Making a slip cast mold involves building a containment structure around your original object, then pouring liquid plaster over it to create a hollow negative. Once cured and dried, the plaster mold absorbs water from liquid clay (slip), building up a solid wall inside the cavity. The process is straightforward, but the details matter: wrong plaster ratios, missing draft angles, or rushed drying can ruin a mold before you ever pour your first piece.
Start With a Model That Will Release
Your mold is only as good as the original object you’re casting around, called the model or pattern. This can be something you’ve sculpted from clay, carved from wood, 3D printed, or even found. The critical rule is that the model must be able to pull cleanly out of the plaster once it sets. That means no undercuts, which are any features that curve back inward and would physically lock the model (or your finished piece) inside the mold.
Think of it this way: if you pressed the shape straight down into soft sand, could you lift it straight back out without disturbing the sand? If not, you have an undercut. Handles on mugs, the belly of a round vase, or any inward-curving lip all create undercuts. You can still cast these shapes, but you’ll need a multi-part mold (covered below) rather than a simple one-piece pour mold.
Draft angle is the slight outward taper on vertical walls that lets a piece slide out of the mold without sticking. Aim for at least 2 degrees of taper on exterior surfaces and 3 degrees on interior features. Textured surfaces need even more. Without enough taper, pieces grip the mold wall and can crack, warp, or tear during removal.
Choosing and Mixing Plaster
Pottery plaster (often sold as USG No. 1 Pottery Plaster) is the standard for slip casting molds. It’s formulated to be porous enough to pull water from liquid clay but strong enough to hold up over dozens of pours. Don’t substitute plaster of Paris from the hardware store. It’s too soft and breaks down quickly.
The standard mixing ratio for most studio work is 70 parts plaster to 100 parts water, measured by weight. This is called a “consistency of 70” and gives you a mold that balances porosity and strength. Changing the ratio shifts the tradeoff: more water creates a softer, more porous mold that absorbs faster but cracks sooner, while less water produces a dense, hard mold that barely absorbs water at all and also sets faster, leaving you less working time.
How to Mix
Always add plaster to water, never the reverse. Weigh your water into a clean bucket, then slowly sift the plaster across the surface by hand, letting it slake (absorb water and sink) for about two minutes. After slaking, mix by hand or with a drill-mounted mixer for two to three minutes until the consistency is smooth and uniform, like heavy cream. Tap the bucket on the floor a few times to release trapped air bubbles, which would leave pits in your mold surface. You typically have 8 to 12 minutes of working time before the plaster begins to thicken and set.
Mix plaster in a well-ventilated area. Some pottery plasters contain small amounts of silite, and the dry powder is irritating to breathe. Exhaust ventilation or at minimum a dust mask is worth using during the sifting stage.
Building the Cottle (Containment Wall)
Before you pour plaster, you need a wall around your model to contain the liquid. This barrier is called a cottle. For simple shapes, you can use strips of linoleum, sheet metal, or foam-core board wrapped around the model and sealed at the base with soft clay. For flat-backed pieces, a simple open-top box made from boards works well.
Leave at least one inch of space between your model and the cottle wall on all sides, and plan for at least one inch of plaster above the highest point of the model. Thinner walls dry faster between casting sessions but wear out sooner. Seal every seam and the base of the cottle with soft clay or hot glue so the liquid plaster doesn’t leak out. Even a small gap will drain your bucket surprisingly fast.
Coat your model with a release agent before pouring. A thin layer of mold soap, petroleum jelly, or spray cooking oil prevents the plaster from bonding to the model surface. Apply it evenly and wipe off excess so it doesn’t pool in details and blur your mold.
Pouring a One-Piece Mold
A one-piece mold, sometimes called a dump mold or drain mold, works for open shapes like bowls, cups without handles, or simple vases. Secure your model face-up on a flat surface, build the cottle around it, apply release agent, then pour the mixed plaster in a slow, steady stream into one corner of the cottle. Let the plaster rise around the model rather than pouring directly onto it, which traps air against the surface.
As the plaster fills the cottle, gently tap the sides or the table to encourage bubbles to rise. Once full, leave it completely undisturbed. The plaster will heat up noticeably as it sets (this exothermic reaction is normal and can reach temperatures warm enough to be uncomfortable to the touch). After roughly 30 to 45 minutes the plaster will have hardened and begun to cool. At that point you can remove the cottle walls and carefully separate the model from the mold.
Making a Two-Piece or Multi-Part Mold
Most objects with curves, handles, or enclosed forms need at least two mold pieces. The principle is the same as a one-piece mold, but you pour the plaster in stages, creating one half at a time.
Start by embedding your model halfway into a bed of soft clay, with the parting line (where the two mold halves will meet) exposed along the surface. The parting line should follow the widest point of the form, so each half can release without catching on an undercut. Build your cottle around this setup and pour the first half. Before the plaster fully hardens, you can carve small registration keys (cone-shaped depressions) into the flat plaster surface around the model. These keys will create matching bumps on the second half, ensuring the two pieces always align perfectly.
An alternative to hand-carved keys is commercial plastic natches. These are small interlocking fittings, commonly 3/8 inch (9.5 mm) in diameter, that you cast directly into the plaster. They create durable, precise alignment points that hold up better than carved plaster keys over repeated use. You can also replace them if they break.
Once the first half has set, flip the whole assembly over, remove the clay bed, and apply release agent to every exposed plaster surface (this is critical, or your two halves will fuse into one solid block). Rebuild the cottle and pour the second half. After it sets, separate the halves, remove the model, and you have a complete two-piece mold.
For complex objects like teapots with spouts and handles, you may need three, four, or more pieces. The same logic applies: plan your parting lines so every piece can pull away from the cast object in a straight line without catching.
Drying the Mold Before First Use
A freshly poured mold is saturated with water and will not absorb slip effectively. It needs to dry thoroughly before its first casting session. At room temperature with decent air circulation, a small mold may be ready in a few days. A large, heavy mold (25 pounds or more) can take two full weeks to dry at room temperature.
You can speed this up dramatically with gentle heat. Setting the mold in a kiln or oven held at 115 to 120°F, with the door or lid propped open for airflow, can bring a mold to usable dryness in one to two days. The critical limit is 150°F. Above that temperature, the plaster begins to lose its chemically bound water (a process called calcining), which permanently destroys its ability to absorb moisture. A calcined mold looks fine but won’t cast properly. Keep a thermometer nearby and stay well under the limit.
Between casting sessions, molds also need drying time. Most casters let molds sit at least overnight, and in humid climates you may need longer. A mold that still feels cool to the touch when pressed against your cheek likely has moisture left in it.
How Slip Casting Actually Works
Once your mold is dry, you pour deflocculated liquid clay (slip) into the cavity and wait. The porous plaster immediately begins pulling water out of the slip, and a firm layer of clay builds up against the mold wall. The longer you leave the slip sitting (called dwell time), the thicker the wall gets. For a typical mug or bowl, two to five minutes of dwell time produces walls in the range of 3 to 5 millimeters, though exact timing depends on your slip recipe and how dry the mold is.
To calibrate your mold and slip combination, pour slip in, leave it for a set time (say, two minutes), then pour the excess out. Once the piece has firmed up enough to handle, trim the rim cleanly with a knife and let the piece release from the mold. Measure the wall thickness, and you’ll know exactly how long to leave future pours for the thickness you want.
After draining, leave the mold sitting with the piece inside until the clay shrinks slightly away from the walls. At that point, for a one-piece mold, you can invert it and the piece will drop out. For multi-part molds, remove the pieces one section at a time.
Common Mistakes to Avoid
- Pouring plaster too fast: A heavy stream traps air against the model, leaving bubble pits in the mold surface that transfer to every piece you cast.
- Skipping release agent between mold sections: The second pour bonds permanently to the first. There is no fix. You will have to start over.
- Eyeballing plaster ratios by volume: Plaster and water must be measured by weight. Volume measurements are unreliable because plaster settles and aerates unpredictably in the bag.
- Washing plaster down the drain: Plaster hardens in pipes and will clog your plumbing. Let waste plaster set in a disposable container, then throw it in the trash. Rinse tools in a bucket of water, let the plaster settle out, and pour only the clean water down the drain.
- Rushing to cast in a wet mold: A mold that hasn’t dried enough absorbs water too slowly, resulting in thin, fragile walls and pieces that stick inside the cavity and crack when you try to remove them.