How to Mix Plaster of Paris

Plaster of Paris has been used for centuries as an inexpensive, durable and easy-to-use art and craft material. It can be use to pour sculpture castings and as a mold-making material. Plaster of Paris is a dry white powder, mainly comprised of gypsum, that is added to water to form a thick liquid mixture. The liquid plaster can be poured, brushed or applied with a spatula into a mold or onto sculpture. The water and plaster, after mixing, undergo a chemical reaction that causes the mixture to harden. Following the steps in this article will ensure success in mixing plaster of Paris.

Things You'll Need

• Measuring cups
• Water (distilled or filtered if possible)
• Plaster of Paris
• Flexible mixing bowl: rubber, plastic or disposable
• Wooden spoon or spatula for mixing
• Newspapers or old rags for cleanup

Instructions

1. Read the directions and safety precautions on the packaging of the plaster material; some manufacturers of hobby materials include additives to the plaster that affect the mixing ratio. The traditional plaster of Paris mix uses two parts plaster to one part water.

2. 

   Measure the volume of water first, and pour it into the mixing bowl. Always add the plaster to the water; do not add water to dry plaster powder. The temperature of the water will affect the setting time: Hot water will cause the plaster to set more quickly; cold water will slow down the setting process. When you're finished measuring the water, dry the measuring cup thoroughly, or use a separate cup to measure the plaster.

3. Measure the volume of plaster. The mix does not need to be exact; for example, adding less plaster will slow the setting time but will result in a less dense and weaker casting. Adding more plaster will speed up the setting time and result in a denser plaster when hardened.

4. 

   Use a spoon or spatula to break up any lumps in the dry plaster and ensure that the powder is an even, fine texture.

5. 

   Add the plaster to the water in the mixing bowl in small amounts, sprinkling or sifting it into the bowl using the spoon. Do not stir the mixture while adding the plaster. Continue adding the plaster to the water. Tap the side of the mixing bowl periodically to vibrate the mixture and release air bubbles. Never use your bare hands to mix or stir plaster.

6. 

   Add the plaster to the mixture so that it settles evenly throughout the depth of the bowl. A rule of thumb is to stop adding plaster to the water when the powder reaches the top surface of the water evenly across and the water can no longer easily absorb the plaster. Continue tapping the side of the mixing bowl for about a minute to ensure that all air bubbles are released from the mixture. Do not stir the mixture yet. Let it set for an additional minute or two.

7. 

   Stir the mixture with the spoon or spatula briefly, with slow side-to-side motions. Do not stir vigorously, as this will add air to the mixture and cause it to harden sooner. Break up any remaining lumps or patches of dry white powder. When the mixture is a smooth, even consistency of a uniform light-gray color, the plaster of Paris is ready to be poured into a mold for a casting. If the plaster is being used to create a mold, it can be brushed on to the surface of a sculpture or applied with a flexible spatula.

8. Allow the unused liquid plaster to harden in the mixing bowl. Never pour liquid plaster down a sink or drain, and never allow powdered plaster to enter a sink or drain, as either liquid or powdered plaster will clog drains severely. Remove the hardened plaster by flexing the bowls to break the hard plaster into pieces, and discard the pieces in the trash. Wipe the plaster thoroughly off the spoons, spatulas or other implements with newspapers or rags, then wash off the residue with hot, soapy water.

Tips & Warnings

• Bowls made of flexible materials such as rubber or plastic are recommended. Disposable containers such as plastic margarine tubs or cottage cheese containers can also be used. Wooden spoons and rubber or plastic spatulas are good mixing implements. I often use old kitchen implements and containers purchased at garage sales that are used only for arts and craft projects and are kept separate in a garage or closet.

• Keep plaster away from food or kitchen and out of the reach of small children. Read all directions and safety precautions on the packaging of any plaster materials that you obtain or purchase. Take precautions to keep dry or mixed plaster off of skin and out of eyes. Plaster undergoes a chemical reaction that produces high heat and can cause severe burns. Do not attempt to cast body parts in plaster. Never mix plaster using your bare hands.

Resources

• Read this Article in Spanish
• Mixing Plaster for Mold Making
• Plaster Craft Molds Online