Out of Stock, Not Out of Luck: Guidelines for Raw Material Substitutions Jeff Zamek

Raw Material Substitutions 

There are several levels of difficulty when considering a glaze material substitution. Even the most closely aligned substitution can cause a different outcome than expected. Many glaze formulas were first developed using a feldspar, clay, or other raw materials, which are no longer in production. Potters can make the mistake of using a raw material in their studio thinking it is still being sold in the ceramics market. For example, Oxford feldspar, a potassium-based alkali alumina silicate, is no longer being mined. If you have a container of Oxford feldspar in your studio and use it in a glaze, there might not be a readily available re-supply. 

In some instances, even if the raw material is still in production, it might have subtly changed in chemical composition, particle size, or organic content, all of which can alter the glaze result. Often, ceramics suppliers are unaware of changes in the raw materials they sell. The best course of action, though time consuming and inefficient, is to test raw materials before committing to a production glaze batch. 

Another potential source of error occurs when potters use an inappropriate substitute material in the glaze or clay-body formula. For example, when requiring Custer, a potassium-based feldspar, do not use Minspar 200, a sodium-based feldspar. Feldspars used in glaze and clay-body formulas should be substituted within their groups, which are sodium, potassium, and lithium. 

Substitute Clays from Within the Same Group 

There are several classifications of clays: 

• Bentonites: One of the smallest-sized platelet clays and extremely plastic. In glaze formulas, 1–2% will keep a liquid glaze in suspension. 
• Earthenwares: Low-temperature clays, some containing high amounts of iron. In clay-body formulas, they contribute to brown fired colors. In glazes, they introduce iron along with alumina and silica. 
• Ball Clays: Very plastic clays when used in stoneware clay-body formulas. In glaze formulas, they keep the liquid in suspension while contributing alumina and silica. 
• Stoneware Clays: Less plastic than ball clays, frequently used in high-temperature stoneware clay-body formulas. They contribute to platelet size variation in clay-body formulas. Less frequently or not at all used in glaze formulas. 
• Kaolins: High-temperature, white clays used in porcelain or white stoneware clay-body formulas. Kaolins are used in glaze formulas for their alumina and silica content. 
• Fireclays: High-temperature clay, not as plastic as ball clays, kaolins, or stoneware clays. Not used in glaze formulas. Fireclays are the most inconsistent in quality control. They can contain carbonaceous materials such as lignite, peat, coal, and tramp materials. Fireclays can also have variable particle sizes of silica, iron, and manganese; the last two metallic oxides can deform and disrupt the surfaces of fired glazes as brown or black blemishes. 
  

Substituting Metallic Coloring Carbonates 

At some point, we have all run out of a metallic coloring oxide when mixing a glaze formula. The general rule is: 1½ times more carbonate will equal one part of the oxide. For example, if a formula calls for 1 part cobalt oxide, we can use 1½ parts of cobalt carbonate, which should yield approximately the same color intensity. While this is not the exact ratio (1.59 is the exact ratio), it will produce functionally correct color intensity.¹ In satin or matte glazes, cobalt oxide can yield a blue field of color with blue specking due to its larger particle size when compared with the smaller particle size of cobalt carbonate. The same ratios in reverse can be used for substituting metal coloring oxides for their carbonate forms. 

Difficult Replacements 

Because of their unique chemistry, some clays are difficult to substitute on a one-for-one basis. Most notable are Albany slip and Barnard clays. While some suppliers have tried to put forth substitutes, at best, some work in limited situations. Newman Red, a high-iron content, low-range stoneware clay, is another clay with unique qualities that has gone out of production. On a broader scale, there are numerous local small-deposit clays that have a limited market or high mining cost. 

Commercially Prepaid Moist Clay 

A discontinued material affects not only potters but also the companies that mix clay for the wider ceramics market. For ceramics suppliers, there is an economic cost in finding and then substituting a material. 

A difficult problem for suppliers is whether to notify customers of the change in material. If the actual clay-body performance stays the same, is a notification warranted? Consistently reliable moist clay is critical in securing a potter’s trust. 

A Ceramics Education 

Since the availability of any given raw material is an unknown, one tool a potter can acquire and sharpen is knowledge of the chemistry and physical properties of materials used in clay-body and glaze formulas. While this seems at first to be an overwhelming subject, one piece of information might get you started: Over 80% of glaze formulas contain just 1 to 12 raw materials in different amounts and ratios, and that narrows the field of study down considerably. 

Acknowledgments: Jim Fineman, my technical editor and a professional potter. 

the author Jeff Zamek started his career 48 years ago. He obtained BFA/MFA degrees in ceramics from Alfred University, College of Ceramics, New York. In 1980, he started Ceramics Consulting Services, a ceramics-consulting firm developing clay body and glaze formulas for ceramics supply companies throughout the US. His books, The Potter’s Studio Clay & Glaze Handbook, What Every Potter Should Know, Safety in the Ceramics Studio, and The Potter’s Health & Safety Questionnaire are available from Jeff Zamek/Ceramics Consulting Services. For technical information, visit www.jeffzamek.com. 

1 Other metallic carbonate to oxide ratios are: 1.4 parts copper carbonate equals 1 part copper oxide, 1.7 parts manganese carbonate equals 1 part manganese dioxide.