[Editor's note: print this document with a monospaced font (Monaco or Courier on a Macintosh) to have the graphs and tables appear in their proper format.] glz21d.txt Clay and Glaze Formulation Robert Fromme ______________________________________________________________ Note: This file contains several sections of my notes on the various oxides which are used in base glaze formulation. #1 glz2d.txt = Lead, Sodium, Potassium & Lithium #2 glz3a.txt = Boron #3 glz3b.txt = Calcium, Zinc #4 glz4a.txt = Magnesium, Barium & Strontium #5 glz4c.txt = Alumina & Silica -------------------------------------------------------------- Notes on Lead, Sodium, Potassium & Lithium The low melting fluxes or bases (RO, R2O) ______________________________________________________________ LEAD (metal) Oxide Formula: PbO Molecular Weight: 223.2 * Melting Range: Lead has been a major fluxing oxide for historical low- temperature glazes and is active as low as cone 022. * Viscosity: Low viscosity, therefore, lead fluxed base glazes are usually smooth, shiny, bright and blemish-free. * Melting and Mixing Characteristics: Lead combines easily with other flux oxides in the melt. When combined with silica, it can make a simple glaze requiring no additional oxides. It can be used in glazes which may be anywhere in the full surface range from matt to shiny. It is also functional in glazes from clear to opaque. *Surface Softness or Hardness: Lead usually results in glaze surfaces which are soft and easily scratched or abraded. * Clay-Glaze fit: It has a fairly low coefficient of expansion, and lead fluxed glazes are relatively easy to fit to a clay body. * Color Response: Lead works with most metals for a strong and pleasant effect on most colorants. Color in lead glazes usually develop a nice, soft warm look. Most base or clear glazes tend to turn slightly yellow when lead is a major flux. * Problems: Lead is not a good choice for glazes designed for reduction kiln atmospheres. Carbon monoxide and free carbon cause glazes with lead to turn dark, boil and bubble as the lead converts to toxic lead oxide. Lead is unstable and begins to burn out or volatilize by cone 6, so it is useless at the higher temperatures. (Volatilize means it begins to change from a molten liquid to gaseous form and move into the kiln atmosphere.) * Precautions for use: Lead should be fritted with silica and used in its fritted form to suppress solubility with acidic foods and gastric acids. Better yet, lead should never be used on ware which might under any circumstances come into contact with acid foods. The laws of many states and nations restrict the use of lead-containing glazes on ware for food. * Poisonous or Toxic: Lead can be very POISONOUS in the unfired state and in the fired glaze. Brief Notes on Lead Glaze History Ancient Lead Surfaces Lead sulfide, or galena, was probably used as a glaze material by ancient Babylonians. The raw lead materials were probably dusted or painted on damp greenware before it was fired. The primitive lead glaze would result as the galena fused with the silica in the clay of the forms to create a shiny surface. This kind of crude lead surface can be found in Europe as late as the Middle Ages. Lead glazes, like those which use primarily Sodium and Potassium, are soft glazes which can turn dull with time and the natural abrasion of weather and wind. Lead glazed surfaces have frequently lost their bright shiny quality by the time the archaeologists find the clay objects. Some glazes which use galena are scummed with a whitish powder due to sulphuration in the firing atmosphere. Raku Glazes Many of the original Japanese Raku glazes were fluxed with lead. The tradition of using lead with raku has continued into our century, however, with the hazards which are associated with its use, most contemporary Raku craftspersons prefer to use other low temperature fluxes like Boron, Sodium, Potassium and Lithium. Majolica and Lusterware Glazes Lead has served as an active low temperature flux for Majolica and Lusterware glazes in the Near East, Europe and Mexico. As with Raku, most contemporary artist-craftspersons choose to turn to other low temperature fluxes in order to remove the lead hazard from their products and their working environment. Enamels Enamels were also originally fluxed with lead. These glaze mixtures are also known as overglaze colors, on-glaze colors or china paints. They are essentially a very low melting glaze base similar to Raku. They usually mature at 700 to 800 deg. C. and are generally applied over a glaze which has already been fired. They provide the potential for bright touches of color. Craftspersons are now turning to other fluxes such as boron and sodium in order to remove lead from their studios and work. Adventurine Glazes A high lead base glaze was usually chosen for low-temperature glazes which had an addition of seven to fifteen percent rust or iron in the mix. The iron melted into the solution during the heating cycle but upon cooling, the iron would begin to crystallize on the surface. The process yielded a soft but flashy red or gold crystal surface. In recent years, fluxes other then lead have been tried for adventurine surfaces with varying effects. Low and Middle Range Crystal Glazes When chrome oxide is saturated into a glaze at up to 12%, a full range of colors from yellow, orange and red crystals are known to develop in low temperature lead glazes (Cone range 010-012). Lead is also a functional flux for glazes which develop crystals in the middle range (cone 4-6). In these glazes, titanium, nickel and copper are often used for color and crystal effects. ____________________________________________________________ SODIUM (alkali) Oxide Formula: Na2O Molecular Weight: 62 * Melting Range: Sodium melts very early and is active all the way through the firing ranges of glazes. It is a good replacement of Lead. Many stoneware and porcelain glazes have sodium as a small portion of the fluxing oxides. In addition to some of the advantages of color response and low viscosity, low melting elements like sodium get the melt moving in the very early stages of the firing before the middle and high temperature fluxes have had a chance to become active. * Viscosity: Sodium encourages a very fluid bright melt with low viscosity, however the crazing in cooling result in the smooth surfaces if sodium is a major flux in the melt. * Melting and Mixing Characteristics: Sodium is similar to lithium and potassium and is only slightly more active then potassium as a low-melting flux. * Surface Softness or Hardness: If Sodium is a primary flux in the glaze, it may give a soft, easily abraded surface. * Clay-Glaze fit: Sodium has the highest coefficient of expansion and contraction of all RO, R2O Oxides and it is used to encourage crackle glazes because of its effect upon the shrinking cooling glaze. * Color Response: Sodium adds intensity and brilliance to the glaze colors which come from metal oxides. It is a vigorous flux and has the ability to encourage rich color responses. When it is used in large amounts, as in the Egyptian alkaline glazes and Egyptian Paste, it can give very rich, vivid, Egyptian and Persian blues and blue-greens when copper is the colorant in the low-temperature glazes. Low levels of alumina in the glaze will be necessary for the best color response in alkaline glazes. Strongly alkaline glazes containing high proportions of potassium, sodium or lithium oxide are noted for their unusual and exciting color response when iron, manganese, cobalt and copper are added to the base glaze. Alkaline glazes have also been used widely because of their potential for rich, bright yellow colors from a variety of metal oxides, including chrome, vanadium, antimony, iron and rutile. Sodium and other alkaline fluxes are important ingredients in copper reduction glazes ranging from pink and red to purple, at higher temperatures. When manganese is used in a soda glaze, it gives a reddish purple color, while if potassium is used in place of sodium, the manganese yields a blue purple. * Problems: Many of the raw materials which are used to provide sodium in a glaze recipe are water soluble so it is frequently introduced into a glaze in fritted form. * Precautions for use: Some of the raw material forms of sodium, such as soda ash, are caustic and can be harmful to your skin while in the glaze bucket. Other forms are very soluble so they must be kept in air tight containers. Many craftspersons prefer to use the soda feldspars and frits to avoid the problems of solubility, etc. At lower temperatures, most alkaline glazes also contain a mixture of other fluxes (boron or lead, for example) in order to help create a more functional glaze surface. * Poisonous or Toxic: Soda Ash is Caustic and can cause irritation and burning of the skin. Brief Notes on Sodium Glaze History Sodium glazes of Antiquity Ancient Alkaline glazes were perhaps the earliest glazes to be use by craftspeople. We have Egyptian Paste, Egyptian Blue Glazes and the glaze faced bricks on the Neo-Babalonian 'Ishtar Gate' to suggest the very ancient origin of alkaline glazes in the Near East. The early Mediterranean alkaline glazes tended to craze and peel away from the early forms. The early glazes also were so charged with sodium or other alkaline flux that the element was often soluble in food acids. Sodium is an important constituent of ordinary glass, which is composed primarily of silica, soda, and lime. The history of sodium glazes and the development of the ancient glass industries are tied together by this ancient and important melting agent. Many formulae for lusterware and majolica include alkaline fluxes like sodium. Sodium can be found in recipes for enamels, Raku, Adventurine and crystalline glazes. Of course, sodium is the primary flux in salt glaze. This technique seems to have originated in Europe of the Middle Ages. Sodium feldspars are frequently included as part of the ingredients for historical and contemporary ash, slip, stoneware and porcelain glazes. -------------------------------------------------------------- POTASSIUM (alkali) Oxide Formula: K2O Molecular Weight: 94.2 * Melting Range: Potassium, like sodium, melts very early and is active all the way through the firing ranges of glazes. It is a good replacement of Lead. Many stoneware and porcelain glazes have potassium as a small portion of the fluxing oxides. In addition to some of the advantages of color response and low viscosity, low melting elements like potassium get the melt moving in the very early stages of the firing before the middle and high temperature fluxes have had a chance to become active. * Viscosity: Potassium encourages a very fluid bright melt with low viscosity, however the crazing in cooling result in the smooth surfaces if potassium is a major flux in the melt. * Melting and Mixing Characteristics: Potassium is similar to lithium and sodium and is only slightly more active then potassium as a low-melting flux. * Surface Softness or Hardness: If Potassium is a primary flux in the glaze, it may give a soft, easily abraded surface. Some craftspersons prefer Potassium over sodium as a low melting flux because it is said to produce a more brilliant glaze than soda, especially when lead is present. It also gives a slightly longer firing range then soda. * Clay-Glaze fit: Potassium has one of the higher coefficients of expansion and contraction. Like sodium,is used to encourage crackle glazes because of its effect upon the shrinking cooling glaze. Potassium is not quite as high as sodium in its effects of expansion and contraction in the heating and cooling melt. * Color Response: Potassium adds intensity and brilliance to the glaze colors which come from metal oxides. It is a vigorous flux and has the ability to encourage rich color responses. Like Sodium, when it is used in large amounts, as in the Egyptian alkaline glazes and Egyptian Paste, it can give very rich, vivid, Egyptian and Persian blues and blue-greens when copper is the colorant in the low-temperature glazes. Low levels of alumina in the glaze will be necessary for the best color response in alkaline glazes. Strongly alkaline glazes containing high proportions of potassium, sodium or lithium oxide are noted for their unusual and exciting color response when iron, manganese, cobalt and copper are added to the base glaze. Alkaline glazes have also been used widely because of their potential for rich, bright yellow colors from a variety of metal oxides, including chrome, vanadium, antimony, iron and rutile. Potassium and other alkaline fluxes are important ingredients in copper reduction glazes ranging from pink and red to purple, at higher temperatures. When potassium is used in place of sodium, manganese yields a blue purple, however, if manganese is used in a soda glaze, it gives a reddish purple color. * Problems: Some of the raw materials which are used to provide sodium in a glaze recipe are water soluble so it is frequently introduced into a glaze in fritted form or a potash feldspar its used in the recipe. When pearl ash or potassium carbonate is use as the raw material source for potassium, it acts as a deflocculant so the glaze may become very runny when a normal amount of water is used in the glaze bucket. * Precautions for use: One raw material form of potash, called potassium carbonate or pearl ash is very soluble and deliquescent (will absorb moisture from the air) so it must be kept in an air tight container. Many craftspersons prefer to use the potash feldspars and frits to avoid the problems of solubility, etc. At lower temperatures, most alkaline glazes also contain a mixture of other fluxes (boron or lead, for example) in order to help create a more functional glaze surface. Brief Notes on Potassium Glaze History Potassium glazes of Antiquity Ancient Alkaline glazes were perhaps the earliest glazes to be use by craftspeople. We have Egyptian Paste, Egyptian Blue Glazes and the glaze faced bricks on the Neo-Babalonian 'Ishtar Gate' to suggest the very ancient origin of alkaline glazes in the Near East. The early Mediterranean alkaline glazes tended to craze and peel away from the early forms. The early glazes also were so charged with potassium or other alkaline flux that the element was often soluble in food acids. Many formulae for lusterware and majolica include alkaline fluxes like sodium. Potassium can be found in recipes for enamels, Raku, Adventurine and crystalline glazes. Potash feldspars are frequently included as part of the ingredients for historical and contemporary ash, slip, stoneware and porcelain glazes. The potassium content of some wood ash may be as high as 12 to 15%. -------------------------------------------------------------- LITHIUM (alkali) Oxide Formula: Li2O Molecular Weight: 30.0 * Melting Range: Lithium, like Potassium and sodium, melts very early and is active all the way through the firing ranges of glazes. It is a good replacement of Lead. Many stoneware and porcelain glazes have lithium as a small portion of the fluxing oxides. In addition to some of the advantages of color response and low viscosity, low melting elements like lithium get the melt moving in the very early stages of the firing before the middle and high temperature fluxes have had a chance to become active. * Viscosity: Lithium encourages a very fluid bright melt with low viscosity. When compared to Sodium and Potassium, crazing in cooling no not result as easily in the smooth surfaces if lithium is a major flux in the melt. * Melting and Mixing Characteristics: Lithium is similar to potassium and sodium as an active low- melting flux. Although the usual alkaline surface is glossy, glassy, fluid, brilliant and clear, it is possible to make a matt alkaline glaze. One will need to make adjustments that will dull some of the brilliance of color and bright, glossy surfaces and encourage the development of surface crystals in the cooling mixture. * Surface Softness or Hardness: If Lithium is a primary flux in the glaze, it may give a soft, easily abraded surface. Lithium, like potassium, gives a slightly longer firing range then soda. When lithium is used as a replacement for sodium or potassium, it reduces the expansion and cooling contraction of the glaze and promotes crystallization. * Clay-Glaze fit: Lithium has one a coefficients of expansion and contraction which is not as high as the other alkalis. It is sometimes used to replace sodium or potassium in order to avoid glaze crazing and encourage glaze fit. * Color Response: Lithium, like potassium and soda, adds intensity and brilliance to the glaze colors which come from metal oxides. It is a vigorous flux and has the ability to encourage rich color responses. By adding only 1 or 2% lithium to a glaze can brighten most of the colors considerably. Low levels of alumina in the glaze will be necessary for the best color response in alkaline glazes. Strongly alkaline glazes containing high proportions of lithium, sodium or potassium oxide are noted for their unusual and exciting color response when iron, manganese, cobalt and copper are added to the base glaze. Lithium helps to encourage copper to yield a blue color. Alkaline glazes have also been used widely because of their potential for rich, bright yellow colors from a variety of metal oxides, including chrome, vanadium, antimony, iron and rutile. Lithium and other alkaline fluxes are important ingredients in copper reduction glazes ranging from pink and red to purple, at higher temperatures. * Problems: Most of the sources of lithium in raw glaze materials are expensive. Lithium Carbonate is also slightly soluble in water while other sources such as Macaloid, Amblygonite and feldspars are less soluble to insoluble in water. Large amounts of lithium can cause a glaze to shrink less then the clay body resulting in glazes that shiver or pots that dunt after cooling. * Precautions for use: Lithium carbonate, a common source of lithium in glazes, is a toxic material - handle with care. The use of amblygonite as a source of lithium tends to cloud the melt and cause boiling and bubbling. (fluoride and phosphorous in the melt create this problem) The high amount of alumina in amblygonite lowers its usefulness at lower temperatures. At lower temperatures, most alkaline glazes also contain a mixture of other fluxes (boron or lead, for example) in order to help create a more functional glaze surface. Brief Notes on Lithium Glaze History Ancient Alkaline glazes were perhaps the earliest glazes to be use by craftspeople. Many formulae for lusterware and majolica include alkaline fluxes like lithium. Lithium can be found in recipes for enamels, Raku, Adventurine and crystalline glazes. Lithium feldspars are frequently included as part of the ingredients for historical and contemporary ash, slip, stoneware and porcelain glazes. ______________________________________________________________ -------------------------------------------------------------- (c) 1994 Robert Fromme For educational use, only. -------------------------------------------------------------- ------------------------------------------------------------- glz3a.txt Clay and Glaze Formulation Robert Fromme ______________________________________________________________ Notes on Boron, the Ambiguous 'Class Clown' of Glazes ______________________________________________________________ BORON Oxide Formula: B2O3 Molecular Weight: * Melting Range: Although Boric Oxide's formula would suggest that it belongs with alumina as a viscous agent and refractory (R2O3) which helps to retard the growth of crystals, that is not the case. In spite of its formula, boron is a major fluxing oxide for the full temperature range of glazes and it is active all through the firing cycle, although it may give a weak, slightly soluble glaze. I am including boron at this point in the lesson so that it will follow the other, more conventional low-temperature fluxes, lead, sodium, potassium and lithium. By introducing boron with the other low-temperature fluxes, it should be easier for you to keep all of them in a clear, functional relationship in you mind. This way, when it comes time for you to choose some of the low- melting fluxes for glaze formulae, you will include boron in the pool of information that you will be considering. * Viscosity: Boron has a medium to low viscosity, therefore, boron fluxed base glazes are usually smooth, shiny, bright and blemish-free. It has less expansion than the alkalies, and has been used frequently with lead, to produce hard, smooth, and transparent dinnerware glazes at lower temperatures. At low temperatures, boron is sometimes used to inhibit crystallization and the devitrification of the cooling glaze. In very low temperatures, some clay artists use boron as a replacement for alumina as a neutral oxide. This is usually attempted when the normal fluxes seem to be less active. * Melting and Mixing Characteristics: Boron is used as a primary flux in low-melting leadless glazes when the amount of alumina is kept to a minimum. In leadless glazes with high boron, it is recommended that several additional fluxes be added to the glaze to insure durability and function in domestic forms. I have also seen boron used as a glass former in low-temperature raku formulae. When Boron is used as a glass former. it yields smooth surfaces with high gloss. The student may find it easy to think of this ambiguous material as the 'class clown' of the ceramic materials. It does not behave the way we might expect. Yet, it is a very important addition to our list of glaze materials. Boron combines easily with other flux oxides in the melt. Its relationship to barium should be noted. When the two are added to the same glaze, an eutectic forms and the glaze will not yield the traditional matt that one would expect from the addition of barium to the base. (Some of us can relate the personal experience of wasting hours of tests in search of a barium matt, only to discover that the glaze seemed to remain glossy and unpredictable due to the inclusion of boron in the formula.) Boron has a reputation for being quite active in the early stages of middle and high-temperature firing. Due to the boiling and bubbling in the melting glaze, it will encourage the development of glazes with mottled surface color. *Surface Softness or Hardness: Boron in large amounts in a glaze usually results in glaze surfaces which are soft and easily scratched or abraded. * Clay-Glaze fit: It has one of the lowest coefficients of expansion and contraction. Boron fluxed glazes are relatively easy to fit to a clay body. When the amount of boron is relatively low, it can help the glaze to resist crazing, however, it is my experience that some crazing will develop if the use of boron is excessive in the glaze mix. * Color Response: Like the alkali, boron works with most metals for a strong and pleasant effect on most colorants. We have mentioned that it will serve at higher temperatures to encourage broken or mottled color effects. When colemanite or Gerstley borate is used a source for boron and calcium, the glaze may develop a wonderful opalescence. Manganese will turn toward violet or purple with certain base mixtures which use boron. Red glazes (except copper reds) seem harder to achieve when boron is used in the base. Copper can work with boron to create a beautiful turquoise blue. Some metals which traditionally yield greens will not work as expected when boron is a flux in the base mixture. Rutile as well as very small amounts of iron oxide often yield mottled milky blues when boron is present in the melt. * Problems and precautions for use: Boric Acid, one source of Boron in a glaze, is deliquescent and must be stored in an airtight container. Boric Acid is also water soluble so one may choose other, less soluble, sources if the glaze will be left in its liquid state for long periods of time. Borax, another source for boron and sodium is also deliquescent and water soluble. Brief Notes on Boron Glaze History Boron in Historical Glazes Like the alkali fluxes, boron has been an active ingredient in formulae of enamel, Raku, Majolica, Lusterware, Adventurine, other Crystalline glazes and even an occasional Copper Red Reduction mixtures. Chun Glazes Chun glazes are not easily understood and some of us find them difficult to define. These are usually slightly opalescent glazes with usually develop when alumina is kept to a minimum and boron is present in the mixture. I have heard several conflicting suggestions concerning the source of opalescence in a glaze. One theory suggests that it is the result of a unique process of devitrification in the cooling glaze. Another theory involves the presence of minute bubbles trapped in the melt, especially those from fluorine and phosphorous when fluorspar and bone ash are raw materials in the glaze. A third suggestion points to the intermixing of several glass structures of slightly different light refraction in the glaze surface. In reality, Chun glazes probably result from a combination of these particular circumstances. Chun glazes are usually a greenish to bluish color resulting from a very small amount of iron in the reduction fired glaze surface. Sometimes the surface color is mottled and boron is usually a critical ingredient in these glazes. 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. __________________________________________________________ ----------------------------------------------------------- (c)1994 Robert Fromme For educational use only. -----------------------------------------------------------