Some information on PLASTER, A.K.A. Plaster of Paris or #1 Moulding Plaster.

What is it?

Plaster is gypsum rock that has been crushed to a powder and calcined (heated to about 900^of.). The calcination chemically dehydrates the gypsum. That is, it drives off water that is part of the gypsum's chemical, not atmospheric, composition (of course, the atmospheric water is driven off too!). When water is added to the dehydrated gypsum (now called plaster), it provides the necessary ingredient for its recrystallization. Put another way, the powder turns back to stone.

How much does it cost?

At this time (2007) a fifty pound bag of plaster costs about $11.75 (plus tax). Smaller amounts purchased in hobby stores may go fo a dollar a pound.

Where do you get it?

Only the larger building supply companies carry plaster as a matter of course. In Lexington, there is only one source at present:

THREE RIVERS MATERIALS
115 Delaware Avenue
Lexington, KY
859-225-8801

What are its uses?

In the classroom, plaster can be used as a vehicle for direct building or modeling, a mold making material, a casting material, and a carving substance.

What precautions should be take in its use?

Health precautions:

Plaster is relatively non-toxic in external use (I've never heard of anyone eating it!). However, it is an alkali and should not be allowed to get in the eyes. Avoid breathing the dust. Some few people may develop a rash if their skin is in prolonged contact with plaster. The solutions to these problems are simple. Gloves, barrier creams (vaseline), eye protection and care in handling are called for.

Cleanliness precautions:

Cover worktables with heavy plastic sheeting or several layers of newspaper. Have students wear aprons or else dress in old clothes. Provide doormats to avoid tracking problems.

Plumbing precautions:

Plaster will definitely plug drain pipes! A thin solution of plaster that is absolutely useless for any practical purpose will diabolically precipitate into a concrete-like mass in your sink drain. The way to avoid this eventuality is to use the "three-bucket" system of plaster disposal/cleanup or else have a specially designed plaster trap attached to the sink. (THREE BUCKET SYSTEM: Bucket #1 a dry waste can with a liner for the initial cleaning of plaster from bowls, hands, etc.; Bucket #2 a bucket half-filled with water for the initial washing of bowls, hands etc.; Bucket #3 a bucket similar to #2 for rinsing the bowls, hands etc. After the three bucket, it's safe to wash in the regular sink. Let the plaster settle in buckets #2 and #3, decant the clear water and dispose of the remaining sludge.)

How is it mixed?

The usual way to mix plaster in the classroom is by hand. The plaster is sifted into water to the extent that it just projects above the water's surface in small peaks and islands. In hand mixing plaster, plaster is always added to water, never the other way round. For mixing, a wide shallow bowl with smooth interior contours (no sharp angle or corners) made of a flexible material is ideal. The width provides a broad water surface which promotes effective slaking of the plaster. Further, a broad bowl makes inadvertent spills, wet or dry, less likely. After the plaster is sifted into the water, it is allowed to stand for a couple of minutes to promote thorough slaking. Then use your hand to completely mix the plaster into a smooth consistency by crushing any lumps that may exist. Don't overmix the plaster unless you wish to shorten its working time. Agitation promotes rapid setting. If you're going to use the plaster for casting, tap the bowl on the table to cause entrapped air bubbles to rise to the surface and break them.

Thick plaster is good plaster.

When the ratio of plaster to water is right, the result will be a smooth material a little thicker than heavy cream. The thickness will be almost as heavy as sour cream. Any mixture thinner than this will result inset plaster that is weak and porous. Plaster of the proper thickness actually takes sharper detail in casting than does thin plaster. Of course, it's possible to mix plaster too thickly, but that's not the usual problem.

How do you calculate plaster volume?

Once you've determined how much plaster you need, start you mix with an amount of water equal to a little more than half the needed volume. Properly hand mixed plaster almost doubles the volume the water occupied. If you are trying to calculate how much water you'll need for a given amount of dry plaster, the reverse works. The water will be a little less than half the amount of plaster. These directions are of necessity imprecise because the condition of the plaster can vary in density and in water already absorbed from the atmosphere.

How long O Lord, How long! (Setting time)

Once it has been mixed, plaster will set hard in 20 to 30 minutes at room temperatures. The time is variable because it depends on such things as exact temperature, atmospheric moisture content, and the plaster's condition. Further variables are mixing techniques and the mineral content of the water. If plaster is being mixed at very low temperatures, it may take hours to set.

So, how can I tell when it's set?

Plaster goes through stages as it sets. At first, it's liquid and pourable. Later, it becomes increasingly thick and can be spread by troweling. This is called the "buttery" state. At the end of its workable stages it becomes lumpy and chalky. It should not be disturbed when it reaches this state because any fissures opened at this time become permanent. Once the surface of the plaster seems set, it can be given the "thumbnail" test. Press your thumbnail into the surface. If it easily makes an indentation, the plaster is not fully set. If it is hard to make a mark, the plaster is probably set. When plaster sets, there is an exothermic reaction (heat is produced). Therefore when the plaster becomes warm, it has set. When the exothermic heat passes off, the plaster has attained approximately 80% of its possible strength. The remaining 20% comes with curing and drying (Days, not hours).

What's the shelf life of plaster and how should it be stored?

Plaster will actually last for many years if it is properly stored. However, in merely humid conditions (like a Kentucky Summer) it will quickly absorb water from the atmosphere and, in a month or so, deteriorate markedly. At first it will cake slightly and will have to be vigorously sifted to return it to powdery consistency. This stage is also marked by a shortened setting time. After more time, the plaster will become lumpy and may prove very difficult to break up for mixing. By this point it may have taken in so much water that it either sets almost immediately or, surprisingly, won't set at all. In either case it is now useless.

To properly store plaster, it must be protected from moisture. Unopened bags of plaster have fairly good resistance to humidity and will protect their contents for at least two or three months when stored indoors. The best way to insure an opened bag's freshness is to put it in a large plastic bag that can be tied off and put this double-bagged package in a covered container (like) a garbage can). I've kept plaster fresh for years using this approach.

Some curious things about plaster.

Cold water slows the setting time of plaster.
Hot water speeds up the setting time of plaster.
Salty water speeds up plaster's setting time.
Hot, salty water really speeds up the setting time.

(The problem with salt is that, some time after the plaster has set, the salt eventually migrates to the surface of the plaster where it causes pitting and the flaking of any paint.)

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