Material Sense

Ceramic glaze and body recipes often need material substitution or just scrutiny. Their chemistry, mineralogy and physical properties all need consideration.

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Material substitution in bodies, slips, engobes and glazes is a constant and ongoing part of most ceramic enterprises. Often, lack of availability, quality issues and price are motivating factors. "Materials sense" can also save much time and effort by enabling recipe logic. With these, potters and technicians can often identify problems with recipes simply by noting unusual combinations of materials, their percentages or whether they even belong in that type of recipe.

In Glazes, Undergazes

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 of the same type (and thus a similar chemistry). This is often possible with different brand names of the same mineral or a refined generic material (e.g. calcium carbonate, zinc oxide, dolomite). The more complex the chemistry of the raw material, the more likely are issues with changes to another (e.g. feldspar, spodumene). In some cases, substitution recipes are recommended where a mix of two or more other materials is said to be an equivalent. In more complicated cases, the substitution involves substituting one material for another and removing a related amount of one or more others (e.g., substituting petalite to, or for, lithium carbonate). Another problem can occur where, although the chemistry of the substitute is very similar, it's physical properties or particle physics are different enough to affect the working properties of the glaze slurry (e.g. a kaolin). Frit substitutions can be more complex, especially when glaze chemistry is needed to supply the same mix of oxides from mixes of multiple frits and other materials. Metal oxide and stain substitutions can be even more difficult; the color shade can be a combination of percentage, the chemistry of the stain and the chemistry of the host glaze.

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 when the substitute brings 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). As noted, frits especially can have complex chemistries and obviously it is more complicated if they are involved in the substitution calculation.

In Bodies, Engobes, Slips

Materials are employed for their physical presence (e.g. plasticity) and firing characteristics (not for chemistry). Thus, testing must be done to see how maturity, drying properties, plasticity and/or forming behavior, texture, firing color and character, thermal expansion and other properties are affected. It is a matter of accommodating a new material while maintaining the needed properties in the body. For example, to substitute a plastic ball clay a combination of a less plastic one with a bentonite addition might be the most suitable. Where a body's fired color depends on an iron-bearing clay, the substitute faces the challenge of not only imparting the same color, but also the same plasticity and fired maturity (thus a multimaterial approach is almost always needed).