Hydrocal is the proprietary name of a gypsum cement that can be used for the same applications as plaster of Paris and industrial plaster. The product comes in several grades, the most common of which is Hydrocal White, used for projects in the arts ranging from fine art sculptures to crafts for school children.
Hydrocal White is sold as a dry powder. Art supply stores and online vendors sell it in packages ranging in weight from 5 to 50 pounds. Once purchased, the product must be stored in a plastic bag or bucket. Hydrocal powder will absorb water and become lumpy and unusable if it is exposed to high humidity.
Hydrocal is stronger and harder than plaster of Paris. It sets up rapidly and has a high green (early) strength, which cuts down on breakage when the casting is being removed from the mold. Hydrocal can be sanded, drilled and painted or colored using other methods such as wet or dry pigments added to the slurry before casting. Hydrocal is suitable for both solid and hollow casting; its strength and hardness mean it is good for hollow-cast items with fairly thin walls. The slurry picks up the details of the mold well, so Hydrocal is an excellent choice for very intricate castings.
Hydrocal can be cast in molds made of latex rubber, alginate, plaster, moulage, urethane rubber, silicone rubber, wax, plastic, plaster bandages (used to make casts for broken bones) and other materials. When choosing a mold material, users take into consideration whether they want to make more than one casting and choose the material accordingly. A wax mold, for example, is good for only one casting, whereas a latex rubber mold can be reused.
Hydrocal is used by mixing a slurry of about 2 parts Hydrocal to 1 part water. The artist sprinkles the Hydrocal into the water, allows it to slake so the powder absorbs the water, mixes the rest of the plaster into the water, and pours the slurry into a mold. After 15 to 20 minutes, the casting is ready to unmold. A drying time of several days is required before the casting is painted or decorated.