Three examples are shown here, a rectangle of clay on wood, plaster on plaster and a round of clay on a plaster base. The type of plaster used is Hydrocal by US Gypsum. It is a smooth, durable and carvable medium.
The sculpture is contained with metal strips “fence” held with grips and sealed with a bead of clay. The border and inside of fence is brushed with petroleum jelly. The surface of the clay is brushed with a thin even layer of mineral oil. Next the sculpture is leveled using a bubble leveler and shims.
Plaster is not highly toxic but can be a health hazard if inhaled repeatedly. Be safe, wear a mask. Plaster is sifted into approximately 70 degree water until it stops sinking and forms islands in the water. The plaster then sits undisturbed to saturate for 4 minutes. Mixing by hand for a full 3 minutes makes a smooth strong cast.
The plaster mixture is slowly poured over the model and then agitated to release trapped air. After 40 minutes the plaster is set enough to part from the model.
When pouring plaster to plaster the model must be treated in order for the new plaster to not stick. The best thing I have found is tinture of Green Soap cut with 40% water. This mixture, when brushed on the surface of the plaster is not a lubricant but actually changes the properties of the surface of the plaster which prevents the new plaster from adhering to the old. After the soap is wiped off, the model is fenced, sealed and brushed with mineral oil.
The plaster is mixed and poured and again sets up for 40 minutes before parting. A metal blade eases the two apart. The plaster is still wet and workable. After being exposed to air for about a week, it will be fully cured.
For round casts the process is the same save for the fence. Here a sturdy flexible metal strip is held in place with a ratcheting strap. Then soap, remove soap, brush with oil, level, pour and part.