How to measure clay shrinkage with a test tile
A step-by-step way to find your clay body's real shrinkage percent using a marked test tile, so wet-to-fired sizing stays accurate.
Why a test tile beats the bag label
Clay suppliers print a shrinkage figure, but the number you get depends on your kiln, your cone and how you work the clay. A tile fired in your own studio captures all of that. Making one takes a few minutes and removes the guesswork from every project that has to hit a specific finished size, from lids that must seat to tiles that must fit a grid. Once you know your true percent, this calculator turns it into reliable wet sizes.
Making and marking the tile
Roll a flat slab of well-wedged clay and cut a strip a few centimeters wide. While the clay is still soft, scribe two fine lines exactly 10 centimeters apart with a ruler and a needle tool, since a round 10 gives easy math later. Mark the clay body and cone on the tile so you can tell tiles apart. Let it dry slowly and evenly to avoid warping, which would distort the measurement you are about to take.
Firing and doing the math
Fire the tile through bisque and then to the cone you normally use for finished work, because most shrinkage happens as the body matures at top temperature. When it is cool, measure between your two lines again with the same ruler. If the marks are now 8.8 centimeters apart, the piece lost 1.2 centimeters, so shrinkage is 1.2 divided by 10, times 100, which is 12 percent. Enter that percent into the calculator to project any wet or fired size.
Using the number in real projects
With the percent in hand, planning gets simple. To build a mug that should finish at 8 centimeters tall from a body that shrinks 12 percent, divide 8 by the 0.88 factor to get a wet height of about 9.09 centimeters. Fire a fresh tile whenever you change clay bodies, switch cones, or notice pieces missing their target, since even a one percent drift adds up on larger work. Keeping a small library of tiles and their percents pays off every firing.