Today, we live in an age of super abundance of ceramic raw materials. Innumerable clays and glaze materials offer us a bewildering array of choices. Far from understanding glazes and these materials as familiar rocks, feldspars, and clays, each with unique personalities, we know them only as white, gray, or brown powders neatly packaged in uniform bags.
But we all know that getting to know them better can only improve our work. Happy learning! – Jennifer Poellot Harnetty, editor.
There are probably as many kinds of clay as there are riverbanks, creekbeds, roadcuts, abandoned coal mines and backyard gullies, but most of the clays that many of us use on a regular basis are commercially mined.
Because not all materials are available through all suppliers, this chart is meant to provide data for the most common clays used in recipes you are likely to come across. You can use these data to compare the materials available through your supplier, or those you have on hand, with materials in the published recipes.
While the satisfaction, discovery and personal control that is possible through prospecting and processing your own clay are certainly valid reasons for the effort, most of us rely on the consistency and (relative) reliability of materials mined in large quantities. Even though the reasons for using commercially mined clays are most often based on a desire for a trouble-free product, the properties of clay as a natural material can make this goal somewhat elusive.
The following chart contains recent information, however, because the chemical and physical makeup of naturally mined materials can change across a given deposit, this chart is meant to be used as a starting point for clay substitutions. In order to precisely recalculate a recipe using a substituted clay, you will need to obtain a current data sheet for all materials you purchase from your supplier.
Please note that the clays are presented in alphabetical order, and the formulae are presented with alumina (Al2O3) in unity (totaling 1). This makes it easier to immediately see the ratio of alumina to silica, and also more accurately compares the relative amounts of all other components in the clays.
|Primary Function of Common Ceramic Raw Materials|
1. Substituting glaze ingredients may alter color,
2. Test and record your results.
3. Materials vary from supplier to supplier and batch to batch.
**First published in 2010