236 TAXIDERMY AND MODELLING 



arranging it for casting ; thus, the plaster being poured over 

 all, the upper and under surfaces of both fins appear on the 

 same block with the fish — an obviously valuable precaution 

 against loss or mistakes. Such a mould is shown by Fig. 3, 

 Plate XII. 



Dry the mould near the fire, but do not bake it to make it 

 chalky and brittle, and, when dry, give it one or two coats of 

 colza-oil, but not sufficient to fill up the pattern or to cause 

 a skin of oil to form upon the plaster. 



Although many authorities recommend treatment of the 

 mould with water previously to making a model, yet this has 

 certain disadvantages not found in oil, and the difficulty of 

 distinguishing which is mould and which model is accentuated, 

 in the case of the water process, in the trimming of the edges, 

 unless the mould be coloured to distinguish it from the model. 



Only simple objects such as those previously described will 

 come out or relieve when plaster is cast into a plaster mould, 

 and although many fishes can be made from such, yet there 

 are others which are too '' undercut " to relieve without breaking. 

 In such cases, either the character or composition of the mould 

 itself must be materially modified, or the substance of which 

 the model is made must be changed. 



The first alternative — that of modifying or changing the 

 character of the mould so that a plaster cast or model may be 

 made — leads to a consideration of — 



How to make Plaster Casts of Undercut Objects 



There are several methods of arriving at a satisfactory 

 solution of this difficulty, which may be summarised under the 

 following headings : — 



1. By waste moulds in plaster. 



2. By piece moulds in plaster. 



