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An innovative way to measure the shrinkage of your clay. Works great when making replacement lids for a casserole dish or cookie jar etc.. This flexible two-sided plastic ruler has 4 clay shrinkage percentages to aid in matching lids to bowls etc. 14% 12%, 10%, 6%.
( The photos show the two sides of the same ruler, 6% and 10% on one side and 12% and 14% on the other side,)
HOW TO USE YOUR SHRINK RULER
Measure the dimension of the finished piece you want using a standard inch ruler.
Now find that measurement on the Shrink Ruler on the scale corresponding to the shrinkage of your clay, and mark or form the wet clay piece to that dimension.
Example: To replace a lid for a casserole dish. First measure the casserole dish with a regular ruler to get its’ final desired dimension, say it is 8” diameter in this example. Determine the shrinkage rate of the clay you are using, (The clay supplier can provide this or you can calculate it using the formula below.) Let’s use 12% clay shrinkage for this example. Locate the 12% expanded scale on the Shrink Ruler and measure off 8” and make the lid to that size in your wet clay. When the piece has achieved final firing it will have shrunk to the correct size, 8” diameter in this example.
HOW TO MEASURE THE SHRINKAGE RATE OF YOUR CLAY
Make a rectangular clay slab as thick as you plan to make your piece and 4” X 12”, then with a regular ruler measure and carve a 10” line in the wet clay.
Let the clay dry, then bisque fire the clay slab. Then fire the slab to final firing conditions.
Measure the length of the same line again with the regular ruler.
Use the following formula to solve for S: S=(1-F/M)x100
M=length of line in original moist clay slab with regular ruler (10”)
S=percentage of shrinkage,
F= length of line in finished clay slab (with regular ruler)