Plaster of Paris is basically the same stuff that doctors often use to create casts for broken bones, so you know it's an incredibly sturdy material. This kind of plaster got its name because of a plentiful supply of gypsum near Paris, which was dehydrated to make powder that could be molded and dried. Thanks to store-bought powder and the easy-to-follow plaster of Paris recipe, working with this material is surely a lot easier now than it was in centuries past.
The Plaster of Paris Recipe
What's nice about using the plaster of Paris recipe for craft projects is that it's really easy to remember and follow. Artists who work professionally with plaster of Paris use different types of plaster mixes and are careful about measuring the plaster and water in specific ratios based on weight. However, when you're using commercial plaster powder, a ratio of two parts plaster powder to one part water should work (though you should always go with the instructions on the package).
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Once you've decided how much plaster powder you'll be using, measure the right amount of water and pour it in a bowl. Put the powder through a sifter or stir it to remove any clumps that may have formed. The key to getting smooth plaster is making sure every powder particle is soaked with water, so clumps aren't good.
Start sprinkling the powder into the water a little bit at a time, aiming to completely cover the surface of the water with powder. Tap the sides of the bowl to get rid of any bubbles and add more powder, repeating this slow process until all the powder has been sprinkled evenly over the water. Let the mixture sit for a minute and then use a spoon to gently stir it with circular motions until the mixture has thickened. At this point, you can mix in colors or pour the plaster of Paris right into your molds.
Making Homemade Plaster
If you don't have plaster of Paris powder, there are a few ways to make homemade plaster for art projects. Both white flour and white glue can be substituted for the plaster powder. The ratio is the same, so use one part warm water and two parts either flour or glue.
If you are using flour, stir it and the water together to make a thick paste. Aim for the texture of thick mashed potatoes and add small amounts of water or flour until the consistency is right. Scoop the mixture into the molds and let it dry for at least a few days.
With glue, the mixture will naturally be very thin. Spoon or pour it into the molds and let it dry for several days. The larger the mold, the longer it will take for the glue to dry all the way through.
Tips for Making Plaster
Make sure your mold is ready before starting your plaster. Always prep the mold with some sort of agent that will prevent the plaster from sticking to it, such as petroleum jelly. Even silicone molds need some sort of prepping before they're filled with plaster. Either talcum powder or cornstarch should work as a mold release in silicone molds, allowing the finished products to slide out easily.
Use a disposable bowl and spoon to mix plaster of Paris. It's really difficult to wash the plaster material off dishes once it starts to harden. To that end, be sure to cover your work space with newspaper or a disposable tablecloth before starting work with any kind of plaster project. Small droplets that land on your table will quickly harden.