Slip casting, from plaster to pot

There is a reason every section here has a video sitting next to a written procedure, and it is not that we could not decide.

Video is the only honest way to show timing. You cannot write down what a slip pour sounds like when it starts running thin, or how fast somebody actually tips a mold over. You watch that once and your hands know something your notes never will.

But video is terrible at the second thing. You are standing over a mold with wet hands, you have forgotten whether the gate gets trimmed before or after the piece releases, and the answer is somewhere in minute nineteen of a twenty-six minute video. Scrubbing back through it is miserable. Scrolling to step six is not.

There is decent evidence that reading and watching together stick better than either one on its own, because they load differently and back each other up. We are not going to pretend we can cite you a study on pottery specifically. What we can say is that everybody who has taught a room full of beginners knows the same thing: they watch the demo, they nod, and then they still want the sheet of paper.

So this page gives you the sheet of paper. Every band pairs a walkthrough you can read at the bench with a video from somebody who casts for a living. And near the bottom there is a stripped-down list built for exactly one job, which is being open on a phone propped against a bucket.

None of the videos are ours. We do not cast, so we have nothing to demonstrate. Every one is credited to the person who made it, and nothing loads from YouTube until you press play.

Plaster is thirsty. Pour liquid clay into a dry plaster mold and the plaster pulls water out of it, so a wall of clay grows inward from the mold face. Leave it ten to twenty-five minutes, until that wall is about an eighth of an inch thick. Tip the rest back out and you are left with a hollow shell in the shape of the mold. As it dries it shrinks a little and lets go of the plaster on its own. Then you tidy the seam, dry it slowly, and fire it like anything else.

Everything else is detail hanging off those six sentences.

Slip for joining is clay and water and nothing else. Casting slip has a third ingredient, and that ingredient is the reason the whole technique works.

Here is the problem it solves. If you just keep adding water to clay until it pours, you end up somewhere north of fifty percent water. Then it dries, all that water leaves, and the piece shrinks so violently it tears itself apart. A dried mud flat is what that looks like.

A deflocculant fixes it. It is an electrolyte that puts the same electrical charge on every clay particle, so they push each other apart instead of clumping. Charged apart, the same clay flows like heavy cream at around forty percent water. Less water in means less shrinkage, fewer cracks, and a faster cast.

The two you will actually meet

Sodium silicate, usually paired with soda ash. Cheap, powerful, traditional, and unforgiving. There is very little room between not enough and far too much, and it slowly eats the surface of your plaster molds, which shortens their life. Always dilute it before it goes anywhere near the bucket.

Darvan 7. A polyacrylate dispersant. More expensive, much harder to overdo, needs no soda ash partner, and it is gentle on plaster. If you are casting from the same molds for a long time, this is the one that keeps them alive. A common starting point is around 0.35 grams of Darvan per 100 grams of dry clay, and iron-heavy stoneware bodies may want closer to double that.

Specific gravity, and how to read it

Specific gravity is just the weight of your slip against the weight of the same volume of water. It tells you the ratio of clay to water, and it is the first number to check when anything goes wrong.

Put a measuring container on a gram scale, tare it, pour in exactly 100 ml of slip, and read the weight. Move the decimal two places left. 178 grams means a specific gravity of 1.78. You are aiming for somewhere between 1.75 and 1.80.

Below that and there is too much water: the slip level will sink hard in the mold, the wall builds slowly, and the mold waterlogs early. Above it and the slip is thick, casts fast, and cracks.

Knowing when you have gone too far

Over-deflocculated slip casts slowly, drains badly, leaves hard glassy spots, keeps the molds wet, and produces pieces that come out chalky and soft. Under-deflocculated slip gels solid in the mold and will not pour back out at all.

Both look alike from outside the bucket, which is why the sequence matters. Mix thoroughly first. Then measure. Then adjust. Most problems people blame on chemistry are actually a bucket that was not stirred long enough.

And if all of this sounds like more chemistry than you signed up for, buying premixed casting slip from a ceramic supplier is completely legitimate and what most people do. It arrives at the right specific gravity, already deflocculated, matched to a firing range.

1. Invert at an angle, not straight downRest the mold upside down on two sticks across a bucket, tilted. Straight down lets the last of the slip pool and hang, and it dries into stalactites on the inside of the base.
2. Leave it until the sheen goesUsually fifteen to thirty minutes. Wet clay looks glossy. When the surface at the gate has gone matte, the draining is done.
3. Set it upright and trim the gateCut the excess flush with the rim while the clay is still soft leather. Cut it, never tear it. A torn gate starts a crack that shows up two firings later.
4. Wait for it to release itselfAn hour, often more. The clay keeps shrinking and pulls away from the plaster on its own. You will see a hairline gap open at the rim. That gap is the signal.
5. Take the mold off the pieceNot the piece out of the mold. Unstrap, lift each half away, support the piece from underneath. Never lever, never pry, never grab it by the rim.
6. Fettle the seamRun a soft metal rib or a fettling knife along the seam line, then go over it with a barely damp sponge. A wet sponge drags the seam back open and softens the detail either side of it.

Is slip cast pottery still handmade?

When one person designs the form, carves the model, builds the mold, mixes the slip and finishes every piece by hand, yes. The mold is a tool, in the same way a wheel is a tool. What people are usually reacting to is factory casting, where none of those decisions belong to the person who packed the box. Plenty of serious ceramic artists cast everything they make.

Can I slip cast at The Clay Hole?

No. Not in a corner, not for a workshop, not as a trial. It is messy across a shared room, it ties up an instructor for a whole cycle, blowouts would cost members too many pieces, and pouring plaster is banned in the building regardless. That last one applies to everybody equally and it is not going to change.

Do I have to make my own molds?

Not at all. Commercial molds for mugs, planters, figurines and dishes are cheap and widely available, and a lot of people cast for years without ever pouring plaster. Buying molds is also the fastest way to learn the casting half of this without fighting two skills at once.

Can I use my regular studio clay as casting slip?

You can turn a clay body into casting slip, and there is a good reason to: everything you make then shrinks the same, fires the same and takes glaze the same. It is not as simple as adding water though. You slake bone-dry clay down, mix it smooth, measure the specific gravity, then deflocculate it carefully. Rebekah Bogard's two-part series on casting slip, linked above, walks through it.

How many casts will one mold give me?

Fifty to eighty is a reasonable expectation from a well-made mold that is looked after. Detail goes soft before the mold actually fails, so the number you get depends on how crisp your piece needs to be. Sodium silicate slowly attacks the plaster surface, so switching to Darvan meaningfully extends a mold's life if you are doing long runs.

What is the difference between a slab mold and a casting mold?

A slab mold is a form you drape a rolled slab of clay over or press it into, so you are shaping solid clay by hand against a shape. A casting mold is hollow and you fill it with liquid. We have a room full of the first kind, and you can read about using them on our slab rollers and slab molds guide.