Knowing how a glaze turns out after a firing isn’t enough for some potters. They need to dig through the why and the how of a glaze. By doing so, a foundation of knowledge can be applied to glazes fired at other temperature ranges and in different
kiln atmospheres.
Defining the Terms
Alkaline or Alkali: A classification of glazes which are glossy in surface texture offering bright color responses. Alkaline-based glazes can craze due to their high rates of expansion and contraction.
Batch Weight: Glaze materials that add up to 100%. Gums, suspension agents, dyes, opacifiers, metallic coloring oxides, and stains are listed after the 100% batch weight. A method used in some instances to compare one glaze with
another.
Coefficient of Expansion: The change in length or volume in a clay body or glaze due to temperature change.
Eutectics: A mixture of two or more materials causing a melting point lower than the melting points of the individual raw materials.
Refractory: A material that can withstand high temperatures.
Unity Formula, Unity Molecular Formula (UMF), or Seger Formula: The formula represents glaze materials in molecules (Mols, units of measurement). In three columns, oxides are listed as fluxes, stabilizers, and glass formers.
This system of classification allows one glaze to be compared to others.
Vitrification: Occurs when fluxes in a clay body or glaze start the glass formation process.
The Building Blocks
What’s in a glaze is a common question we have all considered. When mixing your own glazes or using pre-mixed commercial glazes at some point have you ever wondered what functions specific raw materials serve in a glaze formula, or what amounts
of them will be required to achieve a precise outcome? Finding answers to these questions will offer insights into a greater understanding of raw materials when developing your own glazes or trying to correct glaze defects. Deconstructing a glaze
will examine the basis for its composition with regard to the individual raw materials’ characteristics and their limits based on batch weight guidelines.
One definition among many states glazes are composed of silica which is the primary glass former in pottery glazes. In fact, if a glaze could be formulated just from silica it would be stable in rapid temperature changes, and produce a hard, abrasion-resistant
surface, all characteristics potters are currently trying to achieve. However, silica will not go into a melt until 3092°F (1700°C) a much higher temperature than potters can reach in their kilns. The refractory nature of silica requires other
oxides to bring it into a melt, forming a eutectic and lowering its maturing point. Additionally, using a 100% silica glaze on a clay body will require a compatible coefficient of expansion on the clay body for an adequate glaze fit (both the clay
body and glaze shrinking upon cooling in an acceptable range). Due to silica’s unique ability to withstand high temperatures, a form of it was used in the space shuttle tiles.
Some of the most frequently used oxides in glazes are boric oxide, sodium oxide, potassium oxide, lead oxide (no longer used as it is toxic), tin oxide, titanium oxide, zinc oxide, barium oxide, calcium oxide, magnesium oxide, and zirconium oxide. One
or more of these oxides can be combined with silica to form a useable glaze that can mature within pottery kiln temperature ranges. Additions of alumina, often in the form of clay which is a refractory material, will stiffen the glaze when molten
helping it to remain in place on vertical surfaces and not pool in horizontal areas. The oxides and their carbonate forms can be found individually in raw materials such as zinc oxide (Zn0) or in combinations as found in dolomite calcium and magnesium
(Ca0 and Mg0). The combination and amounts of each oxide will determine the temperature range, opacity, surface texture, color development, and light transmission in the glaze. Simply stated, a glaze is composed of silica fluxed with other oxides
and stiffened with alumina to form a covering glass over a clay body.1
Another factor influencing glaze development is the kiln atmosphere whether it is fired in an oxidation atmosphere (more air than fuel present at combustion), neutral (equal amounts of air and fuel), or reduction (excess fuel than air). In reduction kiln
atmospheres, carbon monoxide (oxygen-hungry gas) develops removing oxygen from metallic oxides such as copper and causes its transformation to a red color in glazes. Reduction kiln atmospheres also flux or melt metallic oxides such as iron and manganese
causing increased degrees of vitrification as opposed to oxidation atmospheres even when fired to the same temperatures.
A potter looking at a ceramics supplier’s catalog of raw materials at first would be overwhelmed by the different clays, feldspars, metallic coloring oxides, and other raw materials. The average list of materials can exceed 120 items. This vast
array of materials presents too many options from which to formulate a glaze. However, this universe can be substantially condensed by knowing that eighty percent of glazes will only require between one and twelve raw materials. Some glazes might
need only flint, clay, and feldspar to function while others could require eight or more raw materials. By understanding just a dozen materials a workable knowledge of how to construct a glaze becomes possible. Other materials can be added as you
become familiar with these twelve.
A central question when looking at any glaze is its raw material content. What are the characteristics of each raw material and how do they influence the fired glaze? Keep in mind raw materials do not act independently but can form eutectic combinations.
Raw material combinations can also alter a glazes’ surface texture, light transmission, color, and maturing temperature.
Functions of Raw Materials in a Glaze
Each raw material used in a glaze contains one or more oxides or their carbonate forms. Some raw materials contain multiple oxides such as Minspar 200 feldspar having sodium, potassium, alumina, and silica components. When possible it is always best to
choose raw materials that have multiple oxide components as long as they fulfill the requirements of the glaze formula as their oxides are integrated more efficiently in nature as compared to using only single oxides. However, in some instances, a
single oxide is needed to complete the glaze formula. Potters have many raw material choices to fulfill a glaze formula. Additionally, the same glaze can be constructed using a different set of raw materials as long as the oxide requirements are met
in the formula. Let’s take apart a glaze and see how each material functions.
Minspar 200 — Na2O K2O SiO2Al2O3The
major flux in this glaze is feldspar. This sodium-based feldspar (the predominate oxide is sodium with lesser amounts of potassium, both are strong alkalis) also contains silica and alumina. Feldspars are an efficient way to introduce alkali oxides
into a glaze in relatively insoluble forms. At cone 9 (2300°F/1260°C) Minspar 200 contains all of the above oxides in near-ideal ratios to form a glaze almost by itself. Minspar 200, aside from being a strong flux at this temperature, also
brings other glaze materials into an active melt.
Silica — SiO2Silica will not melt by itself at the temperatures reached in potter’s kilns; however, combining it with other glaze materials lowers its melting point. For most glazes, 325-mesh silica
is used. However, finer 400-mesh or coarser 200-mesh sizes are available from most ceramics suppliers. As a general rule, the finer the mesh the more complete the melt as additional surface area is exposed in the heating process.
Wollastonite — CaO SiO2This common glaze material contains calcium and silica and is an ideal way to incorporate both materials into a glaze formula. In many glazes, wollastonite can be used in place
of whiting (calcium carbonate) as it does not release carbon dioxide when going into a melt. The release of this gas can cause bubbles in the fired glaze. Either the glaze can be recalculated for the addition of silica contained in wollastonite or,
in some instances, the extra silica will not significantly change the glaze.
EPK — Al2O32SiO22H2O EPK is one of many kaolins
which are a group of primary clays formed on site. They are relatively non-plastic and white firing. Both its silica and alumina content are refractory stiffening the molten glaze and keeping it on vertical surfaces. An addition of any type of clay
in the glaze helps to suspend the liquid glaze in storage.
Gerstley Borate — Na2O 2CaO 5B2O316H2O 2CaO 3B2O35H2O Na2O 2CaO 5B2O310H2O Gerstley
borate is a popular glaze material with a complex chemical composition of ulexite, colemanite, and probertite. Gerstley borate is an
unrefined, hygroscopic (can take on water in the atmosphere in storage), soluble ore with a chemical history that can vary. While these are characteristics that do not lead to reliability, Gerstley borate is found in many past and current glaze formulas
bringing other glaze materials into a melt.3 Gerstley borate is soluble and can leach into the water system of a glaze altering the actual glaze formula. Soluble materials in Gerstley borate can also migrate to the outer surfaces of the
pottery during glaze application causing blisters and rough areas in the fired glaze. However, Gerstley borate can promote a variegated glaze surface which is one reason for its use in glazes. Gerstley borate is no longer being mined but still remains
in potters’ raw material bins. Before formulating any glaze make sure all materials are still currently available.
Zinc Oxide — ZnO While not a strong flux in small amounts it reacts with other glaze materials causing fusion. Zinc oxide also helps prevent crazing (a fine network of lines in the fired glaze due to glaze tension upon cooling)
and has an intensifying effect on cobalt blue colors. Zinc oxide contributes durability and hardness, and promotes a craze-resistant glaze.
Cobalt Carbonate — CoCO3Cobalt carbonate and the more concentrated larger particle size, cobalt oxide is one of the strongest metallic coloring agents in either its oxide or carbonate form. One part
of cobalt in 100,000 parts of white glaze will have a tinting effect. Cobalt can be a strong flux in glazes and dissolves efficiently in high alkaline and boron-based glazes.4
Batch Weight Limits
The batch weight limit formulas are, in part, based on the unity molecular formulas, which detail the specific parts of molecules of the oxides in the glaze. The unity molecular formula is often referred to as the Skeleton Formula as many glazes are based
on this calculation and are then turned into batch weight formulas ready for use.
Based in part on the unity formula batch weight limits for each glaze, materials can be determined. There are no precise cut-off points to the minimum and maximum amounts of materials. In most instances, when a minimum or maximum limit has been reached,
as in silica 5% to 25%, the glaze will not be noticeably different—even if 4% or 26% of silica, or possibly greater percentages, is used.
100% Batch Glaze
The individual glaze materials when added should total 100% in the glaze batch. Gums, suspension agents, dyes, opacifiers, metallic coloring oxides, and stains are listed after the 100% batch weight.5
the author Jeff Zamek started his career 57 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.
1 Jeff Zamek, Ceramics Technical # 39 November 2014 March 2015, Glaze Description and Notation. 2 Val Cushing Handbook, 3rd edition and raw material notes from Alfred University, College of Ceramics 1972–73. 3 Jeff Zamek, Ceramics
Technical #39 November 2014, March 2015, A Simple Glaze 4 Frank and Janet Hamer, The Potter’s Dictionary of Materials and Techniques, A& C Black, University of Pennsylvania Press, Philadelphia, 1986, page 65. 5 Alfred
University, College of Ceramics raw material notes 1973. Acknowledgments: The following sources were used for additional technical information: Ian J. Mc Colm. Dictionary of Ceramic Science and Engineering, second edition, Plenum
Press, NY Tony Hanson’s Digital Fire Insight Program Limit Formula (Ron Roy Limits).
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Knowing how a glaze turns out after a firing isn’t enough for some potters. They need to dig through the why and the how of a glaze. By doing so, a foundation of knowledge can be applied to glazes fired at other temperature ranges and in different kiln atmospheres.
Defining the Terms
Alkaline or Alkali: A classification of glazes which are glossy in surface texture offering bright color responses. Alkaline-based glazes can craze due to their high rates of expansion and contraction.
Batch Weight: Glaze materials that add up to 100%. Gums, suspension agents, dyes, opacifiers, metallic coloring oxides, and stains are listed after the 100% batch weight. A method used in some instances to compare one glaze with another.
Coefficient of Expansion: The change in length or volume in a clay body or glaze due to temperature change.
Eutectics: A mixture of two or more materials causing a melting point lower than the melting points of the individual raw materials.
Refractory: A material that can withstand high temperatures.
Unity Formula, Unity Molecular Formula (UMF), or Seger Formula: The formula represents glaze materials in molecules (Mols, units of measurement). In three columns, oxides are listed as fluxes, stabilizers, and glass formers. This system of classification allows one glaze to be compared to others.
Vitrification: Occurs when fluxes in a clay body or glaze start the glass formation process.
The Building Blocks
What’s in a glaze is a common question we have all considered. When mixing your own glazes or using pre-mixed commercial glazes at some point have you ever wondered what functions specific raw materials serve in a glaze formula, or what amounts of them will be required to achieve a precise outcome? Finding answers to these questions will offer insights into a greater understanding of raw materials when developing your own glazes or trying to correct glaze defects. Deconstructing a glaze will examine the basis for its composition with regard to the individual raw materials’ characteristics and their limits based on batch weight guidelines.
One definition among many states glazes are composed of silica which is the primary glass former in pottery glazes. In fact, if a glaze could be formulated just from silica it would be stable in rapid temperature changes, and produce a hard, abrasion-resistant surface, all characteristics potters are currently trying to achieve. However, silica will not go into a melt until 3092°F (1700°C) a much higher temperature than potters can reach in their kilns. The refractory nature of silica requires other oxides to bring it into a melt, forming a eutectic and lowering its maturing point. Additionally, using a 100% silica glaze on a clay body will require a compatible coefficient of expansion on the clay body for an adequate glaze fit (both the clay body and glaze shrinking upon cooling in an acceptable range). Due to silica’s unique ability to withstand high temperatures, a form of it was used in the space shuttle tiles.
Some of the most frequently used oxides in glazes are boric oxide, sodium oxide, potassium oxide, lead oxide (no longer used as it is toxic), tin oxide, titanium oxide, zinc oxide, barium oxide, calcium oxide, magnesium oxide, and zirconium oxide. One or more of these oxides can be combined with silica to form a useable glaze that can mature within pottery kiln temperature ranges. Additions of alumina, often in the form of clay which is a refractory material, will stiffen the glaze when molten helping it to remain in place on vertical surfaces and not pool in horizontal areas. The oxides and their carbonate forms can be found individually in raw materials such as zinc oxide (Zn0) or in combinations as found in dolomite calcium and magnesium (Ca0 and Mg0). The combination and amounts of each oxide will determine the temperature range, opacity, surface texture, color development, and light transmission in the glaze. Simply stated, a glaze is composed of silica fluxed with other oxides and stiffened with alumina to form a covering glass over a clay body.1
Another factor influencing glaze development is the kiln atmosphere whether it is fired in an oxidation atmosphere (more air than fuel present at combustion), neutral (equal amounts of air and fuel), or reduction (excess fuel than air). In reduction kiln atmospheres, carbon monoxide (oxygen-hungry gas) develops removing oxygen from metallic oxides such as copper and causes its transformation to a red color in glazes. Reduction kiln atmospheres also flux or melt metallic oxides such as iron and manganese causing increased degrees of vitrification as opposed to oxidation atmospheres even when fired to the same temperatures.
A potter looking at a ceramics supplier’s catalog of raw materials at first would be overwhelmed by the different clays, feldspars, metallic coloring oxides, and other raw materials. The average list of materials can exceed 120 items. This vast array of materials presents too many options from which to formulate a glaze. However, this universe can be substantially condensed by knowing that eighty percent of glazes will only require between one and twelve raw materials. Some glazes might need only flint, clay, and feldspar to function while others could require eight or more raw materials. By understanding just a dozen materials a workable knowledge of how to construct a glaze becomes possible. Other materials can be added as you become familiar with these twelve.
A central question when looking at any glaze is its raw material content. What are the characteristics of each raw material and how do they influence the fired glaze? Keep in mind raw materials do not act independently but can form eutectic combinations. Raw material combinations can also alter a glazes’ surface texture, light transmission, color, and maturing temperature.
Functions of Raw Materials in a Glaze
Each raw material used in a glaze contains one or more oxides or their carbonate forms. Some raw materials contain multiple oxides such as Minspar 200 feldspar having sodium, potassium, alumina, and silica components. When possible it is always best to choose raw materials that have multiple oxide components as long as they fulfill the requirements of the glaze formula as their oxides are integrated more efficiently in nature as compared to using only single oxides. However, in some instances, a single oxide is needed to complete the glaze formula. Potters have many raw material choices to fulfill a glaze formula. Additionally, the same glaze can be constructed using a different set of raw materials as long as the oxide requirements are met in the formula. Let’s take apart a glaze and see how each material functions.
Minspar 200 — Na2O K2O SiO2 Al2O3 The major flux in this glaze is feldspar. This sodium-based feldspar (the predominate oxide is sodium with lesser amounts of potassium, both are strong alkalis) also contains silica and alumina. Feldspars are an efficient way to introduce alkali oxides into a glaze in relatively insoluble forms. At cone 9 (2300°F/1260°C) Minspar 200 contains all of the above oxides in near-ideal ratios to form a glaze almost by itself. Minspar 200, aside from being a strong flux at this temperature, also brings other glaze materials into an active melt.
Silica — SiO2 Silica will not melt by itself at the temperatures reached in potter’s kilns; however, combining it with other glaze materials lowers its melting point. For most glazes, 325-mesh silica is used. However, finer 400-mesh or coarser 200-mesh sizes are available from most ceramics suppliers. As a general rule, the finer the mesh the more complete the melt as additional surface area is exposed in the heating process.
Wollastonite — CaO SiO2 This common glaze material contains calcium and silica and is an ideal way to incorporate both materials into a glaze formula. In many glazes, wollastonite can be used in place of whiting (calcium carbonate) as it does not release carbon dioxide when going into a melt. The release of this gas can cause bubbles in the fired glaze. Either the glaze can be recalculated for the addition of silica contained in wollastonite or, in some instances, the extra silica will not significantly change the glaze.
EPK — Al2O3 2SiO2 2H2O EPK is one of many kaolins which are a group of primary clays formed on site. They are relatively non-plastic and white firing. Both its silica and alumina content are refractory stiffening the molten glaze and keeping it on vertical surfaces. An addition of any type of clay in the glaze helps to suspend the liquid glaze in storage.
Gerstley Borate — Na2O 2CaO 5B2O3 16H2O 2CaO 3B2O3 5H2O Na2O 2CaO 5B2O3 10H2O Gerstley borate is a popular glaze material with a complex chemical composition of ulexite, colemanite, and probertite. Gerstley borate is an
unrefined, hygroscopic (can take on water in the atmosphere in storage), soluble ore with a chemical history that can vary. While these are characteristics that do not lead to reliability, Gerstley borate is found in many past and current glaze formulas bringing other glaze materials into a melt.3 Gerstley borate is soluble and can leach into the water system of a glaze altering the actual glaze formula. Soluble materials in Gerstley borate can also migrate to the outer surfaces of the pottery during glaze application causing blisters and rough areas in the fired glaze. However, Gerstley borate can promote a variegated glaze surface which is one reason for its use in glazes. Gerstley borate is no longer being mined but still remains in potters’ raw material bins. Before formulating any glaze make sure all materials are still currently available.
Zinc Oxide — ZnO While not a strong flux in small amounts it reacts with other glaze materials causing fusion. Zinc oxide also helps prevent crazing (a fine network of lines in the fired glaze due to glaze tension upon cooling) and has an intensifying effect on cobalt blue colors. Zinc oxide contributes durability and hardness, and promotes a craze-resistant glaze.
Cobalt Carbonate — CoCO3 Cobalt carbonate and the more concentrated larger particle size, cobalt oxide is one of the strongest metallic coloring agents in either its oxide or carbonate form. One part of cobalt in 100,000 parts of white glaze will have a tinting effect. Cobalt can be a strong flux in glazes and dissolves efficiently in high alkaline and boron-based glazes.4
Batch Weight Limits
The batch weight limit formulas are, in part, based on the unity molecular formulas, which detail the specific parts of molecules of the oxides in the glaze. The unity molecular formula is often referred to as the Skeleton Formula as many glazes are based on this calculation and are then turned into batch weight formulas ready for use.
Based in part on the unity formula batch weight limits for each glaze, materials can be determined. There are no precise cut-off points to the minimum and maximum amounts of materials. In most instances, when a minimum or maximum limit has been reached, as in silica 5% to 25%, the glaze will not be noticeably different—even if 4% or 26% of silica, or possibly greater percentages, is used.
100% Batch Glaze
The individual glaze materials when added should total 100% in the glaze batch. Gums, suspension agents, dyes, opacifiers, metallic coloring oxides, and stains are listed after the 100% batch weight.5
the author Jeff Zamek started his career 57 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.
1 Jeff Zamek, Ceramics Technical # 39 November 2014 March 2015, Glaze Description and Notation.
2 Val Cushing Handbook, 3rd edition and raw material notes from Alfred University, College of Ceramics 1972–73.
3 Jeff Zamek, Ceramics Technical #39 November 2014, March 2015, A Simple Glaze
4 Frank and Janet Hamer, The Potter’s Dictionary of Materials and Techniques, A& C Black, University of Pennsylvania Press, Philadelphia, 1986, page 65.
5 Alfred University, College of Ceramics raw material notes 1973.
Acknowledgments: The following sources were used for additional technical information:
Ian J. Mc Colm. Dictionary of Ceramic Science and Engineering, second edition, Plenum Press, NY
Tony Hanson’s Digital Fire Insight Program Limit Formula (Ron Roy Limits).
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