Material substitutions in ceramic glaze and body recipes must consider their chemistry, mineralogy and physical properties
Material substitution is a constant and ongoing part of any ceramic enterprise that is using clay and/or glaze recipes. Often lack of availability, quality issues and price are the motivating factors. In addition, when recipes need to be used in another locality where the same raw material brands or even types are unavailable, substitutions must be done.
In ceramics, glazes fire the way they do primarily because of their chemistry. The most basic substitution is to simply replace one material for another which has a chemistry similar enough that the fired properties will not be adversely affected. This is often possible with different brand names of the same mineral or refined material (e.g. Calcium carbonate, zinc oxide). The more complex the chemistry of the raw material, the more likely there is to be issues with changes to another (e.g. feldspar). In some cases substitution recipes are recommended where a mix of two or more other materials is said to be equivalent. Another problem can occur where, although the chemistry of the substitute is very similar, it's physical properties or particle size are different enough to affect the working properties of the glaze (e.g. a kaolin) or even it's fired appearance (a metal oxide).
A given chemistry can be supplied by many different mixes of refined and raw material powders, each of these having advantages and disadvantages regarding price, ease of use in production, toxicity, etc. It is common to use glaze chemistry to calculate how to juggle a recipe to substitute one material for another of slightly or even very different chemistry while maintaining the chemistry of the glaze as a whole. The calculation is more complex where the substitute is bringing along other oxides not in the material being substituted (or fewer) or the original or substitute has a very different weight loss on firing (LOI). Frits especially can have complex chemistries and obviously it is more complicated if they are involved in the substitution calculation.
When clay bodies and porcelains require material substitution, the issue is physical properties (which are often not directly related to chemistry). Thus testing must be done to see how maturity, drying properties, plastic and/or forming behavior, texture, firing color and character, thermal expansion and other properties are affected. A series of tests usually must be done to alter the recipe to accommodate the new material while maintaining the needed properties.
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