Plaster Molds
198 plaster molds in flexible silicone - figurines, trays, vases, ornaments, candle holders, jars and wall pieces. Plaster is the cheapest casting material there is and the easiest to paint, which is why most people casting decor at volume reach for plaster molds rather than resin.
Can you use plaster of paris in a silicone mold?
Yes. Silicone is chemically inert and non-porous, so plaster of paris, casting powder and gypsum all cure against it without bonding. That is the whole reason silicone replaced rigid plaster molds: a cast will not stick, and the mold flexes away instead of needing to be chipped or split. No mold release is required for plaster in silicone - adding one usually just softens your surface detail.
Casting powder, plaster of paris and gypsum
These names overlap more than people expect. Plaster of paris is calcined gypsum - gypsum that has been heated to drive off water, so it re-hardens when you add water back. Casting powder is a general trade name for the same family, usually a finer or harder grade sold for detail work. Pottery plaster and dental stone are denser grades again. For mold casting, a finer grade holds edge detail better; a coarser one is cheaper for bulk pieces. Any of them work in the plaster molds on this page.
Keeping plaster from sticking - and other release problems
If plaster is sticking in a silicone mold, the mold is almost never the cause. The usual culprits are demolding too early or a mix that is too wet. Plaster heats as it sets and then cools; wait until the mold is cool to the touch before you demold. A mix that is too thin stays soft and chalky at the surface and tears when you pull it - thicker mixes release cleaner and hold detail better.
Finishing plaster casts
Plaster dries white long before it is actually dry. Give a cast a full day or two before painting, and seal it first if you want acrylic to sit on the surface rather than soak in. Unsealed plaster takes dry brushing and washes beautifully, which is how painted figurines get their depth.
Plaster molds, resin molds and candle molds - the same silicone, three finishes
Almost every mold here is rated for all three materials, so the shape you pick is not locked to plaster. Pour the same vase mold in plaster for a matte, paintable piece; pour it as one of our resin molds for weight and clarity; pour it as one of the candle molds if the cavity suits a wick. Plaster is the one to start with if you are testing a shape - it is cheap enough that a failed cast costs you nothing.
Plaster mold questions we get asked
What water-to-plaster ratio should I use? Roughly 2 parts plaster to 1 part water by weight for detail work, adjusted to a single-cream consistency. Our plaster calculator works out the exact amounts once you know your cavity volume.
How long does plaster take to set in a silicone mold? It firms in 20-30 minutes and is demoldable in about an hour, but it is not dry - it is still full of water. Leave a cast a day or two before painting or sealing.
Why are there white crystals on my plaster mold? That is efflorescence: dissolved salts carried to the surface as the cast dries. Brush it off dry once the piece has fully cured. Using less water in the mix reduces it.
Can I cast plaster in molds sold for resin or candles? Yes, in nearly all cases. Silicone does not care what cures against it. The only shapes to avoid are very deep, narrow cavities, where plaster sets before it can settle to the bottom.
