Plaster-of-Paris and other gypsum-based casting compounds are widely used to make rigid, inexpensive molds and casts, although casts can also be made out of resin, fiberglass, cement, and other materials. Generally, one uses a flexible rubber mold (negative impression of a subject) to make a rigid cast (also called a replica or reproduction) of an original specimen. The original specimen is sometimes referred to as the subject or model. It is also possible to make a hard impression directly from the original subject by pouring or applying plaster directly on it, if the subject lacks undercuts and if the subject is not hard (like a footprint or tire track in dirt) or a release agent is applied.* Realize that this will result in a reverse image of the original (technically a hard mold) rather than a cast replica, which has the same relief as the original.

One may also embed a plastic-coated wire or hook in the plaster before it has dried or "set up" so that the finished cast may be hung on a wall. Plaster can be colored by adding powdered cement pigment, or acrylic paint may be added to water before mixing. However, this tends to weaken the plaster and it is difficult to achieve an exact, desired color, so painting or staining the finished cast may also (or alternatively) be done.

Precautions

Supplies

• Plaster of Paris or gypsum cement
• Water source (water should be reasonably clean and roughly room temperature)
• Mixing cups (preferably flexible plastic, paper or rubber)
• Stirring tools (such as plastic spoons or popsicle sticks)
• Mold or model to cast
• Newspaper and paper towels (to cover work area and clean up)

Optional Supplies

• Release agent (such as spray silicone, or paste wax or Vaseline).
  (Not needed if casting plaster into a rubber mold).
• Windex (to reduce surface tension on rubber molds). Use sparingly.
• Brushes (to spread plaster into nooks and crannies of model)
• Oil-based modeling clay (to create retaining walls if needed)
• Acrylic paint (to color plaster or finished cast)
• Plastic-coated wire (18 to 24 gauge) to embed in plaster as hangers.

Procedures

1. Gather supplies and prepare work area. Have water source nearby.
2. Select and clean subject or mold.
3. If subject is rigid, or the casting material is resin rather than plaster, apply thin coat of release agent. Wipe out any excess agent. This step is not needed if casting plaster into a rubber mold.
4. If using a subject or rubber mold without retaining rims, build a retaining wall around it (clay, cardboard, or plastic strips work well).
5. Measure water into mixing container. Water volume should be about 2/3 the volume of the cast.
6. To color plaster (if desired) add small amount of acrylic paint to water
7. Sift plaster into water until a plaster island remains and does not sink.
8. Allow plaster to sit a minute or two before mixing. If island sinks, add a little more plaster.
9. Stir plaster until smooth and creamy. Mixture should be about as thick as ketchup. Add more plaster if needed.
10. (Optional) To help reduce surface tension and avoid air bubbles in the cast, if subject is a rubber mold, spray a light mist of Windex on the mold. Wipe out any excess Windex.
11. (Optional) To further reduce chances of air bubbles, gently brush a small amount of the plaster slurry over the mold and into all nooks and crannies, before pouring in the rest of the plaster. Tap the mold to settle the plaster.
12. Pour additional plaster into the mold to desired level.
13. Gently tap or vibrate the mold (or base it sits on) to help raise air bubbles and level the cast surface.
14. )Optional) After cast is partially set but still soft, embed a wire hanger if desired. Inscribe one's name if desired.
15. When plaster is completely set (typically 15 min to an hour or more, depending on the plaster product used, and temperature and thickness of the mix), gently remove cast from model.
16. Allow cast to fully dry before painting.**

Notes: