Using an alginate mould is an effective method to produce a plaster cast of an object. Originally developed for dentistry due to the accuracy of moulding teeth, home users most commonly use alginate moulds to craft plaster casts of hands or feet. Using an alginate mould effectively includes using a viable design idea, creating the mould 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 jewellery 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 moulding object, in order to create a strong enough mould 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 moulding container.
Submerge your moulding object into the alginate while simultaneously moving the object within the mixture to release air bubbles on the surface of the object. Keep the moulding object stationary for approximately 10 minutes to allow the alginate mould to set, or according to the manufacturer's instructions.
Release the object from the alginate mould by slowly pulling the object out of the set gel. Inspect the inside of the mould 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 mould slowly to thinly coat the mould and minimise the introduction of air bubbles. Continue pouring the plaster to top off the mould while gyrating the mould to release trapped air. Allow the plaster to dry before peeling off the alginate mould to reveal the finished casting of your object.
Use water above 23.9 degrees Celsius to reduce the alginate setting time, or use water colder than 21.1 degrees Celsius to increase the setting time. Rub a thin layer of a petroleum-based jelly product on moulding objects to reduce air bubbles.
Wear a respirator while mixing plaster-casting powder to avoid chemical inhalation.
Tips and warnings
- Use water above 23.9 degrees Celsius to reduce the alginate setting time, or use water colder than 21.1 degrees Celsius to increase the setting time.
- Rub a thin layer of a petroleum-based jelly product on moulding objects to reduce air bubbles.
- Wear a respirator while mixing plaster-casting powder to avoid chemical inhalation.
Things you need
- Plastic container
- Moulding object
- Wooden spoon
- Electric mixer (optional)
- Mixing bowl
- Plaster-casting powder
- Petroleum-based jelly-type product (optional)