PART III. MAKING CASTS. 



CHAPTER XXXIV. 



PRINCIPLES OF UNIVERSAL APPLICATION IN MAKING 

 MOULDS AND CASTS. 



THE processes employed in making plaster Paris moulds and 

 casts are very simple, and easily learned, even by one who has 

 had ^no previous knowledge of the subject. To be sure, a cer- 

 tain degree of intelligence and skill is necessary in the operator ; 

 but we are not writing for the edification of duffers who do not 

 know how to use their hands, or follow plain directions. 



The first thing to understand is the difference between a 

 mould that will " draw," and one that Avill not. A mould may 

 be made on one side of a base-ball, and it will draw off the ob- 

 ject at once, because there is no point on the ball behind which, 

 or under which, the plaster can catch, and hang fast until some- 

 thing breaks. A mould of one full side of an apple will not 

 draw, because the apple has a hollow at each end, and when 

 these are set full of plaster the mould and the apple are held 

 firmly together. 



A hollow or a protuberance on an object which would prevent 

 a mould from drawing away makes what is calied an " under- 

 cut," and necessitates the making of a separate piece in the 

 mould. To cast several copies of a human head and neck neces- 

 sitates the making of a mould in several pieces, all fitting very 

 nicely together, with countersink joints, to accommodate the 

 undercuts behind the ears, under the chin, the hollows of the 

 eyes, etc. 



A mould made in more than two pieces is called a " piece 



