Understanding Clay Shrinkage: Why Pottery Gets Smaller in the Kiln

You throw a mug that sits perfectly in your hand. The rim is even, the handle lands where you wanted it, and the foot is wide enough to feel stable. Then the glaze firing ends, you lift the lid off the kiln, and there it is: the same mug, only smaller. The handle now feels a touch high, the lid you threw for the jar beside it drops a few millimetres too deep, and a plate that started life at 28 cm measures closer to 25.

Nothing went wrong. Clay shrinks, every time, in a fairly predictable way. Once you know how much your clay body moves, sizing becomes a matter of arithmetic rather than guesswork — and the kiln stops surprising you.

Two Shrinkages, Not One

Most people first notice shrinkage after firing, but the process starts much earlier. It helps to think of two separate events.

Drying shrinkage

This happens between throwing and bone dry. Clay particles are flat and plate-like, and each one carries a film of water around it. As that water leaves, the films thin, the plates settle closer together, and the whole mass contracts. Drying shrinkage typically accounts for roughly half to two-thirds of the total, and it is entirely unavoidable. It is also why pieces warp, curl at the rim, or crack off the base if they dry too fast: the outside stiffens while the inside still wants to move.

Firing shrinkage

In the bisque firing, the remaining chemically bound water is driven off, organic material burns away, and clay particles begin to sinter — touching, then fusing at their edges. In the glaze firing, the body densifies further as pores close and a glassy phase develops. Porcelain bodies can end up around 14–15% smaller than they were on the wheel; many stonewares sit near 10–12%; earthenware red bodies are often gentler, somewhere in the 5–8% region. Treat those as rough territories to check, not fixed rules, because your body, your water content and your firing cycle all shift the number.

Clay doesn't shrink to be difficult. It shrinks because it is turning into something else.

Why Shrinkage Is Never Quite Even

A single figure for shrinkage is useful, but it hides some stubborn realities. Shrinkage is not uniform across a piece, and that is where trouble usually starts.

Throwing compresses clay in the direction the wheel spins and slightly aligns the particles. A bar rolled from a slab can behave differently from a thrown cylinder of the same body — sometimes a percentage point or two apart. Attachments are another pressure point: a handle made from wetter clay than the mug it sits on will shrink more, and the join carries the strain. Thin walls and thick walls drying side by side move at different rates, which is why a heavy foot ring and a paper-thin rim in one pot is asking for a crack.

Add gravity and you get warping. A wide plate that shrinks freely is fine; a wide plate dragged slightly by friction on the shelf will lift at one edge. Nothing here argues for fussing over every pot, but it does explain why consistent working habits — even thickness, even moisture, patient drying — pay off in the kiln.

How to Measure Shrinkage in Your Own Studio

Published figures for a clay body are a starting point. Your version of that body, mixed with your water and fired on your schedule, deserves its own number. The test takes ten minutes and a couple of firings.

1. Make a test piece from the same clay you use for real work. A flat bar or tile about 1 cm thick and 12 cm long is ideal. Use the same wedging, the same water, and roughly the same working method as your pots.
2. Mark a 100 mm line while the clay is still soft. A needle tool and a steel ruler give you a clean, repeatable mark. Measuring 100 mm makes the maths trivial later.
3. Label it. Scratch the date and clay name into the bar. If you use several bodies, keep a bar for each and never mix them up.
4. Dry it the way you dry your pots. If your work dries under plastic for a week, do the same here. The test should mirror your routine, not shortcut it.
5. Measure at three points: bone dry, after bisque, and after the glaze firing. Wipe the ruler clean each time and measure along the same line.
6. Do the sum. Subtract the final length from 100 mm, then express the difference as a percentage. A bar that finishes at 89 mm has shrunk 11%.
7. Record it and repeat. Two or three bars averaged together beats a single reading, especially with a body you haven't used before.

A neat trick: write the shrinkage figure on the fired test bar in underglaze and keep it on the shelf next to the clay. New bags, new batches, a change of supplier — you only need to test again when something changes.

What the Number Does for You

Knowing your body shrinks 11% lets you size pieces backwards from the finished result. A dinner plate that must measure 26 cm across needs to be thrown at about 29.2 cm. To get there, divide the target dimension by one minus the shrinkage as a decimal: 26 ÷ 0.89 = 29.2. Multiplying by 1.11 gives a very similar answer and is fine for mugs and bowls, but for lids, galleries and stacked boxes, use the division — small errors that matter are exactly the ones simple multiplication introduces.

Many potters make themselves a shrinkage ruler, marking a normal ruler's increments up by the appropriate factor so they can measure a soft pot and read the fired size directly. It is a genuinely useful half hour of work.

The places where shrinkage bites hardest:

• Lids and galleries. Lid and jar shrink together, so the fit you test in leather-hard state is not the fit you'll get. Allow a little clearance and remember the lid sits higher once everything has moved.
• Slab boxes and sets. Panels cut from a template shrink in both directions. A box that must hold something specific needs its internal dimensions computed, not eyeballed.
• Handles and spouts. Joins between parts of different moisture or thickness are the first to crack. Attach at the same stage, with clay of similar firmness.
• Commissions and repeats. If a customer wants a second mug matching the first, you need the fired dimension plus the shrinkage figure, not a memory of how the first one felt on the wheel.

When Shrinkage Catches You Out

Two mistakes account for most of the frustration. The first is measuring a test bar while it is still slightly damp — the number comes out too low and every subsequent piece is oversized. The second is assuming two clay bodies behave alike. A porcelain and a grogged stoneware can differ by three or four percentage points, which on a 30 cm platter is more than a centimetre. Mixing bodies within one piece, or swapping suppliers mid-project without a fresh test, is the quiet route to a lid that doesn't seat.

A Simple Studio Routine

Keep one test bar per clay body, fired alongside your normal work. Note the total shrinkage on the bar and in your notebook. Make a shrinkage ruler for each body and hang it where you wedge. Before throwing anything that must fit something else — a lid, a set of nesting bowls, a slab box — write the finished dimension on your workboard and calculate the wet size before you touch the clay. It takes a minute, and it removes almost all of the guesswork.

Shrinkage is not a flaw to fight. It is a property of the material, as consistent as the way clay smells after rain. Measure it once, write it down, and the kiln becomes a place of small confirmations rather than surprises.

Photo: Yan Krukau / Pexels