1. Determine What Percentage You Will Measure
Different clay bodies shrink at different rates. These can be as little as 4%, but can also go higher than 15% for some clay bodies. Even a single percentage point can make a dramatic difference in the final product, depending on its overall size and your need for precision.
You will need to know exactly what your clay's shrinkage is for the temperature to which you fire it. If you are using a commercial clay body, this information is often given as part of the clay body's description. If this information wasn't included in the clay's description, you may want to call or email the dealer or manufacturer. If you still can't get the shrinkage rate or if you are using your own clay body recipe, you can run your own test to find out.
2. Do the Math
Once you know your clay body's percentage of shrinkage, it is time to determine the actual change in measurement. Let's use my own favorite low-fire white clay body as an example. At cone 04 (maturation), this clay body has a shrinkage rate of 6%.
- When wet, the piece was 100% of its size. Subtract the loss from 100%. 100% - 6% = 94%
- Convert from percentage to decimal. 94% = 0.94
- Divide. 1 / 0.94 = 1.06
- 1 inch fired equals 1.06 inches wet.
Let's do another example. I also use a mid-range stoneware that has a shrinkage rate of 12.5% at cone 6 (maturation). Here are the steps:
- 100% - 12.5% = 87.5%
- 100% = 1.00 :: 87.5% = 0.875
- 1 / 0.875 = 1.14.
- 1 inch fired equals 1.14 inches wet.
3. Make the Ruler
You will need
- A length of 1 x 3 inch firing strip from sixteen to eighteen inches long
- a fine-tipped permanent marker
- a ruler that measures in decimal increments.
With the permanent marker, mark out sections on the wood corresponding to the wet dimension that equals one after firing. For the 6% shrinkage clay body, our new ruler would be marked out in segments of 1.06 inches each. For the 12.5% shrinkage clay body, our new ruler would be marked out in segments of 1.14 inches each.
Divide each segment in half then in quarters, marking each sub-segment. Label your new ruler's segments and sub-segments as if they were a regular ruler. So, the first segment mark will be labeled "1", the next sub-segment mark will be "1-1/4", and so on.
4. Label and Store Your New Shrinkage Ruler
After making your new shrinkage ruler, be certain to write directly on the ruler what shrinkage rate it is for. A simple "6%" written on the non-measuring side can alleviate a lot of frustration in the future.
One of the best ways I have found for storing shrinkage rulers is to drill a hole at one end, thread a thin chain (such as the ball chain used for lights), then hang the rulers all together off the end of a shelf in the studio. This keeps them easily accessible while also remaining out of the way. I also add a piece of wood to the chain with a list of my clay bodies and their percentages. That way in case I forget which is what, the answer is right there, too.