Why clay is plastic, and why that is the same thing as why it shrinks

Grind quartz as fine as you like and it will never throw. Grind feldspar to the same fineness and it will not throw either. Both stay powders that slump when wet and crumble when dry. Only clay does the thing clay does, and the reason has surprisingly little to do with how fine the particles are.

It is a matter of shape. A quartz grain is blocky, because its atoms sit in a three-dimensional framework: however small you break it, the fragment comes out roughly equidimensional, and two of them meet at a point or along an edge. Clay minerals are built from stacked sheets. They break into thin flat plates called platelets, and two of them meet across their whole face. That difference in how the particles touch each other is where plasticity comes from, and the rest of this follows from it.

The platelet and the water film

Clay platelets are small, typically two micrometres across or less, roughly a quarter the diameter of a red blood cell. The fineness matters, but it matters mainly for the surface it creates. A kaolin presents something like 8 to 15 m²/g of particle surface to the water around it. A sodium bentonite presents 150 to 200 m²/g, ten to twenty times more. The milled quartz and feldspar in the same body present about 1 to 2 m²/g. Almost the whole relationship between a clay body and water therefore belongs to its clay fraction, and most of that to the finest part of it.

Water settles along those flat faces as a thin film, and the platelets slide over one another instead of locking. That sliding is plasticity. The word names internal mobility, not softness in the everyday sense. When you pull a wall on the wheel you are rearranging the clay more than compressing it, platelet over platelet, the way a deck of cards shears when you push the top of the stack sideways.

Michael Cardew, following Rosenthal, gave the cleanest account of it: three causes acting together, the plate-like shape of the particles, their very small size, and the electrical relationship between water and anything that fine. The electrical part is worth a sentence, because it explains behaviour the mechanical picture cannot. Each platelet carries a negative charge across its flat faces and a weak positive charge along its broken edges. In water, a cloud of positive ions gathers around each face. While that cloud is thick the platelets repel each other and slide freely. When it collapses, the positive edge of one platelet pins to the negative face of the next, and the clay stiffens into an open scaffolding that colloid chemists call a card house. That collapse is why a slip gels when it stands and loosens when you stir it, and why a spoonful of the wrong water can change how a body handles.

Shrinkage is the same mechanism running the other way

Then the water has to leave, and it leaves from the surface first. Capillary action draws more out of the interior to replace it, and the surface tension of the thinning films pulls the platelets towards each other as it goes. The whole mass contracts. Nothing chemical is happening yet. The piece gets smaller because the water that was holding the platelets apart is no longer there.

That contraction stops at a definite moment. The platelets run into each other, pack, and cannot close any further. Water still fills the gaps that are left and still evaporates, but the solid structure has locked and the dimensions hold. Ceramic engineers call that turning point the critical moisture content. Potters call the state leather hard and find it with a fingernail. Everything that can go dimensionally wrong in drying happens before it, and nothing after it, which makes it the most useful moment to be able to recognise in the whole process.

So plasticity and shrinkage are not two properties you trade against each other when you adjust a recipe. They are one property seen at two moments. Whatever lets the platelets glide while you work is exactly what lets them close up when the water goes. And the closing is permanent: soak a bone-dry pot and it will slake apart, but it will never return to the size it was on the wheel.

The plastic clays are the ones that need the most water

The traditional measure of this is water of plasticity, the percentage of water, by weight of dry clay, that a clay needs to reach a workable consistency. Ranked by fineness, the studio clays sit roughly here:

Clay	Water of plasticity
Ball clay	30 to 40%
Plastic kaolin	25 to 35%
Plastic stoneware clay	20 to 26%
Earthenware clay	18 to 24%
Fire clay	12 to 18%

Sodium bentonite sits at 200 to 600%, off the end of the scale, which is why it goes into a body at 1 to 3% as an additive rather than as an ingredient in its own right. A 1% addition measurably raises green strength, 3% changes how a body throws, and much beyond that the work cracks as it dries. Most working studio bodies land between 22 and 28%. Above 30% a body throws with more water than it can lose without warping; below 20% it is short and fights you when you pull a tall wall.

All of that water has to come back out, and the piece gets smaller by roughly the space it occupied. Measured as linear shrinkage from wet to glaze-fired, earthenware totals about 9 to 12%, stoneware 10 to 14%, and porcelain 16 to 18%. A 30 cm (12 in.) porcelain bowl can come out of the glaze fire closer to 25 cm (10 in.). You learn to build oversized, and to distrust the family averages when a dimension actually matters.

Those totals cover two separate phenomena, and they are worth splitting. Of porcelain’s 16 to 18%, only about 6 to 7% is drying; the remaining 10 to 12% is vitrification in the fire, a different mechanism with a different cause. The part this post is about, the platelet-packing part, is the 5 to 7% that every one of these bodies loses on the drying board.

Cracks are not caused by drying

They are caused by uneven shrinkage during drying, and the difference matters. A piece can dry quickly and come through intact, so long as every part of it passes through the shrinkage phase together. What opens a wall is one area getting ahead: it pulls against the parts still holding their size, and the stress has nowhere to go.

This is why cracks turn up where the thickness changes, at rims and edges, and at the joint where a handle meets a wall. A handle has air on every side and very little mass. The wall it is attached to holds far more water. Leave the pair uncovered and the handle will finish shrinking while the wall has barely started.

Not all of that stress announces itself at the time. Some of it locks into the piece and waits. The pot dries sound, passes through bisque still looking sound, and then opens during the glaze firing, when the body softens just enough for the trapped force to let go. The crack arrives weeks after the mistake that caused it, which is what makes drying faults so hard to learn from.

The remedy is dull and it works. Cover the piece with thin plastic and let the moisture equalise through the wall, uncover it to let some water go, and cover it again before any part gets too far ahead. Protect the thin sections and leave the thick ones open. Cover it again after trimming, because a freshly cut surface is wet clay exposed to the room. Plenty goes wrong in pottery from rushing. Nothing goes wrong from drying too slowly.

Every touch writes a direction into the clay

One more consequence of the plate shape, and it is the one that catches people out. Because the particles are flat, any force you apply turns them. Compression lays them flat in the plane of the load. Shear aligns them along the direction of sliding, which is what wedging, rolling a slab and pulling a wall all do. Tension aligns them along the pull. The result is called preferred orientation.

It matters because a stack of platelets loses much more height than width as it closes. Shrinkage therefore has a direction, and the alignment sets it. A piece with one consistent alignment throughout simply contracts more in one axis than the other and survives that. The trouble starts when two parts of the same piece carry different alignments.

Rolling a slab from one side only is the clearest case. The upper surface takes the roller’s alignment while the underside, cushioned by the clay above it, stays more random, so the two faces contract by different amounts and the slab curls. Roll from both faces, and across more than one direction. A mug handle is the same problem in another geometry: the wall’s platelets run one way from throwing, the handle’s run along its own length from pulling, and the joint has to hold two contractions that disagree. Scoring and slipping is more than glue for that joint. The slip is a layer with no alignment at all, and it gives the mismatch somewhere to go. Use more of it than looks necessary.

The same fact has a pleasant side. Burnishing leather-hard clay with a smooth pebble adds nothing to the surface. It simply turns the top layer of platelets face-up, so light meets flat faces instead of a forest of edges. That soft waxy sheen on polished unglazed pottery is platy geometry made visible, and whole traditions rest on it.

Measure your own

Every number above is a family average, and the bag on your shelf is not a family. The test that settles it takes ten minutes and a ruler: roll a strip, scribe two lines exactly 10 cm apart, let it dry evenly, and measure the gap again. The difference, times ten, is the drying shrinkage as a percentage. Most studio bodies land between 4 and 8%. Above 10% you have a body that will punish any form with a thickness change in it. Run the same strip through bisque and the glaze firing and you have the total, which is the only figure worth trusting when you throw to a target size.

That number is worth more than getting the size of a bowl right. It is the closest reading you can take of the trade this whole post has been about: the water a body needs to become workable is water it has to give back, and giving it back is what moves the walls. The clay that feels most generous under your hands is the one that will move furthest while you are not watching it.

This is an abstract of one chapter of The Form of Clay, a book I am writing on the craft and the physics behind it. The book carries the drying curve, the five tests for characterising a clay body, the shrinkage ruler, and the mineralogy underneath all of it. It is close, but not finished. If you would like to know when it is out, subscribe and I will email you, or leave your address in the form below.