Plaster of Paris is used to make a variety of different shapes and moulds. At home, it is often used to make hand casts and other simple moulds. Children love to work with plaster of Paris because it is fun, slightly messy, and can be moulded into a myriad of different shapes. Plaster of Paris can be heavy, however, so the right ratio must be used to create a lightweight plaster that will not be extremely heavy.
Heat the water to between 23.9 and 37.8 degrees Celsius. The lower the water temperature, the longer the plaster will take to mix and set. It does not matter for the weight of the plaster what temperature of water you use, so just decide whether you want the plaster to set fast or slowly. Generally, if you're trying to mould something with details, such as a handprint, you will want the plaster to set quickly.
Determine the ratio of water to plaster you want to use. A 50-50 split will make the lightest weight plaster, while any mixture between 50 and 75 parts of water to 100 parts plaster will make a stronger mould. Use the scale to measure the plaster and water ratios, or you can use a measuring cup. Since you are making lightweight plaster, make a 50-50 split between water and plaster.
Pour the water into a plastic or disposable container. Do not add the plaster yet. Slowly pour the plaster into the water, stirring as you add the plaster. This is the best way to prevent lumps from forming in the plaster. Make sure the water absorbs each bit of poured powder before adding more plaster. Once you have poured all the powder into the water, the mixture should be somewhat stiff and hard to stir.
Pour the mixture into the mould you plan to use. Shake the mould gently to release any air bubbles that might be present in the plaster. If you're making a handprint or some other body mould, spray your hand with cooking spray before pressing into the mould. This will prevent the sticky substance from sticking to your hand.
Set the mould aside and do not disturb it for 24 hours. Do not let children or pets touch the mould while it is drying.
Remove the plaster from the mould by pulling at the edges of the mould container. The plaster should drop right out. If it does not, you can use a butter knife to release the plaster from the mould.