----------------------------------------------------------- glz3b.txt Clay and Glaze Formulation Robert Fromme __________________________________________________________ Notes on Calcium, the 'Good Old Work Horse' Glaze Flux __________________________________________________________ CALCIUM (Alkaline earth) Oxide Formula: CaO Molecular Weight: 56 * Melting Range: With the relatively high melting point of 2575 F.,calcium is not an active flux at lower temperatures and is seldom used to encourage melting below cone 4. By adding calcium to a low temperature boron glaze can produce mattness. Calcium is not an active melting flux until the middle and higher temperature ranges. At very high temperatures, such as cone 12 or cone 13 porcelain, calcium is a dependable glaze ingredient. * Viscosity: Calcium is use to reduce viscosity in glazes which are high in silica, but if the melt is too fluid, devitrification may take place. * Melting and Mixing Characteristics: Most glazes contain some calcium because it is a very common and inexpensive substance and usually adds only good characteristics to a base glaze. In the middle and high temperature ranges, calcium is rated as the most dependable fluxing oxide. When calcium is used in large quantities, it can help to create matt surfaces in glazes. High molecular amounts, .8 or more, in a lower alumina glaze gives matt qualities through the formation of calcium silicate crystals or lime matt glazes. These surfaces can be very pleasant as long as the zinc content is kept low to avoid the formation of large disruptive crystals in the cooling surface. Even when the craftsperson has worked to produce a transparent glaze mixture, calcium seldom produces a extraordinary high gloss or 'brilliant' surface. However, in high-fired glazes which contain calcium oxide in the range of .5 to .7 molecular equivalents tend to become transparent and fairly bright. These are hard, fairly smooth, and remind one of polished jade surfaces. Functional high-fired glazes can be created from very simple combinations of 83 to 86 parts feldspar to 17 to 14 parts whiting. Another simple plan suggests the addition of 60 parts feldspar, 18 to 20 parts flint and 20 to 22 parts whiting. Without the addition of clay or bentonite, however, these mixtures will quickly settle out of suspension in the glaze bucket. *Surface Softness or Hardness: When calcium is added to a glazes for any firing range, it has increased durability and hardness. This is particularly important when it is added to low-fired glazes which have a high quantity of lead, sodium or potassium. The Calcium renders the mix harder and less soluble. * Clay-Glaze fit: Calcium tends to react with the clay body under the glaze to create a good interface and encourage the clay-glaze fit. It has a middle to slightly high expansion and contraction rate in glazes. CaO promotes a lower expansion then the alkalies, sodium and potassium, so it can add stability and fit in combinations of these fluxes and silica. In very large quantities, it may develop crazing and glaze fit problems. At lower temperatures, CaO and Wollastonite are used in clay bodies as fillers which have the effect of lowering effects of firing shrinkage and thermal shock failure. One traditional method of gaining a good clay-glaze fit involves the use of similar materials in both the glaze and the clay body. This practice is particularly applicable for the porcelain craftsperson. Many porcelain formulae can be changed into a glaze composition by the addition of 18% to 22% additional feldspar and 9% to 12% additional calcium. * Color Response: Calcium works with most metals for a strong and pleasant effect on most colorants. Some clay artists have noted a slight tendency for calcium to have a bleaching effect on iron oxide in some glaze mixtures. When calcium is used with potassium and sodium, copper reds are possible in reduction atmospheres at middle and high temperatures. Calcium has long been a choice flux along with large amounts of barium to encourage the development of celadon green from iron in reduction atmospheres at middle and high temperatures. When colemanite or Gerstley borate is used a source for boron and calcium, the glaze may develop a wonderful opalescence. * Problems and precautions for use: One will seldom find calcium creating a problem in a glaze and it is usually the safest and most reliable inclusion at the middle and high temperatures. If a glaze has excess calcium or if it is too low in viscosity, the glaze will easily devitrify, forming crystals of calcium feldspar. A matt glaze will result unless the alumina content is increased to compensate. We should note that one source for calcium, bone ash is slightly soluble in water and gives off phosphorous oxide which may cause extra bubbling some glazes. Fluorspar, another source for calcium, emits fluorine during firing and can cause extra bubbling and the loss of gloss in some glaze surfaces. Although Calcium Chloride is not usually thought of as a source for calcium in a glaze, a mixture for glazing which has had too much water added may be made thicker by the addition of three to four teaspoons per gallon of glaze. Brief Notes on Calcium Glaze History Calcium in Historical Glazes Calcium is a favored ingredient in most glazes throughout history and involving a wide range of temperatures and processes, from low-temperature lead glazes the intense temperatures of porcelain. Calcium Borate Mixtures When glazes contain formidable amounts of Gerstley borate, colemanite or a calcium borate frit and are cooled slowly, the slight recrystallization will yield beautiful clouds of lighter or milky, opalescent surface over a glossy, more vivid background melt. Some clay artists prefer the use of a calcium borate frit for the milky opalescence because, as we have noted, colemanite and Gerstley borate boil and bubble in the early melt, creating a problems of mottling and sputtering in the melt. __________________________________________________________ Notes on Zinc Oxide __________________________________________________________ ZINC OXIDE (metal) Oxide Formula: ZnO Molecular Weight: 81.4 * Melting Range: With the relatively high melting point of 1800 C.,zinc is not an active flux at lower temperatures and is seldom used to encourage melting below cone 01. Zinc is not used as an active flux, but in smaller quantities it can be used as a replacement for lead as a flux at the lower temperatures. (See Bristol Glazes) It has a fairly broad firing range from medium to low temperatures to normal limits for higher temperature ceramic production. At very high temperatures, zinc will become volatile, creating some increased bubbles and boiling. Zinc is usually introduced into glazes as an auxiliary or secondary flux, especially in leadless glazes fluxed with feldspars or boron. Since it is not a powerful flux at the lower temperatures, it will act a refractory in glazes maturing below 1050 C.(1920 F.) At higher temperatures, it is valuable to provide a smooth transition from sintered to a molten state. * Viscosity: Large quantities of zinc in a glaze can create problems ow crawling due to the high viscosity of zinc. When gas bubbles move through a high zinc glaze, the viscosity may not allow for the surface to smooth out or heal, resulting in pitting, pinholing and crawling. * Melting and Mixing Characteristics: Zinc will function as a catalyst in slip glazes and in small amounts its contribution to glaze mixtures is usually positive. In certain glaze mixtures it is a very powerful flux with small to medium additions to the glaze. Small additions seem to reduce boiling during firing and in combination with titanium in a mixture which has very low amounts of alumina, zinc has the tendency to help the formation of crystals. The zinc-titanium glazes usually require a soaking or very slow cooling temperature around 1500 F. or 817 C. and then a very rapid final cooling period. Zinc works well to encourage a dull matt when it is used in larger quantities at most temperatures. Zinc oxide can produce opacity and whiteness at low temperatures, if the calcium content of the glaze is kept low. When used with boron, zinc can have amphoteric (neutral or viscous) qualities at the lower temperatures. * Surface Softness or Hardness: Zinc can lend strength and durability to a glaze mixture. * Clay-Glaze fit: Zinc is in the middle of the ceramic flux list of coefficients of expansion and contraction, so its addition in a glaze usually works to improve crazing and related clay-glaze fit problems. * Color Response: Many metal oxides will produce dramatic color responses when zinc is used in a glaze. Cobalt in a zinc fluxed glaze will yield a very intense blue. Copper greens are encouraged by the addition of zinc in the melt. Some metals do not respond well with a zinc fluxed glaze and become dull and neutral. It is a poor flux in iron brown glazes. It works against the formation of chrome greens, turning them brown, khaki or olive drab brown. It is not a good addition in copper red glazes and it may cause some high tin white glazes to blush to a pink color. Migration from copper to white glazes which are high in zinc and tin have caused some crafts people to avoid using these glazes in the same kiln with copper red glazes. * Problems and precautions for use: Zinc oxide is poisonous. Be careful with its use. Clay crafts people prefer to use zinc oxide in its calcined form because water can lead to excessive drying shrinkage and similar problems in the early stages of the firing. Remember that large additions of zinc can create problems with pitting, pinholing and crawling. Brief Notes on Zinc Glaze History Zinc in Historical Glazes Bristol Glazes The so-called Bristol glazes represent attempts to replace lead oxide by zinc oxide when the poisonous nature of raw lead compounds had become a health hazard in the growing pottery industry. The use of the zinc fluxed glazes seems to have developed near Bristol, England and were given the name from that location. In these glazes, zinc served as the primary flux and calcium, magnesium, and barium were used as auxiliary fluxes. Microcrystalline glazes These are glazes which often contain zinc, in which crystals are so small that we are not able to detect them without magnification. Examples are the satin-vellum and matt glazes formed by crystallization from solution of zinc, calcium or barium silicates and titanates. Macrocrystalline glazes Zinc is also used in macrocrystalline glazes of many types. These are glazes in which the crystals are sufficiently large to be readily detectable by the naked eye. The category includes the zinc-titanium crystal glazes and the adventurines glazes. Satin-vellum glazes Satin-vellum glazes often contain zinc oxide as an ingredient. Since true matt glazes often tend to have rough surfaces which are easily marked by metal and which are difficult to clean, the satin-vellum glazes provide a functional alternative. The much smoother semi-matt surface of the satin-vellum glazes have a most attractive satin sheen surface and are produced by the addition of 3 to 4% zinc, titanium and tin oxides in a majolica or low-earthenware region (1100 C. (2110 F.). __________________________________________________________ ---------------------------------------------------------- (c) 1994 Robert Fromme For educational use, only. ---------------------------------------------------------- ----------------------------------------------------------- glz4a.txt Clay and Glaze Formulation Robert Fromme __________________________________________________________ Notes on Magnesium as a Glaze Fluxing Oxide __________________________________________________________ Magnesium (Alkaline earth) Oxide Formula: MgO2 Molecular Weight: 40.3 * Melting Range: With the relatively high melting point of 2800 C., Magnesium oxide is not an active flux at lower temperatures and is seldom used to encourage melting below cone 3. Adding magnesium oxide to a low temperature glaze can produce mattness. * Viscosity: At very high temperatures, magnesium oxide encourages a more fluid glaze, however, at lower temperatures large amounts can work with other viscous ingredients to produce problems with pinholing and crawling. * Melting and Mixing Characteristics: Magnesium is used to produce beautiful opaque matte surfaces in the higher temperature range. Reduction firing helps to produce high temperature glazes which have the 'egg-shell' and soft 'buttery' mattes. * Surface Softness or Hardness: When magnesium is added to a glazes at higher firing ranges, it increases durability and hardness. * Clay-Glaze fit: Magnesium tends to react with the clay body under the glaze to create a good interface and improves glaze adhesion. It the lowest coefficient of expansion and contraction in glazes. * Color Response: Magnesium has a negative effect in the traditional celadon and copper red reduction glazes. It is a critical ingredient in many for the beautiful white tin opacified glazes. If part of the kiln load includes copper glazes in higher reduction temperatures, a pink blush may result in a white glaze which contains magnesium. The copper becomes volatile and often moves into the surfaces of the tin and magnesium glazes near it. When glazes contain magnesium and cobalt, one can expect the cobalt to develop more of a purple or violet than blue. 3 to 9% copper in a high magnesium glaze can produce a warm gray in reduction and a cool gray in oxidation atmospheres. * Problems and precautions for use: Because some of the sources of magnesium are slightly soluble, it may act as a flocculent with clay content of glazes mixtures. Although magnesium sulfate is not a traditional raw material for glazes, clay artist frequently use several teaspoons of it to help thicken a bucket of glaze which has too much water in relation to the other raw materials. Brief Notes on Magnesium Glaze History Magnesium has long been an important flux for glazes which develop the buttery micro-crystalline matt surfaces at high temperatures. __________________________________________________________ Notes on Barium as a Glaze Flux __________________________________________________________ BARIUM OXIDE (alkaline earth) Oxide Formula: BaO Molecular Weight: 153.4 * Melting Range: Barium oxide does not melt until 1923 C. and serves as a useful flux at the higher temperatures. It seems to be able to withstand the highest ceramic temperatures where other fluxes would begin to volatilize and boil. * Viscosity: Although barium is slow to melt and is active only at higher temperatures, it seems to become a very active flux creating very fluid mixtures at those temperatures. When it is used in excess it can cause the glaze to seem more viscous due to its refractory nature in the lower temperatures. * Melting and Mixing Characteristics: Ceramic artists have noted that barium seems to encourage an additional quality of brilliance to those glazes which develop glossy surfaces. Barium has long been noted as an important glaze ingredient when developing a satin matt surface. When the mixture is over saturated, however, the same glaze will take on a rough and unpleasant quality of surface. These 'stony mattes' are not only less then pleasing to the eye, they are usually very dangerous if used in utilitarian ware because of the toxic qualities of barium when a glaze is permeated with it. The eutectic between boron and barium will halt barium matt development in a glaze and encourage fluid transparent surfaces at higher temperatures when the two materials are included in the same formula. Barium has functioned as an important flux in certain unique glazes where, at specific temperatures, the surface will develop patches of bright gloss broken up by areas of matt and opaque glaze. Barium oxide seems to be able to function as a dependable flux at very high temperatures where other ingredients tend to volatilize. * Surface Softness or Hardness: Like calcium, barium has been included in controlled amounts to add durability, strength and acid resistance to utilitarian glazes. * Clay-Glaze Fit: Barium is seldom cited as a source for problems with the fit between clay and glaze. It does not seem to cause as much crazing as sodium, potassium and calcium and is usually ranked near lead and lithium in the middle of the fluxing oxides for expansion and contraction characteristics. * Color Response: Most of the metal oxides seem to produce colors which are quite intense in glazes which contain some barium. The color response of cobalt and the iron and titanium in rutile in glazes with barium are usually extremely rich. Mixtures of 1 to 5% cobalt in a barium fluxed glaze can create unique ultramarine blue surfaces. Small amounts of iron oxide in a barium glaze will often produce iron blues. When less then 3 1/2% iron oxide is used as colorant in reduction glazes containing calcium, barium can be added to help produce the traditional celadon green. In other high barium glazes, the addition of 4 to 5% iron with around 2% stannous (tin) oxide can produce pale yellow and cream colors. In the higher temperatures, these are frequently called 'iron yellows' and they seem to work best when they are opacified with titanium and or zirconium. In high temperatures oxidation atmospheres 1 to 10% Copper in a barium glaze can create beautiful turquoise and blue colors when the clay content is kept low. When the clay content of the base glaze is increased, the copper will encourage the color to take on an apple green quality. Nickel in a crystalline, barium and zinc base glaze will yield the traditional 'Wedgwood blue' to surfaces which are a cool blue gray. In oxidation, 1 to 5% nice can produce colors ranging from Prussian blue to Indigo and violet. At very low temperatures (cone 018-011) nickel in a glaze with barium has produced surfaces from pink to mauve. Clay artists often mix two or three metal oxides in glazes containing barium to produce mottled or broken colors in a surface which may break from gloss to matt in the same glaze. Barium has been known to encourage mottled and streaked color effects in some glazes. One raw material source, barium chromate, although very toxic, is used in low temperature glaze and enamel mixtures to achieve bright lemon yellow, green and greenish yellow surfaces. * Problems and precautions for use: Barium Carbonate is poisonous (an ingredient on rodent poison) and many glazes containing barium are not stable even after being fired. Always use caution when using barium and avoid its use in glaze surfaces which are intended for food service. Have any questionable glazes which contain barium tested for stability if they are to be utilized for food or drink. In recent years, may ceramic artists choose to replace the barium content in their studio glazes with strontium oxide because it gives similar properties to the glaze without the risk of toxicity in the raw material form and in the glaze. When drying slip mixtures and other clay bodies develop scumming from soluble salts moving to the surface with the evaporating water, additions of 2% barium carbonate have been used to control this problem. In theory, this addition converts the soluble salts to insoluble forms through a process called 'double decomposition'. Brief Notes on Glaze History Barium has served as a secondary flux in the middle and high temperatures. Barium is an occasional flux in copper reduction glaze bases in the cone 8 to 10 range. In the lower temperatures, it has been used as a refractory and opacifier in some Raku and Majolica mixtures. It is an important ingredient in the Micro- crystalline satin matt glazes for stoneware and porcelain. As the toxic nature of the material becomes known, we can expect to see it used less frequently with strontium used as its replacement. _________________________________________________________ Notes on Strontium __________________________________________________________ STRONTIUM OXIDE (alkaline earth) Oxide Formula: SrO Molecular Weight: 103.6 * Melting Range: Strontium oxide is refractory with a melting point of 2430. When used in a base glaze with other materials it will function as a flux, although it requires a slower firing or soaking in order for its participation in the glaze melt. Its useful fluxing range spans from cone 04 to cone 12. * Viscosity: When the temperature and composition is sufficient for strontium to participate in the melt, it usually helps to produce surfaces which are free from bubbles and pits. If its content is too high, it forces the melt to become viscous with potential for surfaces which are not even. Like zinc, when strontium content is too great in the mixture, it can cause crawling. * Melting and Mixing Characteristics: Ceramic artists have often noted that strontium shares many of the melting and mixing characteristic of calcium and barium. Because of the toxic nature of barium, strontium seems to be a reasonable replacement in middle and high temperature glazes. Strontium and calcium are frequently used in glazes which develop micro-crystalline matt surfaces. * Surface Softness or Hardness: When strontium is used as a flux in the glaze the melt usually results in surfaces which are hard and durable * Clay-Glaze fit: Strontium has a low expansion and contraction coefficient so it is one of the best choices to avoid crazing and related problems of glaze fit. * Color Response: Calcium and Strontium seem to share many of the same characteristics of color response. Calcium has been noted for a tendency to dampen or 'bleach' iron color, however, in a strontium fluxed glaze iron will usually yield its traditional color range with slightly more intensity. Strontium also shares many of the color responses of barium. When from 5 to 10% copper is used in a high barium or strontium glaze, it can produce dark green in oxidation atmospheres. In other high barium or strontium base glazes, 1 to 5% iron or rutile can produce colors ranging from pale yellow and cream to yellow ocher surfaces. * Problems and precautions for use: Strontium is not toxic and can be used to replace the toxic flux, barium in the middle to high temperatures. The primary raw material source, strontium carbonate, is usually a little more expensive than other ingredients and it is slightly water soluble. Brief Notes on Glaze History Strontium was seldom used in ancient pottery and it seems to be a fairly recent addition to the list of fluxing oxides which are available to the potter. Its use will increase as it serves as an effective replacement for the toxic barium raw materials. --------------------------------------------------------- (c) 1994 Robert Fromme / For educational use, only. _________________________________________________________ --------------------------------------------------------- ---------------------------------------------------------- glz4c.txt Clay and Glaze Formulation Robert Fromme __________________________________________________________ Notes on Aluminum as a Glaze Refractory or Neutral Oxide __________________________________________________________ Aluminum Oxide (metal) Oxide Formula: Al2O3 Molecular Weight: 103 * Melting Range: The melting point of alumina oxide is around 2050 C.. In a glaze, however, the addition of fluxing or base oxides can cause the refractory inclusions such as alumina and silica to enter the melt at very low temperatures. The clay artist should keep in mind that by controlling amounts of alumina and silica in a glaze, in relation to the amount and type of base or fluxing oxides acts to control the maturing temperature of the glaze. By adding alumina to a glaze at a given maturation temperate, the refractoriness, as well as the opacity and the viscosity of the mixture is increased. * Viscosity: The viscosity of the glaze is one of the primary characteristics in the melt which can be traced back to the amount of alumina in the mixture. As we have mentioned, the glaze becomes more viscous or less runny as the alumina content in the mixture increases. In turn, as the alumina is removed, the glaze becomes more fluid. * Melting and Mixing Characteristics: Alumina is both acidic and basic in its behavior. It can combine with the acid, silica, the neutral, boron, and the bases or fluxes. With this behavior, like that of boron, alumina is classified as an intermediate, neutral or amphoteric oxide in glaze chemistry. In addition, other terms such as 'viscous agent' and 'crystal retardant' have been used to describe alumina in the glaze. Artists and technicians who work with glass will quickly point out that simple mixtures of fluxes and silica will produce glass. If those mixtures are applied to the surface of a clay object and used as glazes they will not work well. They will melt very suddenly and they would become so fluid that most of the molten mixture would end up in a puddle of glass on the kiln shelf below and under the vessel. In the cooling kiln, the glassy nightmare would begin to crystallize or 'devitrify' into a rough, dry, surgery surface. Glazes are made more stable and useful to the clay artist through the addition of alumina to the glassy melt. With small amounts of alumina the material is made more viscous so that it is slower to move as it melts, so the alumina improves the chances for the melt to remain on the clay object where it is intended to function. With the addition of alumina, the process of change from a solid to liquid phase is slowed and extended over a much longer temperature range. This has the effect of making the molten glaze last several hundred degrees beyond the temperature that the same materials (minus the alumina) would normally melt off the walls of the clay form. In addition to improving the melt by adding viscosity and functional range for its maturation temperatures, alumina also serves a very important role by resisting the devitrification or crystallization of the cooling glaze. * Surface Softness or Hardness: Alumina also helps glazes to become hard and durable. Since the amount of alumina in most glazes increases as the maturation temperature goes up, we can notice that alumina will work to improve the hardness or resistance of glazes as their maturation temperatures increase. * Clay-Glaze fit: Alumina ranks in its very low expansion and contraction with silica and boron as one of the best ingredients to improve the fit a glaze to the clay. The theoretical formula for clay (Al2O3.2SiO2.2H2O) suggests that if alumina is also used in the glaze, our chances of improving fit between the glaze and the clay will improve. * Color Response: In general, most metal oxides which provide color in glazes are not effected by the addition of alumina to a glaze. One exception to our general assumption of color responses for alumina oxide involves those base glazes which are intended to produce a red glaze from copper in reduction atmospheres. Alumina content should be kept low in these base mixtures in order to keep from hindering the formation of metallic copper in the reduced glaze. Alumina also has been known to darken and deaden chrome green colors in certain base mixtures. With the addition of chrome in a high alumina glaze, one can expect the color to turn toward brown. * Problems and precautions for use: The amount of alumina in a glaze is usually much less than the amount of flux and silica. When the alumina content is too great, the glaze will become dry and unpleasant to the touch. In extreme cases, the glaze will simply refuse to melt. Feldspars and Kaolins are usually the raw materials choices for introducing alumina into the glaze. Some situations require the clay or kaolin to be calcined prior to use in order to avoid cracking and shrinkage of the fresh glaze coat as it dries. Other sources for alumina have some characteristics which one must consider. Alumina Hydrate tends to settle in the raw glaze storage container and it is more expensive then other sources. Amblygonite and cryolite have volatile inclusions of fluorine and phosphorous which can increase the boiling and bubbles which can result in the formation of pinholes, pitting and crawling in some mixtures. __________________________________________________________ Notes on Silicon (flint) an acid or glass forming oxide __________________________________________________________ SILICON OXIDE Oxide Formula: SiO2 Molecular Weight: 60.1 * Melting Range: The forms of silicon oxide, quarts and cristobalite melt at 1713 C. while tridymite, another form of silicon melts at 1670 C. In glazes, base or fluxing oxides are used to lower the melting range of refractories such as alumina and silica. The clay artist should keep in mind that by controlling amounts of alumina and silica in a glaze, in relation to the amount and type of base or fluxing oxides acts to control the maturing temperature of the glaze. By adding silica to a glaze at a given maturation temperature, the refractoriness of the mixture will increase. * Viscosity: Silicon, when mixed with a correct amount of flux, will result in a fluid melt. Because of the refractory nature of silicon, larger amounts in a glaze will tend to decrease fluidity. * Melting and Mixing Characteristics: Silicon oxide is the principal glass-forming ingredient in nearly every glaze and it is indispensable for the creation of a useful glaze. This oxide usually makes up between 45 and 70% of the oxides in a glaze The glass-forming oxide combines easily with the bases or fluxes as well as the neutrals or amphoteric oxides such as alumina. * Surface Softness or Hardness: Silica bestows rigidity and durability as well as tensile strength to glass and glazes. * Clay-Glaze fit: The glass-forming acid has a very low coefficient of expansion and contraction. It works to reduce crazing and improve glaze to clay fit. * Problems and precautions for use: Very small amounts of silicon oxide have no effect in glass formation and the mixtures usually remain dry with no indication of chemical change in the heat. On the other hand, very large quantities do not flux to create glass and the surface will remain dry and unmelted. Extensive breathing of silicon can create the disease known as 'silicosis'. Portions of the lung begin to harden and breathing becomes difficult. The clay artist should avoid working in environments which are filled with silica dust and a protective dust mask will help to avoid the accumulation of the dust in the lungs. _________________________________________________________________ ----------------------------------------------------------------- (c) 1994 Robert Fromme For educational use, only. ------------------------------------------------------------------