Consider using a disposable mixing bowl and disposable spoon (or spatula to mix your plaster, since it’s difficult to wash off of dishes. Tap the sides of your container periodically as you sprinkle in plaster powder. This helps disperse the powder and eliminate air bubbles. Some people prefer to use hot water because it speeds up the drying process. To do this, use a microwave or stove to heat the water to 100 °F (38 °C). However, use room-temperature water if you want to take your time.
The exact amount of plaster powder you use can vary by brand, so try stirring and adding plaster until you can see that the powder is not easily absorbed by the water anymore. Wear gloves and an apron while you work to prevent any spills or splashes, and lay down newspaper or a disposable tablecloth where you plan to work. Plaster of Paris powder is made with dehydrated gypsum. Its technical name is calcium sulfate hemihydrate, and it’s designed to harden when exposed to moisture.
Poster paint or acrylic paint can also be used as coloring agents if you have either of them on hand. You can add color to any of the 3 plaster of Paris recipes. Just remember that the best time to add coloring is when the mixture is all stirred but before it’s poured into the molds.
Use a non-stick agent even if you’re using a silicone mold. Talcum powder or cornstarch are both suitable for this. Wait 1 day before using a newly-made silicone caulk mold; if you use it sooner, the out-gassing of the silicone might create bubbles in your plaster.
Be sure to wash it off immediately if it comes into contact with your skin. Clean your bowl and mixing utensils immediately after you’re done with them (if you’re not using disposable dishes).
Don’t throw any leftover plaster away by washing it down the drain, as it can harden and clog your pipes. The easiest way to deal with leftover plaster of Paris is to let it harden in the mixing container, then throw the scraps away in the trash can.
You can also heat up your water before doing this, but keep in mind that the same rule applies as before: the warmer your water is, the faster it’ll harden.
Work with the plaster within 10 minutes. You’ll notice the plaster mixture will start to solidify as you work with it, so move on to the next step as soon as you’ve finished stirring it for the best results.
Use lukewarm water if you want to speed up the drying process slightly. Otherwise, use regular room-temperature water.
Making plaster with wet glue will make it have a slightly longer drying time, but you should still work with it as soon as you finish mixing it!
Don’t use any part of your body as a mold; it’ll be very hard to get that body part out of hardened plaster.
