Why Your Pottery Cracks (and How to Stop It)

Nothing is more discouraging for a new potter than finding a crack in a piece you spent hours on. The good news is that cracks are rarely mysterious. They come from a short list of causes, and once you know the list you can prevent most of them.

Uneven thickness

Clay shrinks as it dries, and thick sections shrink more slowly than thin ones. If a wall goes from thick at the base to thin at the rim, the two parts pull against each other and something gives. Aim for even walls. Run your fingers along the inside and outside together to feel for thick spots, and thin them before the clay stiffens.

Drying too fast

The surface of a pot dries first and starts shrinking while the inside is still wet. In a warm, draughty room this happens quickly enough to crack a rim. Cover fresh pieces loosely with a plastic bag for the first day or two so they dry slowly and evenly. Rims and handles dry fastest, so wrap them a little more carefully.

Weak joins

When you attach a handle, a coil, or two slabs, the pieces need to become one piece of clay. Score both surfaces with a fork or needle tool, apply slip, which is clay thinned with water to a cream, press firmly, and smooth the seam on both sides. Joins that are only pressed together will open as the clay shrinks.

Trapped air

Air pockets expand in the kiln. Wedge your clay before you start, which means kneading it in a rocking motion to push air out and even the moisture. If you fold clay over itself while working, you trap air. Press, do not fold.

Sharp corners and stress points

Where a flat base meets a vertical wall at a sharp right angle, stress collects. Round that internal corner with a finger or a wooden tool. Where a handle meets a mug body, the same applies. Soft curves survive. Sharp angles crack.

When a crack still appears

A hairline crack in damp clay can often be closed by pressing from both sides and smoothing with a little slip. A crack in dry greenware is harder. Mixing paper fibre into slip gives a repair paste that sometimes holds. A crack after firing is permanent, but the piece can still be a planter, a pencil holder, or a reminder of what to do differently next time. Every potter has a shelf of those.