Plaster of Paris is similar to quick-dry cement with one huge difference. Unlike sidewalk cement, hardened Plaster of Paris can be sanded and carved into any shape you want, making it invaluable for sculptures, modeling, and casts.
Plaster of Paris is a powdery compound made up of a mix of gypsum cements. The powder is so fine and light that it almost looks like very fine cake flour. But, once the powder is mixed with plain water, the compound turns into a white paste and will start to set up as quickly as 30 minutes. Plaster of Paris is very easy to work with, but does take a bit of common sense and some safety measures. These tips will help:
Precautions You Should Read Before Starting
Avoid breathing the dust. Those minute particles of Plaster of Paris dust is not something you want to be breathing which is why it’s recommended to make cast in a well ventilated area and wear a face mask.
Do not pour leftover liquid Plaster of Paris or the dust down the sink, toilet, or bathtub. This quick drying cement will harden in the pipes and clog your drains.
Use flexible containers to mix the plaster. Containers that work best for mixing wet plaster and EZ cleanup are flexible plastic buckets such as ice cream buckets, yogurt containers, or a plastic mop bucket. With a flexible bucket, dry plaster can be “popped” out of the bucket for easy clean-up. Avoid using your mom’s glass mixing bowls or metal buckets since the inflexible sides mean a lot of outdoor scraping and scrubbing to get these containers clean.
How to Mix and Pour Plaster of Paris
Flexible sided mixing bucket
Wooden stir stick, such as the kind you get from the hardware store.
One cup measuring cup.
Graduated measuring cup, the type with the volume written on the side.
Plastic covered work area.
A mold. Molds can be anything made of a flexible material, such as a milk jugs or carton with the top cut off; styrofoam meat trays, or a plastic mold purchased from the store.
1. Determine how much will be needed. Before mixing the plaster, figure out how much will be needed for your mold. One easy way to calculate is by pouring water in the mold and measuring it. For every two cup of liquid the mold holds, one “batch” of Plaster of Paris will be needed.
2. Working on the plastic covered work area, mix the plaster in a 2:1 ratio with water. To make a single batch of Plaster of Paris, measure 2 cups of dry plaster and place it in the plastic container. Wipe out the cup with a paper towel. Measure one cup exactly of water in the glass measuring cup and pour into the dry plaster. Stir until the lumps are completely dissolved. For larger molds, you may have to make three or more “batches” all at the same time. Don’t make more than what you can use in 30 minutes.
3. Pour the plaster into the mold and let set for 24 hours. Carefully pour the plaster into your mold. Let dry until the next day. To remove the plaster from the mold, flex the side of the mold and gently ease the cast out onto a cushioned surface, such as an old towel. The cast will break if drop so be careful.
4. Decorate your cast. The cast can now be sanded, carved, and painted any way you like.
5. Clean up the mess and throw everything away in a garbage sack. Anything that needs to be rinsed should be rinsed outside to avoid damaging drains.
Plaster of Paris can be purchased at all craft stores and hardware stores, in quantities as small as a one pound bag to a 25 pound bag. Because it never goes bad, unmixed plaster of Paris can be kept indefinitely in a cool, dry place. A 25 pound bag runs about $19, small “half gallon” sized containers usually cost about $3 or so.