Slip casting calculator
Three sums every caster does: the plaster for the mould, the slip that goes into it, and how long it stays there.
1 · The mould
How much plaster and water
Pottery plaster at consistency 70. Have the cottle sealed before the plaster goes in.
2 · The slip
How much slip, and what is in it
Fill it with water to the top of the spout and measure.
Dry materials taken at 2.6 g/cm³, the middle of Cardew's range for clays, feldspar and quartz.
3 · The cast
When to drain and demould
Typical figures. Time your first cast on a dry mould and put yours here.
The wall grows with the square root of time: twice the wall takes four times the wait.
4 · The session
Cast by cast, as the mould wets
An estimate, not a measurement. The first cast runs at your test-cast rate on a dry mould. Each cast after it leaves water in the plaster, so the rate falls until the cast past the book's session limit takes twice as long as the first; release slows with it. Dry the mould overnight, below 49 °C (120 °F).
This calculator covers slip casting from the bucket of plaster to the cast coming out of the mould. It weighs out the plaster and water for a mould, mixes or corrects a casting slip to a target specific gravity with its deflocculant, and tells you when to drain and when to demould, cast by cast across a session. The figures come from the moulds chapter of The Form of Clay and the sources it draws on, chiefly Andrew Martin, Vince Pitelka and Michael Cardew.
How much plaster do I need for a mould?
Measure the space you are filling, take away the model, and work out how much plaster and water fill that volume at your mix; then add about 10% for what never reaches the mould. Set plaster is close to volume-additive: the batch fills the space the water and the powder take up between them, with the powder at a solid density near 2.7 g/cm³.
At the calculator's default, a round cottle (the wall the plaster is poured inside) 18 cm across, filled to 16 cm (7.1 × 6.3 in) around a 940 mL (31.8 fl oz) model leaves 3.13 L (191 in³) to fill. At consistency 70 with 10% spare, that is 3.22 kg (7.1 lb) of plaster sifted into 2.25 L (2.4 US qt) of water.
The book's quick sizing rule makes a check: divide the volume in cubic centimetres by 1400 for litres of water, then multiply by 1.5 for kilograms of plaster (or divide cubic inches by 80 for quarts, and multiply by 3 for pounds). For the same mould it gives 2.24 L and 3.36 kg, at the stiffer 3:2 mix. Mix the whole pour as one batch: plaster poured onto a batch that has started to set leaves a layer that peels away.
What plaster to water ratio for slip casting moulds?
For pottery plaster, about 70 parts of water to 100 of plaster by weight, which plaster makers call consistency 70. Less water, such as the 3 parts plaster to 2 of water (consistency 67) many studios use, gives a denser, harder mould that takes more casts before it wears. More water, up to about 72, drains faster and casts more per session, at the cost of a shorter life.
Below about 60 the plaster is too dry to set properly; above about 80 it sets so soft it crumbles. The imperial shorthand of 3 pounds of plaster per quart of water is not the same mix as 3:2: a US quart of water weighs 2.09 lb, so it works out at just under consistency 70.
Whichever you choose, sift the plaster into the water, never the other way round, and dry finished moulds below 49 °C (120 °F). Hotter than that drives off the water that gives plaster its pore structure, and the mould stops casting without looking any different.
What specific gravity should casting slip be?
Between about 1.72 and 1.82, depending on the body: 1.75 to 1.80 for a mixed studio body, 1.72 to 1.76 for a slip high in ball clay, 1.78 to 1.82 for kaolin and porcelain, and 1.72 to 1.78 for a high-iron red earthenware. Measure it with a scale: 100 mL of slip that weighs 178 g is 1.78. A hydrometer misreads a deflocculated slip.
Gravity tells you how much clay the slip carries. At 1.78, with dry materials at 2.6 g/cm³, the slip is 71% dry materials by weight and its water is 40% of the dry weight, the same 40% as Martin's test batch. The default batch, enough to fill four 940 mL moulds with 15% to spare, is 5.48 kg (12.1 lb) of dry materials, 2.22 L (2.3 US qt) of water and 16.4 g of Darvan.
Gravity says nothing about how the slip pours; that is the deflocculant's job. Start low, at 0.3% of the dry weight for Darvan or Dispex (0.5% for high-iron bodies) or 0.25% for sodium silicate, and stir it into the water before the dry materials go in.
How do I fix a casting slip that is too thick or too thin?
If the gravity is too high, add water; if it is too low, add dry materials and measure again after a couple of hours. The amounts follow from the gravity alone. Five litres (1.3 US gal) at 1.84, brought down to 1.78, need 385 mL (13 fl oz) of water. The same five litres at 1.70, brought up to 1.78, need 1.27 kg (2.8 lb) of dry materials. Pouring off about 515 mL (17 fl oz) of clear water would do the same, but a deflocculated slip seldom leaves that much on top overnight: pour off what there is and make up the rest with dry materials.
Check the gravity before you reach for the water. A slip that sits on the right gravity and still pours thick is under-deflocculated, and it wants a few drops of deflocculant: water would thin it too, but it would pull the gravity below target and give you a wetter, slower cast that shrinks more. A slip that has gelled in the bucket has too much deflocculant or too little; the FAQ below has the one-drop test that tells you which.
How long should slip stay in the mould?
For a normal 3 to 4 mm (1/8 to 5/32 in) wall, usually 10 minutes to half an hour, and the only reliable figure is one you time yourself. The wall grows with the square root of time, because water has to cross the clay already built before it reaches the plaster, so twice the wall takes four times as long. One test cast on a dry mould fixes the rate for that slip and that mould.
From a test cast of 3 mm (1/8 in) in 12 minutes, the calculator puts a 3.5 mm (0.14 in) wall at 16 minutes in the mould, and demoulding 15 to 30 minutes after draining, so 31 to 46 minutes from the pour. One more millimetre of wall would cost another 11 minutes.
Each cast also leaves water in the plaster, so a mould slows through a session. With 4 cm (1.6 in) of plaster the book gives six or seven casts before the mould needs a night to dry. Martin makes his tumbler moulds thick so they take six or seven casts a day, and about ten with a thin, fast slip. The calculator models the slowdown so that the cast past that limit takes twice as long as the first: by the seventh cast the 16-minute hold has stretched to 29. That is an estimate, and the page says so. The casts you time yourself are the figures to trust.
Common questions
Should I add plaster to water, or water to plaster?
Plaster to water, always. Weigh the water into the bucket, then sift the weighed plaster onto the surface a handful at a time, letting each one sink before the next; by the last handful the surface looks dry. Let it slake, undisturbed, for two or three minutes, then stir gently from the bottom. Pouring water onto plaster traps dry pockets that set unevenly and leave the mould with patches that absorb faster or slower than the rest.
How long does plaster take to set, and how long can I pour it?
About thirty minutes for pottery plaster at consistency 70, once the plaster is in: three minutes to slake, three to stir, then a pouring window of only about three minutes. It turns thixotropic around minute 9, can be carved while it is plastic until about minute 14, hardens, and from about minute 19 warms through a heat cycle that runs to about minute 29, during which it must not be moved. For a big pour, 15 mL of white vinegar per litre of water (a tablespoon per quart) buys ten to fifteen minutes.
How thick should a plaster mould be?
At least 4 cm (1.6 in) over every load-bearing wall. Thinner moulds saturate within two or three casts and have to dry between short sessions. At 4 cm a mould runs six or seven casts a session; production moulds at 7.5 cm (3 in) run ten to fifteen, and weigh a good deal more.
Why does my casting slip gel in the bucket?
Too much deflocculant, or too little. Test a cupful with one drop of deflocculant: if it thins, the slip wants more; if it thickens, it is past its optimum, where water will not bring it back and a few drops of vinegar or magnesium sulphate will. If it keeps happening, use less ball clay and more kaolin in the recipe, and soft or distilled water.
Can I dry plaster moulds in or on the kiln?
No. Above 49 °C (120 °F), repeated or prolonged heating drives off the chemically bound water that gives plaster its pore structure. The damage is cumulative and silent: the mould looks and feels normal, and no longer casts. Dry moulds at room temperature with a fan blowing across them, or in a drying box with a thermometer keeping it below that limit.
Should I use Darvan or sodium silicate?
Sodium silicate, often paired with soda ash, gives finer control over how the slip behaves, but it leaves deposits that slowly block the mould's pores. Darvan 7 and Dispex, both sodium polyacrylates, do not attack plaster, so moulds last noticeably longer, and they are more forgiving between too little and too much. Darvan is used alone, at about 0.2 to 0.5% of the dry weight.
Keep going
- How the slip casting calculator works The plaster, the gravity and the casting-time arithmetic, worked by hand.
- What makes clay plastic, and why it shrinks The shrinkage that pulls a cast away from the plaster is the same water leaving.
- The studio looks clean, and the air is not Fettling dry casts and sweeping up spilled slip put silica in the air.
- Energy calculator Once the casts are dry: what firing them costs.