Drying, firing and total
Clay shrinks twice: as it dries, and again as it’s fired. Total shrinkage runs from the freshly made (wet, or plastic) size to the fired size, and it’s the number you need to make a piece the right size. Ceramics Monthly notes that some clays shrink 8% and others 14% in total.
Digitalfire points out that you can’t simply add the drying and firing percentages together, because they’re measured from different starting sizes. Firing hotter also shrinks most clays more, up to a point.
Measuring it
Draw a line of a known length, such as 10 cm, on a fresh test bar. Measure it again after firing.
Shrinkage % = (wet length − fired length) ÷ wet length × 100
If the 10 cm line fires to 8.8 cm, the shrinkage is 1.2 ÷ 10 × 100 = 12%. Clay makers often publish a figure too; check whether it’s total or firing shrinkage, and for which cone.
Making a piece the right size
Divide the size you want by (1 − shrinkage). For a 10 cm fired tile from clay that shrinks 12%: 10 ÷ 0.88 = 11.4 cm wet.
Ceramics Monthly gives the same idea as an enlargement percentage for a photocopier: (100 ÷ (100 − shrinkage)) × 100. For 12%, that’s 100 ÷ 88 × 100 = about 114%.
Test with your own clay and kiln before an exact job, such as a replacement lid.