Plaster of Paris, or simply plaster, has been used for making molds for a long time. As a molding material, plaster has many attractive attributes. It offers great flexibility in design and ease of manufacture. It is inexpensive and readily available. It is easy to mix and sets fast. Its cast surface captures minute detail. And it can be carved, shaped, smoothed and worked with simple hand tools and sandpaper.
However, there are a few challenges to working with plaster. Freshly cast plaster holds a great deal of moisture, and bare plaster is highly porous. When casting concrete directly against bare plaster, the concrete can stick to the plaster, making mold removal difficult or impossible without destroying the mold or damaging the concrete. The high porosity of the plaster can also affect the color of the concrete. If the plaster is very moist when the concrete is cast, the damp plaster acts as a reservoir for moisture and the concrete will be slightly darker where it touches the plaster. If the plaster is dry, the opposite will tend to occur, resulting in lighter colors. Plaster is also rigid and breakable.
Sealing plaster to prevent direct contact with the concrete will help to prevent color variations and ease mold removal. Sealing plaster also permits the use of concrete form release agents that otherwise would soak into the plaster, rendering them ineffective. Common sealers for plaster are acrylics, but epoxy and urethanes have been used too. One important step in sealing is to dry the plaster because the high moisture content could prevent the sealer from sticking or even drying.
Finally, because plaster is rigid, the mold design and shape must allow for easy removal from the concrete. Otherwise the plaster mold gets locked into the concrete. In this situation, plaster molds can break or shatter, resulting in having to remake the mold.
