You want to try your hand at mixing your own clay body from dry ingredients. You may have even found a formula or recipe that sounds just like what you are looking for.
Now what? How do you understand what you are looking at?
What Is the Triangle?
Many potters use a triangle as shorthand for the word "cone". Let's say, for example, that you are looking at a clay body recipe that says "Creamy Tan ∆6". It means that this clay body is a mid-range, light tan clay body that matures at cone 6, or at 2165⁰ F (1190⁰ C). If it said ∆4-6, that would indicate that the clay body is mature (but not overfired) in the range of temperatures from cone 4 through cone 6.
What Is the System of Measurement?
Like those of home cooks, potters' recipes are not necessarily consistent across the board. When a potter develops a new clay body, measurements for the ingredients may be given in kilograms, pounds, or in simple percentages. The first thing you will need to know is what system of measurement the recipe you want to use is following.
If the recipe does not indicate what measurement is being used, start by adding the numbers. If they add up to 100, it is safe to assume those are percentages. If not and no weight abbreviations are given, do the math to translate the numbers into their 100% equivalents.
Converting to Percentage
Let's say you have a recipe that has the numbers 58, 38, 4, and 10. Add them together to get 110. Divide 110 by 100 to get the number you should divide all the other numbers by. In this case, it's 1.1. Divide all the other numbers by 1.1, rounding as you go, to get 52.7, 34.5, 3.6, and 9.1. You'll be off by one tenth of a percentage, but that should be accurate enough.
What if you have a clay recipe in percentages and it adds up to be 100%, but at the very end it has one or more other ingredients tacked onto it (usually marked as percentages)? For example:
Plastic Orange Brown
Fire Clay 30
A lot of times the main ingredients are calculated to the 100%, with colorants and grog tacked on as percentages of the main batch afterwards. (Adding the grog can take some clay bodies from throwing to hand building clays in one easy step, so to speak.) Don't worry about the mathematical weirdness; treat the recipe as 100%, then get your mathematician friends to look the other way as you throw in the last few add-ons.
What Are These Ingredients?
Many recipes are given using specific names. For example, EPK is a type of kaolin and Custer is a particular feldspar. Find out more about specific mined clays:
- Dry Earthenware Clays and Slip
- Dry Stoneware and Fire Clays
- Dry Ball Clays Used in Pottery
- Dry Kaolin Clays
Be sure to get the specific ingredient listed in the recipe. For example, although EPK kaolin and Georgia kaolin are very similar, they do have different properties that effect how they work in clays and glazes.
If you still have questions, don't be shy about asking. Ask other potters through the forum.