When all the wires are in position on the board, a box is made the right size for the base of the finished solid model, and with sides as high as the solid model is to be made. This box serves as a mold into which the wet plaster of Paris is poured. Care must be taken to have the bottom edges of the box fit well on the board so that the liquid plaster of Paris may not leak out.

The powdered plaster of Paris may be obtained from any store selling building supplies, or from a drug store. Water is added and the mixture carefully stirred until it is free from lumps and of about the consistency of very thick cream. The fluid is poured into the box up to the desired height and allowed to stand for several hours

GENERAL METHODS

or over night until it becomes thoroughly hard. The box is then removed. In order to get the shape of the model as seen in Fig. 237, the plaster of Paris is very carefully scraped away with a piece of tin or some other simple tool until the ends of the vertical wires just show through. Before the plaster of Paris is poured in, care should be taken to mark the different sides of the board so that it will be

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