Linda Moyer, age 9, of Fullerton, Pennsylvania, for her question:

What kind of rock is used to make plaster?

Rocks are minerals and every mineral is made of its own special atoms. Its spe¬cial ingredients make it behave differently from all the other minerals. In New Mexico, there is a snow white stretch of desert called White Sands. Its crusty frosting is made from the mineral gypsum, called calcium sulfate because the ingredients in its molecule packages are atoms of calcium and sulfur. Gypsum which is to be used for plaster is first dried and ground to powder. When water is added to the powder, a pasty mixture is formed which can be applied to a surface. The mixture dries and becomes solid white plaster.

We call this solid gypsum Plaster of Paris and use it to make plaster casts for broken bones. A few other rocky minerals can also be used to make plaster. A spread¬ able paste made of powdered limestone dries and forms plaster. If sand is added to this limestone mixture, the plaster is stronger. Several rocky minerals are mixed to¬gether and baked to make powdered cement. Many wall plasters are mixtures of sand and lime, gypsum and powdered cement. If hairy threads are added to the moist paste, they bind the brittle plaster and make it less likely to crack.