Why clay shrinks
Clay shrinks twice. First as it dries, when the water between the clay particles evaporates and the particles move closer. Then in the kiln, as the clay vitrifies: particles fuse and pores close. The total shrinkage depends on the clay body and the firing temperature.
Measuring your own clay
The most reliable number comes from a test bar made from the same clay you use:
- Roll a bar of plastic clay and mark two lines exactly 10 cm (or 4 in) apart.
- Let it dry fully, then fire it to your usual temperature.
- Measure the distance between the lines again.
Shrinkage (%) = (wet length − fired length) ÷ wet length × 100
Sizing a piece
To get a finished piece of a given size, divide by what remains after shrinkage:
Size to make = target size ÷ (1 − shrinkage)
Example
A test bar marked at 10 cm measures 8.8 cm after firing: shrinkage = 1.2 ÷ 10 = 12%. For a plate 20 cm across when fired, make it 20 ÷ 0.88 ≈ 22.7 cm wide. In inches, an 8-inch target means making the piece about 9.1 inches.
Note that you cannot just add 12% to the target: 20 × 1.12 = 22.4 cm would come out slightly small.
Volume shrinks much more
Shrinkage happens in all three dimensions, so volume shrinks by 1 − (1 − s)³. With 12% linear shrinkage, the volume drops by about 32%. A mug that holds 400 mL when thrown will hold only about 270 mL when fired, which matters for tableware made to a set capacity.
Typical shrinkage
| Clay body | Total shrinkage |
|---|---|
| Low-fire earthenware | about 5% to 8% |
| Mid-fire stoneware | about 10% to 13% |
| High-fire stoneware | about 11% to 14% |
| Porcelain | about 12% to 17% |
Check the supplier’s data sheet, but trust your test bar: firing temperature, the kiln and even how wet the clay is all change the result.
Tips
- Shrinkage is often a little greater across the width of a thrown piece than its height; measure test bars in both directions if precision matters.
- Make lids and pots from the same clay and dry them together so they shrink alike.
- Uneven drying causes warping and cracks: dry slowly and evenly, especially flat pieces.
- A shrink ruler, scaled up by your shrinkage, lets you measure wet pieces directly in fired dimensions.