DiamondCore Tools Shrinkage Ruler  £21.60

An innovative way to measure the shrinkage of your clay. Works great when making replacement lids for a casserole dish that has been bisque fired. 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 both sides of the same ruler, 6% and 10% on one side and 12% and 14% on the other side,)

Directions: HOW TO USE YOUR SHRINK RULER

To replace a lid for a casserole dish that has been bisque fired, measure the casserole dish with a regular ruler.  Using that same measurement, measure your wet clay with the Shrink Ruler on the side with the closest shrinkage rate to your wet clay.

To replace a lid for a casserole dish that has been glaze fired, measure the casserole dish with a regular ruler.  Using that same measurement measure your wet clay with the Shrink Ruler on the side with the closest shrinkage rate to yours.

Example: If you want a piece to measure 8” after 12% shrinkage, use the 12% scale on the Shrink Ruler to measure 8” on the wet clay.

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. 
Bisque fire the clay slab. 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) S=percentage of shrinkage, F= length of line in finished clay slab (with regular ruler). 
Fire the clay slab to maturity and repeat steps 3 and 4 (this will allow you to determine the shrinkage of your clay after both bisque and glaze fired).