bodies
In the bone-dry or bisque state, fingerprints can disturb fragile soluble-salt deposits on the clay surface, causing a noticeable flashing or discoloration on the fired clay. High concentrations of soluble salts on bisque clay surfaces can result in fused areas, which stunt absorption of glaze and result in uneven glaze deposits. It can also create a mechanical disruption of the clay body/glaze interface, which can result in the glaze crawling due to the fluxing action of the soluble salt on the clay body. The relatively low melting point of soluble salts can obstruct the release of organic material in the clay, resulting in carbon trapping at higher temperatures. The melting action of soluble salts on the clay surface can also fuse pottery to kiln shelves.
Any clay body, whether the potter mixes their own clay or buys commercially mixed clays, can produce a defect-free batch of clay many times, but not the next. This is due to the specific level of soluble salts found in one or more of the clays used in the body. In most instances, soluble salt deposits on the clay body surface do not come from water used in the clay mixing process—provided the water does not originate from a static source, which, through evaporation, can concentrate salts found in the water. Ceramics suppliers or companies that mix clay are often not always aware or cannot test for clays having soluble salt contamination.
Clays with Soluble Salts
Eliminating Soluble Salts in Clay Bodies
There are several ways to eliminate soluble salt migration.
Soluble Materials in Glazes
Whenever possible it is best to use insoluble materials in a glaze as they are relatively inert in the glaze water. Soluble materials dissolve in the glaze water to varying degrees depending on the material, the pH (acidic or basic) balance of the water, and the wet-glaze storage conditions relative to heat and time in the liquid state. Simply stated, some soluble materials will break down into the water system of a glaze faster and to a greater concentration than others.
Why use any soluble materials in a glaze? They can contribute higher levels of sodium or potassium and other oxides as compared to natural materials that are insoluble. Some glaze formulas contain soluble materials, and the potter does not know how to substitute insoluble materials. At that point, they are committed to using the original formula. In many instances, a soluble material such as Gerstley borate or Gillespie borate in a glaze formula produces a unique variegated glaze, which is difficult to obtain with insoluble materials.
Soluble materials are dispersed in the glaze water and are an actual part of the glaze formula. The formula can change as water is poured off or added during glaze mixing procedures. If glaze water evaporation occurs in the bucket, the glaze formula becomes more concentrated. In the glaze application process, water in the glaze (containing soluble material) is wicked into the bisque-fired pot. As evaporation occurs, water moves to higher areas of the pot in ridges and handles, causing a different glaze formula as compared to other areas of the pot. Soluble migration results in blisters, discoloration, and excessive disruption of the fired glaze surface. Potters may experience no problems in one glaze batch and encounter defects in the next.
Most Common Soluble Glaze Materials
Gerstley Borate
When we encounter glaze problems with soluble materials they often originate with Gerstley borate. This naturally occurring, evaporate mineral deposit suffers from the sin of complexity, meaning there are many forces acting before, during, and after when it is used in a glaze formula. Gerstley borate is composed of ulexite (Na2O 2CaO 5B2O3 16H2O), with small amounts of colemanite (2CaO 3B2O3 5H2O) and probertite (Na2O) 2CaO 5B2O3 10H2O), resulting in low levels of pore water and much higher levels of chemically bound water. Due to its erratic chemical composition it is roughly described as a hydrated sodium calcium borate, making it difficult to substitute with frits.
Gerstley borate is a low-grade, inconsistent mineral that was popular with potters from the 1940s until its recent demise. Gerstley borate found its way into many glaze formulas over the years and it produced variegated effects. Today, many published glaze formulas still list it as an ingredient and it can still be found in many potters’ raw material bins.
When used in a glaze it produces phase separation, a unique quality when two or more glasses are formed at slightly different temperatures within the molten state, producing different viscosities and surface tensions. As the glaze cools, random instability occurs, producing the unique variegated quality. This reaction produces light areas of opalescence and color shifts in the glaze.
Gerstley borate has always been subject to variations in particle size, chemical composition, moisture content, and gangue (pronounced “gang”) a tramp material that encompasses bentonitic impurities. Specifically, the small black specks in the material, which can be iron-bearing minerals, manganese, particles of clay and shale, and/or carbonaceous materials not removed in the mining process, all of which can alter the fired glaze result.
Gerstley borate is a strong flux in cone 06 to cone 6 glazes, but is unstable in its diverse chemical composition. Potters were looking for a capricious glaze effect as created in hydrocarbon-fueled kilns instead of flat colors most often but not exclusively produced in electric kilns. When used as part of a glaze formula, it can gel in the glaze water inconsistently from one batch to the next, resulting in application problems.
Gerstley borate used in a glaze can yield excessive fired shrinkage, causing crawling. Additionally, when a kiln is fired too fast in its initial stages, the glaze can pop off the clay body surface resulting in the sudden release of vaporized water. This can yield areas devoid of glaze on the pot and a “halo” of glaze debris on the kiln shelf caused by the high chemical water in the mineral being rapidly released in the first part of the firing.
Gerstley borate is partially soluble and hygroscopic, resulting in the possibility of inaccurate amounts being placed into a glaze formula. Due to its solubility, some of the borate can dissolve into the glaze water, causing an alteration of the glaze as it dries on the pot. The water wicks into the interior of the bisque-fired pot during application. During drying, the water in the glaze containing soluble material migrates to the ridges or high areas of the form causing blistering and dry glaze areas.
The recommended substitute for the no-longer-mined Gerstley borate is Gillespie borate, produced by Hammill & Gillespie, is carried by many ceramics supply companies. It is a blend of Ulexite, colemanite, and clay-like materials acting as a boron flux. It has a consistent particle size with fewer impurities, producing a brighter color response when used as part of a glaze formula. As with any substitute material always test before committing to a large glaze batch.3
Soluble Materials in Glaze Formulas
Soluble materials can be used in glaze formulas, provided several precautions are taken.
Gaining knowledge of materials and how to store and use them in glazes will aid in preventing or identifying defects as they occur.
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 Potter’s Health & Safety Questionnaire are available from Jeff Zamek/Ceramics Consulting Services. For technical information, visit www.jeffzamek.com.
September 2026: Table of Contents
Must-Reads from Ceramics Monthly
Unfamiliar with any terms in this article? Browse our glossary of pottery terms!
Click the cover image to return to the Table of Contents