Thickness against time is not a straight line
The wall a plaster mould builds is limited by how fast water gets pulled through the layer already deposited. Early on that layer is nothing and the wall grows quickly. Later every additional thousandth of an inch has to push its water through everything before it, and growth slows down. The usual description is that thickness goes as the square root of time, which means time goes as the square of thickness.
Put the defaults in and the point lands hard. A test cast reached an eighth of an inch in eight minutes. The target is two tenths, which is 1.6 times the wall — and 1.6 squared is 2.56, so it wants 20 minutes, not 13. If you had scaled the time linearly you would pour off two and a half minutes early on every piece for the rest of the run.
What goes in and what comes back
A 3.4 by 4.2 in cylinder holds 38.13 cubic inches of slip. Twelve percent of topping up as the level drops brings the pour to 42.71. Of that, the cast wall keeps 10.79 cubic inches, 2.56 stays down the mould as film, and 29.36 comes back to the bucket — 69 percent of what you poured. So a piece consumes 13.35 cubic inches of slip, about 383 grams at a specific gravity of 1.75, and the piece that comes out weighs about 336 grams wet.
That return figure is why slip casting is not as wasteful as it looks the first time you pour half a bucket into a mould and watch it disappear.
Moulds or hands
One mould with the defaults runs a 76 minute cycle — 4 to fill, 20 of dwell, 45 firming, 7 to demould — and then has to dry for twelve hours, so its turnaround is 13.27 hours and it gives one cast a day. Six moulds give six pieces. Meanwhile the attended work is only eleven minutes a cast, so in an eight hour day one person could handle 43 casts. The day is limited by moulds by a wide margin, and nothing about working faster changes it.
Turn the mould drying down to two hours — a much drier room, or thinner moulds — and turnaround falls to 3.27 hours, seven casts a mould, 42 pieces from six moulds against the 43 one person can attend. Now hands and moulds are within one piece of each other and the shop is balanced. The point is that the answer flips, and which side it is on decides whether the next spend is moulds or help.
The mould you tested is not the mould you use
Every number here comes from one test cast, and a test cast taken from a bone dry mould first thing in the morning is the fastest cast that mould will ever give. By the third pour the plaster is carrying water and the wall builds slower for the same dwell. If you time the whole day off a morning test you will pour thin all afternoon and not know why. Take the test at the point in the cycle you intend to run.
What this cannot tell you
Nothing here says whether your slip is right, whether it is deflocculated, whether the wall will crack as it firms, or whether the piece will release. Those are the actual craft of casting and they are learned in front of the mould. This is the arithmetic underneath them, which is worth having straight so the craft has somewhere firm to stand.
Questions people ask
How long should I leave slip in a mould?
Long enough for the wall you want, and that comes from your own test cast rather than from a rule. Cast one, cut it, measure the wall and note the minutes. Then scale: at the square-root behaviour a dry mould shows, the time goes as the square of the thickness ratio. An eighth of an inch in eight minutes means two tenths in about twenty, not thirteen.
Why does a thicker cast take so much longer?
Because the water has to come out through the clay already deposited. The layer that has formed is itself the thing slowing the next layer down, so growth is fast at the start and progressively slower. At the square-root behaviour the form defaults to, doubling the wall costs four times the dwell, which is why thick drain casts eat a day and why hollow work is usually designed thin. Fit your own exponent from two test casts and that multiplier changes with it.
How much slip does one casting use?
Much less than you pour. With the default cylinder you pour 42.71 cubic inches including top-up, the wall keeps 10.79, film loss takes 2.56 and 29.36 goes back to the bucket. The piece has cost you 13.35 cubic inches of slip — around 383 grams at a specific gravity of 1.75. The rest is working stock that keeps circulating.
How many moulds do I need to cast twenty pieces a day?
Divide your target by the casts one mould gives in a day, and that comes from the turnaround: the fill, the dwell, the firming, the demould and then the drying time before it can take another. With a 12 hour mould drying time the default mould manages one cast a day, so twenty pieces needs twenty moulds. Shorten the drying and the number falls fast.
Is the drying time or my own time the limit?
The page tells you which, and it flips more easily than people expect. With long mould drying the moulds are the limit and working faster achieves nothing. With short drying the limit becomes how many casts one person can start and demould in a day. Adding moulds to a shop that was already limited by hands buys nothing, and it is a common way to spend money on the wrong thing.