A common casting medium, plaster is also a popular pick for making molds. Using this substance, which is manufactured in powder form, is ideal when you want a rigid structure to make casts as an alternative to using silicone or polyurethane molds. Many mixtures are called plaster of Paris—so named for the abundant deposits of pure gypsum near the eponymous city, originally used to make plaster—and are typically easy to prepare and easily workable. Other plasters as well as the plaster compound gypsum might be used for more specific or more demanding molding processes. No matter your pick, it’s important to exercise safety when handling plaster, which can be dangerous if inhaled.