How to Make Molds Out of Plaster
Learning to make molds out of plaster is an easy introduction to the art of casting, a common method for making sculptures and a variety of commercial products. This plaster mold can be used for casting or molding wax, paper, or latex. Depending on your process, the mold may be broken after casting or saved to use again.
Things You'll Need
- Modeling clay
- Carving tools
- Shoe box
- Large old paint brushes
- Plaster powder
Carve an original object you want to make a mold of, out of modeling clay. The model should have a limited number of small parts and sharp angles -- the closer to a solid block or sphere it is, the easier it will be to make a mold out of plaster.
Figure out where the mold will need to be divided in order to come off the cast object. For example, the mold of a sphere should be divided in half so one piece can pull off of each side. Objects with parts pointing in various directions need to be divided into more sections.
Make a thin sheet of clay and attach it around the model wherever the mold must be separated. For example, an approximate sphere should have a sheet attached all the way around its middle, to divide the mold you will make into two halves.
Mix the plaster with water in the bucket according to the package directions, mixing with the stick. Fill the shoe box partially with plaster (but make sure it's deeper than the part of your model that will be submerged), and let it set just slightly, enough that the model doesn't sink in the plaster. Push the model halfway into the soft plaster (up to the main clay sheet).
Scoop and spread more plaster onto the other half of the model with an old paintbrush. Make it several inches thick if possible, but try to avoid covering the dividing sheets completely. Wait for the plaster to dry completely.
Remove the shoe box if necessary, and separate the pieces of the mold at the clay sheets. If any of the dividing sheets are completely buried in the plaster, chip down toward them carefully from the outside of the mold, using carving tools.
Remove any extra clay and hold or glue the sections back together to create a complete plaster mold. You will need to leave an opening to add a casting material, and after casting you will need to either separate the pieces again or break the mold.
- A more complicated form may require a mold with many different pieces. This isn't necessarily a problem, but it's harder to do correctly, and may be worth saving until you have a bit more mold-making experience.
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