The consistency number is the whole page
Plaster comes with a consistency figure on the datasheet: parts of water by weight per hundred parts of plaster. It is not a mixing suggestion, it is the number the plaster was characterised at, and it is what lets you convert a volume of mould into a weight of powder. A hundred grams of plaster at 2.75 g/cm3 occupies 36.4 cm3. Add 70 grams of water and the slurry is 106.4 cm3. Add a fifth of a percent of linear setting expansion, cubed, and the set plaster is 107 cm3.
So one gram of plaster makes 1.07 cm3 of mould, and 100 grams makes 107. Everything else follows from that single ratio.
What the default cottle takes
A 9 by 7 in box filled 4 in deep is 252 cubic inches. Take out 38 cubic inches of model and 214 remain, which is 3,507 cm3. At 1.07 cm3 per gram that is 3,277 grams of plaster, and 8 percent for what stays in the bucket brings it to 3,540 grams — 7.8 lb of plaster into 2.62 quarts of water. That is most of a small bucket and about a sixth of a fifty pound bag.
Water buys absorbency and sells strength
Of the 70 parts of water, 18.6 become part of the set gypsum and never leave. The other 51.4 evaporate and leave their volume behind as pore space, which is 48 percent of the mould by volume. That pore space is the entire working principle of a plaster mould: it is what pulls water out of a slip and builds a wall against the mould face.
Run the same mould at a consistency of 80 and the arithmetic moves in one direction on all three counts at once. The mould takes 3,235 grams of plaster instead of 3,540, so it costs less. Porosity rises from 48 to 52.5 percent, so it drains a casting faster. Dry density falls from 1.108 to 1.013 g/cm3, so it is softer and wears out sooner. There is no setting where you get more absorbency and more durability together. Every studio has picked a spot on that line, usually without writing down which spot.
Why the arithmetic is only a starting point
The volume model here says the slurry is the powder plus the water and then adds setting expansion. That is close enough to batch a mould with and it is not a chemistry claim. What it cannot know is your bucket, your mixing habit, how much you always throw away and whether your cottle leaks. Mix one batch, see what it filled, and adjust the waste percentage until the page agrees with reality. After two moulds it will.
Things plaster does that nothing else does
It gets warm setting, which stops being a curiosity if a hand is in it. It sets in drains. And a chip of it in a clay body does not go away — it turns up much later in a fired wall. Studios that run both clay and plaster keep the tools, the buckets and usually the benches separate for that reason, and it is a rule that gets learned the expensive way otherwise.
Questions people ask
How much plaster do I need for a mould?
Work out the volume of set plaster in cubic inches, convert to cm3 by multiplying by 16.39, and divide by the volume one gram of your plaster makes. At a consistency of 70 with a particle density of 2.75, one gram makes about 1.07 cm3. A 9 by 7 by 4 in cottle less a 38 cubic inch model needs about 3.3 kg of plaster and 2.3 kg of water, plus whatever you always waste.
What does consistency mean in plaster?
Parts of water by weight per hundred parts of plaster. A consistency of 70 means 70 grams of water to 100 grams of plaster. It comes off the datasheet for the specific plaster you bought, and different plasters are quoted at very different numbers, so it is not a figure to carry from one product to another.
Does more water make a plaster mould more absorbent?
Yes, and weaker at the same time. Only part of the mixing water combines chemically with the plaster; the rest evaporates and leaves its volume behind as pores. More water means more pore space, which means faster draining, and it also means less solid material per unit volume, which means a softer mould that wears faster. The two always move together and you cannot have both ends.
How much does the setting expansion change the volume?
It is quoted as a length, so the volume gain is roughly three times the number. Two tenths of a percent linear is about six tenths of a percent by volume. On a small mould that is a few cubic inches and it does not decide anything, but it is in the arithmetic because it is free to include and because linear-against-volume is the trap that catches people twice on the same page — the same thing happens with clay shrinkage.
Can I reuse old plaster or plaster that has been open a while?
That is a question for your plaster supplier and your own testing, not for this page. Plaster is sensitive to how it has been stored and the datasheet numbers this calculator uses assume the material behaves the way the datasheet says. If a batch behaves differently from the arithmetic, the material is telling you something the calculator cannot.