PLASTER CASTS. 7 



lightly oiled or greased on the surface to prevent it sticking to the 

 plaster ; and when the slip begins to thicken and solidify is 

 carefully placed on the surface of the thickened slip and gently 

 pressed down. After being left awhile, the plaster will have 

 hardened sufficiently to permit of the coin being cautiously picked 

 off by means of a pin or a pair of tweezers, &c., when an accurate 

 impression of the lower face of the coin will be obtained. In this 

 impression, as with an ordinary seal, the hollows and projections 

 of the original coin will be reversed ; so that, for example, the head 

 on a half-crown piece will appear not in "relief" as on the coin, 

 but sunk in, or in " intaglio." 



Another mode of proceeding is to put the coin, &c., on a table 

 or plate, and then pour over it the fluid slip, taking care that no 

 air-bubbles are left sticking to the coin, which would damage the 

 impression. In operating in this way it is often convenient to 

 tie a strip of paper round the coin so as to form a sort of border, 

 sticking up a quarter of an inch or more, which prevents the slip 

 from flowing over the edge of the coin, and causes the cast to be 

 of the same diameter as the coin itself. If the cast is required 

 to be larger than the coin, the latter may conveniently be placed 

 in a pill-box of the required size, lying flat on the bottom of the 

 box and concentric with the sides, and the slip then poured in 

 over the coin so as partly to fill the box. 



After the plaster has become solid, by slowly drying it and then 

 well oiling it, or better still, soaking it in melted paraffin wax 

 until completely imbued therewith and allowing to cool, the cast 

 is obtained in such a condition that fresh plaster slip will not 

 permanently stick to it ; so that by taking a second cast from the 

 mould thus prepared, in the same way as before, an exact dupli- 

 cate of the original object is obtained, the hollows and relief being 

 now reversed a second time. 



Statuettes and similar solid objects are cast in plaster by pre- 

 paring hollow moulds of metal or other material in two or more 

 pieces hinged together, so that when the slip is poured into the 

 hollow of the closed-up mould, it solidifies therein and is finally 

 removed as a solid block by taking the outer mould to pieces 

 (vide Expt. 5). 



By mixing with the plaster a little Prussian blue, yellow-ochre, 

 or other coloured pigment in proper proportion, tinted casts may be 

 prepared of any required shade according to taste. A polished 

 appearance may be given by applying strong soap-water to the 

 surface and allowing it to dry, and then gently rubbing with a 

 soft cloth. The appearance and almost the hardness of marble 

 may be given by applying solution of alum in the same way, or 



