How to Test Clay Shrinkage in Your Studio

A small, repeatable test tells you how much a particular clay body changes in your own making and firing process. Use the same marked length at each stage, and calculate the wet-to-final result when sizing work from its finished dimensions.

Make and mark test bars

Take clay from the batch you want to understand and wedge it as you normally would. Make several narrow, straight bars of consistent thickness; repeating the test helps reveal variation between pieces. While the clay is still workable, draw two fine, permanent reference marks exactly 100 mm apart. Measure between the marks, not from the ends of the bar, which may warp or chip. Label each bar so you can connect the result to its clay batch and firing.

Measure through the process

Record the distance between the same marks while the bar is wet. Let the bars dry evenly and completely to bone dry, then measure again. If you want to separate drying shrinkage from firing shrinkage, keep this bone-dry measurement. Fire the bars using the schedule you want to evaluate. After they are fully cooled, measure between those original marks once more. Do not measure a hot piece.

For each bar, calculate shrinkage from a chosen starting length with 100 × (starting length − later length) ÷ starting length. For wet-to-final shrinkage, use the wet measurement as the starting length and the cooled fired measurement as the later length. If the marks began 100 mm apart and finish 88 mm apart, wet-to-final shrinkage is 12%.

Keep useful records

Write down the clay body and batch or lot, date, how the clay was prepared and the forming direction, the measured wet, bone-dry and cooled fired lengths, and any visible warping or cracking. Record the kiln and firing details, including the cone schedule and whether the bars were placed near the kiln's top, middle or bottom. Clay response can vary with preparation, thickness, drying, firing conditions and position; one result is not a universal property of every batch or kiln load. Compare multiple bars and repeat the test if a result looks unusual.

When entering a value in a wet-to-final sizing calculator, use the percentage based on wet length and cooled final-fired length—not the separate dry-to-fired percentage. Use measurements from the body and process relevant to your work.

What this test does not tell you

Shrinkage testing is a dimensional test, not an absorption, durability or food-safety test. It cannot establish whether a fired vessel is suitable for food contact. Use appropriate testing and guidance for those questions.

For a reference procedure, see Digitalfire's SHAB test notes. Try the shrinkage calculator, browse all guides, or read drying vs. firing shrinkage.