Drying vs. Firing Shrinkage: What the Percentages Mean

Shrinkage percentages describe a change relative to a starting length. Drying shrinkage and firing shrinkage use different starting lengths, so their percentages cannot simply be added to get total wet-to-fired shrinkage.

Three stages, two kinds of shrinkage

Wet length is the measured dimension of shaped, workable clay. As water leaves during drying, the piece contracts to its bone-dry length. Drying shrinkage is the change from wet to bone dry, expressed as a percentage of the wet length.

In ordinary shrinkage testing, the bone-dry piece contracts further during firing to its cooled final-fired length. Firing shrinkage is the change from bone dry to final fired, expressed as a percentage of the bone-dry length. It is not a percentage of the original wet length. Total shrinkage means the change from wet length all the way to cooled final-fired length, expressed as a percentage of wet length.

Why the percentages do not add

Suppose a piece starts at 100 mm wet, measures 94 mm bone dry, and measures 88 mm after firing and cooling. Drying shrinkage is (100 − 94) ÷ 100 × 100 = 6%. Firing shrinkage is (94 − 88) ÷ 94 × 100, or about 6.38%. These percentages have different bases: the first uses 100 mm and the second 94 mm. Adding them (6% + 6.38%) would give the wrong total.

The correct relationship is total = 100 × (1 − (1 − d/100) × (1 − f/100)), where d is drying shrinkage and f is firing shrinkage, each entered as a percentage. Here, 100 × (1 − 0.94 × 0.9362) is approximately 12%, matching the direct wet-to-fired change from 100 mm to 88 mm.

Size wet work from a finished target

If you know total wet-to-final shrinkage and want a finished dimension, divide the target by the fraction that remains: wet size = target ÷ (1 − total/100). For a 150 mm finished target and 12% total shrinkage, 150 ÷ 0.88 = 170.45 mm. The wet dimension should be about 170.45 mm, then checked against your own test results and making tolerances.

A firing-only shrinkage figure cannot correctly size wet work, because it leaves out the contraction from wet to bone dry. Use total wet-to-final shrinkage for wet sizing. If only a dry-to-fired result is available, it describes that later stage and is not a substitute for a measured wet-to-final value.

If a bar expands rather than shrinks, check the measurements and investigate the body and firing separately; this calculator does not model expansion.

Use process-specific measurements

There is no universal shrinkage preset that reliably fits all clay bodies and studio processes. Body formulation and batch, preparation, drying and firing conditions can affect results. Measure the same marked test bar wet, bone dry and cooled after firing; record the body, batch and firing schedule. For more detail on test setup, see how to test clay shrinkage. Digitalfire also explains why firing shrinkage uses a different basis in its firing shrinkage glossary entry.

Use the shrinkage calculator, visit all guides, or return to the studio test guide.