If we mix our own glazes, at some point it usually becomes necessary to use a substitute raw material. Who among us hasn’t been out of a critical material at a critical time (usually when getting ready for a big show or sale)? In an ideal world, we would plan months ahead to restock glaze materials; however, unforeseen events sometimes force substitutions. Some comprehension of which materials can be used when a favorite feldspar or metallic coloring oxide is not available is an asset to the independent potter.
In the long run, the most common reason for glaze material substitutions is one of economics. Potters make up less than VSY of 1% of the raw material purchasing market in the United States. When your favorite Super X feldspar is no longer available, it does not imply Super X has been mined out of existence. On the contrary, there is frequently enough feldspar still at the mine to keep potters supplied for hundreds of years. But mining the material for industrial customers is no longer profitable. The large buyers of materials dictate the market decisions, not the potters. Over time, many of our favorite glaze materials will become obsolete. Potters do not have enough economic power to demand that a mine or processing plant maintain production on any specific raw material. (See “Economics and Raw Materials” in the January 1989 CM.)
Many times potters are faced with the need to substitute a raw material when testing glaze recipes from a magazine or book. Often a particular feldspar or frit can no longer be obtained. Other variables, such as different interactions of glaze and clay bodies, various kiln heating and cooling rates, glaze application thickness or individual glazing techniques, already affect results, so any substitution can have a significant effect on the recipe. When more than one material is substituted, the odds of the other variables throwing off the glaze go up. The general rule in such situations is, the more replacement materials required in a glaze recipe, the further removed from the original will be the fired result.
Moreover, an insidious and sometimes subtle change can occur due to geological changes of raw-materials deposits. The transformation can happen from one bag of material to the next. That old bag of Kentucky Ball Clay (OM 4) you’ve had in the studio for a while might not yield the same results as a new bag. The mines do an excellent job of maintaining quality control— remember, they are supplying large industries who demand consistency—but changes in materials for any number of reasons can and do happen. Always be prepared to make a substitution due to a “shift” in a raw-materials composition. Just because the name on the bag is the same doesn’t always mean what’s inside the bag is the same every time.
Good studio organization involves ordering and stocking raw materials before beginning a glazing cycle. It is always better not to be in the position of needing a replacement material; however, when it is time to reorder materials, always be exact as to the chemical name, common name, mesh size, and the name of the mine or processor.
Be aware that the ceramics supplier orders the material from a processor, and sometimes several processors sell versions of a raw material. Each company can market a product with a slightly different chemical composition or particle size, but still refer to the material by the same generic name. Occasionally, the slight difference is distinct enough to affect the glaze.
The ceramics supplier should be able to identify the producer of every raw material. Try to order whole bags of glaze materials, as the company name will often be on the bag. Ordering by the bag will ensure continuity and consistency of materials; besides, the price should be lower on a per-pound basis.
In most instances, substitution of a raw material because of its expense is not an effective cost-cutting strategy. In rare exceptions, such as the extensive use of cobalt oxide, cobalt carbonate, tin, nickel oxide, nickel carbonate or specific stains, the most expensive part of any glaze recipe is the time and effort required to put the glaze on the pot. Using a less expensive material will not yield a true savings if it causes a higher defect rate or if it does not produce acceptable results. The most economic way to judge raw materials is through their reliability and ease of use, not their initial cost to purchase. Substituting one feldspar for another to save 1 <£ or Tg- per pound is a false economy if the substitute feldspar does not function properly. Such “savings” could “cost” a production delay.
Another example of a false economy is not using a commercial gum, such as CMC, or any other “expensive” glaze additive in the glaze mix. If any additive saves one pot from a glaze application or firing defect, or makes the glazing operation more efficient, it more than pays for itself. The amount needed represents a small percentage of the total glaze cost, and just a fraction of the cost of your time and effort. Don’t waste time chasing pennies; chase dollars.
Before mixing any new glaze in your studio, check to make sure all the materials are still available. Keep up to date on which materials have changed or are no longer being produced. Ask yourself if it is worth the time and effort to mix a glaze with a material that can no longer be easily obtained. We all know of potters holding onto their last pound of Albany slip or Oxford feldspar. At some point, they will have to find an adequate substitution for these once-popular materials.
Some potters will mix a 30-gallon batch of the glaze with a substituted material, use it on all of their pots, then, when the fired glaze does not meet their expectations, wonder what went wrong. Never use an untested material on a whole kiln load of pots. The potential for loss from such “experiments” is too great. A better method is to mix up a small sample, then test the glaze on several vertical tiles throughout the kiln. Vertical tiles indicate if a glaze has a tendency to run down the tile surface. Placing the same glaze in different locations in the kiln will show how a glaze reacts to variations in temperature and/ or atmosphere throughout the kiln. Using an old kiln shelf under the test tiles will prevent shelving damage if the glaze runs. Test pieces should be fired in the same kiln as the regular production pots to assure consistent results.
Achieving a perfect substitute for every possible glaze material is impossible. Many times, alternative materials will have trace elements that might slightly change the color or texture of the original glaze. Several other factors can hinder the substitute material from yielding similar results, such as a difference in the materials particle size, chemical composition or processing methods. While a precise equivalent material will not be possible in all situations, the substitutions listed here will work in a high percentage of glazes.
Raw materials used in glazes can be classified in many different ways. Chemical composition and particle size are two useful indicators in choosing a replacement material. Always look for similarities in both when trying to substitute one material for another. Ask the ceramics supplier for a chemical analysis sheet for every raw material ordered. It will list the mesh size and the chemical composition of the material. Then, when substituting raw materials, analyses can be compared to select a material with the closest chemical composition and particle size as compared with the original glaze material. Remember, most raw glaze materials look like white or of T-white powder and feel similar to one another. Telling the difference in mesh sizes of powders is almost impossible, so rely on the chemical analysis sheet for this critical information.
Frequently, clay is a component of glazes. It contributes silica and alumina and can be classified in groups; the most common are earthenware, stoneware, kaolin, fireclay, ball clay and bentonite. Some groups can be divided into subgroups, such as plastic or nonplastic kaolin. Always choose the replacement clay from the same group or subgroup of clays that are available. This will ensure the optimum glaze match.
Listed below are raw material substitutions and an explanation on how and when to use each substitute material. Raw materials and clays that were not included either do not have a practical substitution or involve glaze calculation to arrive at a close material match. Groups of materials enclosed in parentheses are direct substitutes; for example,(Custer feldspar and G-200 feldspar). Both are potash feldspars that can be substituted for one another in a glaze.
Glaze Material Substitutes
Albany slip», a dark-brown-firing, earthenware type, high-iron clay found in Albany, New York, is no longer being mined. The land on which Albany slip is found is more valuable as real estate than for its clay deposits. Many ceram- ics-supply companies have developed Albany-like substitutes, including Alberta Slip, Seattle Slip, Sheffield Slip Clay Formula, A.R.T. Albany Slip Synthetic, Laguna Clay Company Albany Slip Substitute, with varying degrees of success. Matching true Albany slip with any of the substitutes will depend on the total amount required in the original recipe, the firing temperature of the glaze and the kiln atmosphere.
Alumina hydrate is used in glaze recipes to promote hardness and opacity. Alumina oxide can usually be used as a substitute on a one-for-one basis. Calcined alumina can also be used for either alumina hydrate or oxide, but it might cause raw glaze fit problems.
Ball clay contributes silica and alumina to the glaze formula. Due to its small platelet structure, ball clay also acts to suspend the liquid glaze in the glaze bucket. The higher amount of iron and manganese in each type of ball clay will contribute to a darker fired ball clay color. However, it might not have an effect on the fired color of the glaze, as the percentage of ball clay in most recipes will be low. Therefore, the effect of a dark- versus a light-firing ball clay will not be significant in the fired result of colored glazes. In clear or white glazes, dark-firing ball clay can tint or shade the fired glaze. Some light-firing ball clays are (Tennessee #1, SPG#1, Tennessee #10, Coppen Light, H. C. Spinks C&C, Old Hickory #5, Old Hickory #1 Glaze Clay). Off-white or cream- colored ball clays are (Foundry Hill Cream, #1 Glaze Clay, Jackson, Kentucky OM 4, Kentucky Special, Kentucky Stone, M&D, Thomas, Taylor, XX Sagger, Tennessee #9, Spinks HC5, Gold Label).
Barium carbonate (Chinese and German) acts as a secondary flux, helping to bring primary fluxes into a melt. High amounts of barium carbonate can cause opacity in glazes. Strontium carbonate, using ¾ of the amount of barium required, will make an adequate substitute when color or texture responses are not mandated. When less than 6% barium carbonate is used in glazes that do not contain metallic coloring oxides or stains, there is often no noticeable change in the glaze when all or part of the barium is removed.
Bentonite can be used in a glaze recipe to keep the liquid batch in suspension. Usually, 1% to 2% bentonite is added. Higher percentages are used in once- fire glazes to ensure better fit with the raw clay body. Light-firing bentonites used in glazes are (HPM-20 air purified, 325-mesh Western Bentonite, 200-mesh Western Bentonite, Bentonite B, Bentolite White GK129). Ibex-200 is a dark-firing bentonite that can be used in darker colored glazes. Other, more effective, suspension agents can also be substituted for bentonite: (Epsom salts [magnesium sulfate], Macaloid, Veegum T and Veegum CER that contains a combination of Veegum and CMC). While CMC can be used as a glaze suspension aid, its primary function is that of a glaze binder.
Bone ash, whether natural calcined animal bones (calcium phosphate) or synthetic bone ash (tri-calcium phosphate) produced from other calcium phosphate materials, can be used interchangeably to contribute opalescence and opacity to glazes. However, in some glaze recipes requiring natural bone ash, synthetic bone will modify the color.
Borax is a soluble flux that, when utilized in amounts of more than 10%, has all the inherent application and firing problems associated with any soluble material. An insoluble form known as (fused borax or calcined borax) can be used as a substitute, using ½ the total amount of borax.
Dolomite contains approximately one-half calcium and one-half magnesium. Frequently equal parts of whiting and magnesium carbonate can substitute for dolomite. For example, if the glaze recipes calls for 10% dolomite, 5% whiting and 5% magnesium carbonate can be substituted. This will not be an exact substitution, but a close match. Trace elements in natural deposits prevent an exact match.
Feldspar, one of the most common materials found in glazes, can be classified into three groups: potash feldspars (Custer, G-200, K200 and Primas P—no longer available are Buckingham, Oxford, Yankee, Clinchfield #202, Keystone, Maine, Madoc H, A-3 and El- brook); sodium feldspars (Kona F-4, Nepheline Syenite 270x, Nepheline Syenite 400x, Calspar, Primas S, NC-4, Unispar 50, C-6 and Minnspar 200— no longer available are Eureka, Bainbridge, #56 Glaze Spar, Lu-Spar #4, Minpro #4 and Clinchfield #303); and lithium feldspars (spodumene, lithospar and petalite—lepidolite is no longer available). Choose a feldspar from within its own group for a substitution.
Flint or silica, is one of the most common raw materials found in glazes. It is sold as 400, 325 and 200 mesh, all of which are suitable for use in glazes. (Siltex 44 and Silica IMSIL A-25) are fused amorphous silica; because of their low-expansion rates, their use can cor rect crazing defects. Both can be used in place of 400-, 325- or 200-mesh flint, but if a glaze recipe calls for 200-mesh flint, 325-mesh flint would be the best option. Some glazes are sensitive to finer grind materials. The smaller mesh materials can increase glaze melt—increased surface area produces more of a reaction with other glaze materials—which might cause a glossier surface. Another possible result of using finer-mesh flint is in removing craze lines from a glaze, or preventing glaze pinholes.
Frit contains oxides predetermined as to quantity and type, which are then melted, fast cooled and ground into a powder. In a sense, frits are “man-made” feldspars. They can contain soluble oxides in an insoluble form; they can also contain toxic materials in nontoxic glassy matrixes. The chart below lists commonly used frits and their equivalents.
Gerstley borate, a calcium borate ore, contributes a strong fluxing action and can create opalescent opacity in the fired glaze. In many glaze recipes, colemanite can be substituted without a noticeable difference; however, both colemanite and Gerstley borate are variable and cannot be depended on for consistent glaze results.
Kaolin can be classified as plastic and nonplastic. Because its primary purpose is to contribute silica and alumina to a glaze, any of the kaolins can be substituted for one another; however, it is always best to substitute from the same group. The plastic kaolins include (Edgar Plastic Kaolin, Grolleg, Kaolex D-6, McNamee, 6 Tile, Pioneer, Laguna #1, Sapphire, Treviscoe and T-7); nonplastic kaolins are (Kaopaque 20, Ajax P, Delta, SnoCal 707, Kingsley, English China Clay and Velvacast). Some raw kaolins, such as Avery kaolin, cannot be readily substituted; however, any kaolin that has been calcined—heated to remove its chemical water (Glomax LL or Ajax- SC)—can be used interchangeably. Cal cined kaolin can be produced by firing any kaolin past dull red heat—approximately 1100 F. Calcined and regular kaolin can be used interchangeably but the reduced shrinkage of the calcined kaolin can change the raw glaze fit as it dries on the pot.
Soda ash, sodium carbonate, is highly soluble and not usually found in glaze recipes; however, common baking soda (sodium bicarbonate) can be used as a substitute, as it changes to the carbonate form when heated.
Talc contributes silica and magnesium to a glaze, and moderate amounts will cause opacity. Not all talcs are the same. On the East Coast, NYTAL HR100 Talc is commonly used; a West Coast equivalent would be Pioneer- 2882. There are many other talcs, including Sierralite, high-alumina content; Soapstone 78SS, dirty for use in glazes; TDM 92, high-organic matter; or Talc 80/20, a partly calcined material that can be used in dry-pressed clay bodies.
Whiting, calcium carbonate, is a high- temperature flux that can make a glaze harder; it can also develop a chemically resistant glaze surface. It is produced in various mesh sizes under different trade names by many companies. Several kinds that can be interchanged successfully are (Snowcal 40, Vicron 2511, York White, Whiting 55 C, Whiting 3 C Calcium Carbonate, and Goldbond Whiting, #10 White). Always choose a replacement calcium carbonate with approximately the same mesh size as the original. Atomite, a fine-particle-size whiting, might produce a transparent, glossy, clear glaze, as opposed to a coarser grade of whiting that would not thoroughly dissolve in the molten glaze, thus producing a white, opaque glaze.
Oxides, Carbonates and Opacifiers
Cobalt oxide is one of the most potent metallic coloring oxides used in glaze recipes. The smallest amount will create a blue tint in almost any glaze. Cobalt oxide is one and one-half times stronger than cobalt carbonate, so allow for the difference in any substitution. For example, if the recipe calls for 1% cobalt oxide and cobalt carbonate is being substituted, use 1.5% cobalt carbonate. When cobalt oxide is substituted in glazes requiring cobalt carbonate, it sometimes produces blue specks in a blue field. This is because cobalt oxide is coarser and has a larger particle size than cobalt carbonate. The blue specking is more likely to occur in satin matt or matt glazes where the larger particle of cobalt oxide is not fully incorporated into the molten glaze. Cobalt oxide blue specking is less likely to occur in gloss or transparent glazes.
Copper oxide, including black cupric oxide and red cuprous oxide, is one of the most reactive metallic coloring oxides. The range of glaze colors produced depends in part on the kiln atmosphere and the composition of the base-glaze recipes. -Copper oxide and copper carbonate can produce greens, browns, blacks, turquoise and reds in glazes. Both the red copper oxide and black copper oxide can be used interchangeably, with the red tending not to mix completely in the liquid glaze. Some red copper oxide always floats on the liquid glaze surface but does not affect the color or texture of the fired glaze. Both red and black copper oxides can cause a speckled color in some glazes. Copper carbonate can be substituted for copper oxides by using one and one-half times more than the amount of copper oxide required in the glaze recipe. Because of its smaller particle size, copper carbonate disperses readily into the fired glaze.
Manganese dioxide produces purple or brown in glazes. To substitute manganese carbonate, use one and one-half times more manganese carbonate than the amount of manganese dioxide in the recipe. Granular manganese dioxide is not suitable for glazes, as it will not disperse into a glaze melt because of its large particle size.
Nickel oxide, including nickel oxide black and nickel oxide green, is a strong coloring agent, which can produce browns, grays and, under some conditions, yellows and violets. Nickel oxide black and green can be used interchangeably, while nickel carbonate should be substituted at one and one-half times the required amount of nickel oxide.
Red iron oxide can yield browns, grays, greens, yellows and many other variations of earth tones. Spanish red iron oxide is an ore. Synthetic iron oxides are numerous with differing strengths of iron content and purity; (Iron oxide red #2199, Iron oxide #84, Iron oxide #98) can be substituted for Spanish red iron oxide, but the exact ratio of substitution should be tested. Keep in mind the synthetic brands of iron oxide can be stronger than natural iron oxide ores and a one-to-one substitution will not always produce an exact match. Other types of iron oxide derivatives, such as (Red NR #4686, Red NR #4284) also have to be tested to match. Black iron oxide is a slightly coarser grind than red iron oxide and produces more greens and browns than the red iron varieties.
Rutile is a titanium and iron combination, which produces pale tans, light browns or blue in glazes. The glaze color will depend in part on the base-glaze recipe, application thickness, kiln atmosphere, firing temperature and the time it takes to reach that temperature. Light rutile is a fine light brown powder. (Milled Ruflux 61 or dark rutile) is coarser in particle size and a darker brown in the raw state. A one-to-one replacement of light rutile to Ruflux 61 is possible, with the understanding the fired glaze color will be darker.
Zirconium silicate is used to increase opacity in glazes. Several types in various particle sizes are produced. The smaller the particle size, the less opacifier is required in the glaze recipe. The smallest particle size starts with (Excelopax, Superpax A, Superpax Plus, Zircopax Plus, Superpax, Zircopax A). (Opax, Ultrox) are also opacity-producing materials that can work as direct substitutes. The best match will occur when choosing the nearest particle size. Zircopax Plus and Superpax Plus have slightly higher zirconium contents than the other opacifiers, and less should be used to get the same level of opacity when replacing them with other opacifiers. Because Zircopax is no longer being produced, (Zircopax Plus and Zirconium silicate-RZM) are appropriate substitutes; however, (Zircon G Milled and Zirconium Spinel) are coarser and tend not to be a close match. For a soft or “butter fat” quality, as opposed to the “refrigerator” whites produced with the zirconium silicates, use tin oxide.
Substitution Risks and Rewards
The best position to be in when mixing glazes is not to need to substitute any materials, but when reduced to using a substitute material, keep in mind that many other unforeseen variables may affect the fired result. Statistically, some glaze material substitutions do offer less risk, though. Just remember: in ceramics, nothing is perfect and nothing stays the same. The best results come about when advance planning takes place. Testing should be carried out over a series of separate kiln firings to ensure confidence in the results.
Learning the basic composition of each material and how it works in a glaze gives the potter greater flexibility in developing a glaze palette. It also offers the freedom to experiment with various materials to achieve new textures and colors. Often more than one possible substitute material is available. The correct choices are dependent not on chance but a thorough knowledge of the materials.
This article was excerpted from the November 1997 issue of Ceramics Monthly.