Almost every one has to mix up gypsum or plaster of paris once in a while, but few know how to do it so as to make a smooth cream, or thin dough, without lumps. The trick is not to pour the water on the plaster, but to turn the latter gradually into the water, spreading it about in shaking it in, and to avoid stirring until all the plaster has been added. The proper quantity of gypsum is usually enough to peep out over the surface of the water over the greater part of the area; that is, about equal volumes of each ingredient. The addition of glue-water to the mixture retards setting. Robert Grimshaw.
For experimental purposes and where but a few castings of medium and light weight are required, plaster of paris has many good advantages as a material for pattern making. It is light, it can be given a smooth surface, it is easily given any required shape and it can be added to indefinitely. While it is brittle, this is more than offset by the saving in first cost and the quickness with which the pattern may be prepared. Plaster of paris sets in from three to six minutes, but if for any reason it is desired to keep the mass plastic for a longer period, one drop of glue to a five-gallon mixture will keep it soft for a couple of hours. Plaster of paris mixed with cold water has an expansion of about 1-16 inch to the foot when hardening. Should this be undesirable, mix with warm water or lime water and there is no expansion. Donald A. Hampson. Middletown, N. Y.
Citric acid will delay the setting of plaster of paris for several hours. One ounce of acid, at a cost of about five cents, will be sufficient to delay the setting of one hundred pounds of plaster of paris for two or three hours. Dissolve the acid in the water before mixing the plaster.
Indianapolis, Ind. Otto L. Lewis.