Sometimes a potter finds that it makes more sense to create detail using a mold than it does to handsculpt ornaments individually. For these cases, potters use slip molds. Slip is a viscous solution of water and clay that you pour into a mold. Over time it dries, just like regular clay, but it takes on every detail of the mold. Most slip molds are slightly larger than they need to be because clay shrinks as it dries.
Lay newspaper across your work surface and don your gloves and face mask. Rub the surface of your prototype down with a thin veneer of liquid soap. The soap will keep your mold from sticking to your prototype ornament.
Pour pottery plaster into a bucket. Pottery plaster comes as a white powder in plastic bags or tubs. Add enough of it to completely cover your prototype.
Carry the bucket over to a sink and add water. Stir the water and plaster with your gloved fingers. Stop adding water when the plaster has the consistency of mud. Stir it for an extra 10 seconds to make sure that there are no lingering bits of powder.
Pour the plaster sludge over your prototype. Guide it with your finger so that it fills every crack. Make sure that the pottery plaster is at least 1 inch thick at any given point.
Let the plaster harden overnight.
Turn the mold over, peeling it away from the newspaper, and gently work the prototype out of the mold. Go over the mold with a damp paper towel to clean off any soap residue.
Let the mold fully dry for a week before you use it to cast your slip.