There are two shrinkages, not one
Clay shrinks in two distinct phases. The first is drying shrinkage: as water leaves the body, the clay particles pack closer together and the pot gets smaller. This happens between wet and bone-dry, and it accounts for roughly half of the total movement.
The second is firing shrinkage: as the body is heated, fluxes melt, particles fuse and the mass densifies. This happens in the kiln. Because the two phases have different causes, they cannot be treated as one number — a body that shrinks a lot when drying may shrink relatively little when fired, and vice versa.
- Drying shrinkage5–7% of wet dimension (typical stoneware)
- Firing shrinkage4–6% of dry dimension
- Total shrinkage10–13% wet to fired
- Where drying shrinkage happensWet → bone dry
- Where firing shrinkage happens600–1200 °C
Why this changes how you work
Every dimension you care about is a wet dimension that will not survive the kiln. If you want a 10 cm finished plate, you must throw it around 11.2 cm wide. If you want a lid to fit a jar, both must be measured and cut at the same stage, and both must come from the same body.
| Finished size you want | Wet size to make (12% total shrinkage) | Difference |
|---|---|---|
| 10.0 cm | 11.4 cm | +1.4 cm |
| 15.0 cm | 17.0 cm | +2.0 cm |
| 20.0 cm | 22.7 cm | +2.7 cm |
| 25.0 cm | 28.4 cm | +3.4 cm |
A 1% shrinkage error is 1 mm on a 10 cm pot and 5 mm on a 50 cm pot. This is why large platters and architectural pieces demand a measured shrinkage test, not an assumed figure.
Measure your own body
Never trust a general shrinkage figure for a body you are working with seriously. Measure it. The test takes an afternoon and pays back for years.
Throw or press a test bar
Make three flat bars about 15 cm long and 2 cm wide, from the same body and the same batch you will be using.
Mark and measure wet
While still wet, press two needle marks exactly 100 mm apart into each bar. Record the measurement with calipers.
Measure dry
Let the bars dry completely — a week in open air, or overnight in a warm dry room. Re-measure between the marks.
Bisque, then measure
Bisque fire, then re-measure. The change from wet to bisque is your drying shrinkage.
Glaze fire, then measure
Glaze fire to your working temperature and measure again. Wet to fired is your total shrinkage.
Once measured, write the figure on the clay bag and in your log. It is the single most useful number in your studio.
Controlling and living with shrinkage
- Grog reduces shrinkage. A body with 20% grog moves noticeably less than a smooth body, which is why sculptural bodies crack less on large forms.
- Porcelain shrinks more than stoneware — often 14–16% total. Budget for it when matching a lid to a jar.
- Uneven drying causes uneven shrinkage, which causes warping and cracking. Dry slowly, under plastic, especially in the first 24 hours.
- Always join parts at the same moisture level. A wet handle on a dry body will crack as it shrinks.
- Match slip and glaze shrinkage to the body, or the surface will craze or flake.
Most cracked pots are not a throwing problem, they are a drying problem. Cover work loosely with plastic for the first day and let it dry from the inside out.
Ceramic work involves respirable crystalline silica, kiln temperatures up to 1300 °C and glaze materials that can be toxic in powder form. This guide is an educational reference — it is not a substitute for the instructions supplied with your equipment. Always ventilate, wear appropriate respiratory protection when handling dry materials, and read our studio safety guide.
