Items you will need
- Plastic container
- Molding object
- Wooden spoon
- Electric mixer (optional)
- Mixing bowl
- Plaster-casting powder
- Petroleum-based jelly-type product (optional)
Using an alginate mold is an effective method to produce a plaster cast of an object. Originally developed for dentistry due to the accuracy of molding teeth, home users most commonly use alginate molds to craft plaster casts of hands or feet. Using an alginate mold effectively includes using a viable design idea, creating the mold and producing a plaster cast.
Base your plaster cast design on such considerations as cost, time available and what you want to do with the plaster cast once you make it. Think up designs such as using a clawed hand for a Halloween event, a spread-fingered hand to hang jewelry upon, a closed fist or a thumbs-up fist to use as an artistic piece.
Get a clean plastic container that is large enough to submerge the object you want to cast, such as a plastic jug for your hand. Ensure the container size allows for a minimum of 1/4-inch thickness of alginate to surround your molding object, in order to create a strong enough mold to withstand the casting plaster.
Mix the alginate powder according to the manufacturer’s directions. Commonly, weigh a 3-to-1 ratio of powder to water in a mixing bowl to produce the correct consistency. Use a wooden spoon or an electric mixer to quickly create a smooth alginate mixture because the chemical reaction of alginate salt with water causes the setting process to begin immediately.
Lift and bang the mixing bowl firmly on a hard surface to release any air bubbles within the liquid. Pour the alginate mixture into your molding container.
Submerge your molding object into the alginate while simultaneously moving the object within the mixture to release air bubbles on the surface of the object. Keep the molding object stationary for approximately 10 minutes to allow the alginate mold to set, or according to the manufacturer’s instructions.
Release the object from the alginate mold by slowly pulling the object out of the set gel. Inspect the inside of the mold for rips before you begin to plaster cast in order to avoid an unsuccessful casting.
Mix the plaster-casting powder according to the manufacturer’s instructions. Usually, you use the same method for mixing as you did with the alginate powder, but with a different powder to water weight ratio, such as 10-to-4 for example.
Pour the plaster into the alginate mold slowly to thinly coat the mold and minimize the introduction of air bubbles. Continue pouring the plaster to top off the mold while gyrating the mold to release trapped air. Allow the plaster to dry before peeling off the alginate mold to reveal the finished casting of your object.
Use water above 75 degrees Fahrenheit to reduce the alginate setting time, or use water colder than 70 degrees Fahrenheit to increase the setting time. Rub a thin layer of a petroleum-based jelly product on molding objects to reduce air bubbles.
Wear a respirator while mixing plaster-casting powder to avoid chemical inhalation.