How to Reclaim Clay: Reclaim Clay Calculator

How the reclaim clay calculator works

Water content in clay is usually given as a percentage of the damp weight. Digitalfire defines it that way: weigh a wet sample, dry it completely, weigh it again and work out the percentage lost. So 10 kg of clay at 21% water is 2.1 kg of water and 7.9 kg of dry clay.

When you reclaim, the dry clay stays the same and only the water changes. The calculator works out the dry clay in your scraps, then the weight that dry clay would be at your target water content: dry clay divided by (1 minus the target). The difference between that and the weight of your scraps is the water to add, or to dry out.

For 10 kg of bone-dry scraps, the dry clay is 10 kg, so at 21% the reclaimed clay weighs 10 divided by 0.79, about 12.66 kg. That means adding about 2.66 litres of water, since a litre of water weighs a kilogram.

How wet are your clay scraps?

The calculator needs to know how wet your scraps are now. Bone-dry scraps are close to 0% water. The states in the menu give rough figures for trimmings, soft clay and slurry, but only the throwing figure is published, so the others are our estimates and the result is marked as approximate.

For an exact figure, measure it. Weigh a lump of the scraps, dry it completely in a warm place or a low oven, and weigh it again. The weight lost, divided by the first weight, times 100, is the water content. Enter it and choose “I measured it”.

Many potters skip the measuring and reclaim by feel, which works well once you know your clay. The calculator is most useful when you’re reclaiming a lot at once, or want consistent clay every time.

Choosing the target water content

Digitalfire gives typical throwing clays at about 21% water, and notes that a porcelain that throws well at 21 to 22% can become short and difficult at 19%. Bodies for modelling and hand building typically sit between about 19.5 and 22.5%. The default target here is 21%, a good starting point for the wheel.

Go a little stiffer for hand building and large pieces, which need clay that holds its shape, and a little softer for soft slabs and pinching. Digitalfire also points out that water content isn’t a perfect measure of stiffness, because two clays at the same percentage can feel quite different, so let your hands make the final call.

Softer clay also shrinks more as it dries, because there’s more water to lose. If you’re making pieces that need to match, keep your reclaimed clay at the same consistency as fresh clay from the bag.

How to reclaim clay, step by step

Start by letting your scraps dry out completely. It sounds backwards, but bone-dry clay slakes, or breaks down in water, much faster and more evenly than damp clay, which stays in rubbery lumps. Break larger pieces up so they dry through.

Put the dry scraps in a bucket and add the water the calculator gives you, or simply cover them with water and pour off the excess later. Leave them for a day or two without stirring until there are no hard lumps. Then mix to an even slop.

Spread the slop on a plaster bat or a canvas-covered board, about 3 to 5 cm thick, or hang it in a canvas bag to drip. Plaster draws water out fastest. Turn it every few hours and check by feel. When it no longer sticks to your hands, wedge it thoroughly to even out the moisture and remove air, then bag it.

Reclaiming slop and slurry

Slop from the wheel and slurry from the splash pan are already wet, so the calculator tells you how much water to dry out rather than add. Pour off any clear water on top first, because that’s the easiest water to remove.

Slop dries much faster on plaster than in the open air, and drying times vary with the weather, the thickness of the layer and how much plaster you have, so there’s no reliable rule for how long it takes. Weigh it as it dries if you want to hit the target exactly: when the scale shows the reclaimed clay weight from the calculator, it’s ready.

Reclaim clay at home or in a small studio

You don’t need special equipment to reclaim at home. Two lidded buckets work well: one for dry scraps waiting to slake, and one for slop. A plaster bat is the most useful extra, and you can cast your own by pouring pottery plaster into a shallow tray. Old pillowcases or canvas bags hung over a bucket also work, just more slowly.

Reclaim in batches rather than a little at a time. A full bucket of scraps slakes just as quickly as a handful, and the drying and wedging take about the same effort however much you do. Many potters reclaim once a month, or whenever the scrap bucket is full.

Weigh the clay as you bag it and write the weight and date on the bag. When you come to use it, our clay weight calculator and the weights on the bag tell you exactly how many pieces it will make. If a batch feels slightly too soft or stiff, mix it with fresh clay from the bag while wedging, which evens out the consistency quickly.

Keep reclaim clean and safe

Keep each clay body separate. Mixing a stoneware with an earthenware gives a clay that doesn’t fire properly at either temperature. Keep plaster crumbs and other debris out too, because a fleck of plaster in fired clay can pop out later.

Dry clay scraps and dried slop make dust, and clay dust contains crystalline silica. Keep scraps in a lidded bucket, avoid crushing dry clay where dust can spread, and clean up wet rather than sweeping. Our clay weight calculator helps you weigh out reclaimed clay for each project, and a kiln firing service can fire your finished work.