MOULDING AND CASTING. 291 



stand about Iialf an hour, and it will in that time 

 grow so hard, that you may safely take it ofFj then 

 pare it smooth on the back and round the edges 

 neatly. It should be dried, if in cold or damp 

 weather, before a brisk fire. If you cover the 

 face of the mould with fine plaster, a coarser sort 

 will do for the back : but no more plaster should 

 be mixed up at one time than can be used, as it 

 will soon get hard, and cannot be softened without 

 burning over again. 



Sulphur must not be poured upon silver medals, 

 as this will tarnish them. 



To prepare this mould for casting sulphur or 

 plaster of Paris in, take half a pint of boiled lin- 

 seed oil, and oil of turpentine one ounce, and mix 

 them together in a bottle ; when wanted, pour 

 the mixture into a plate or saucer, and dip the 

 surface of the mould into it; take the mould out 

 again, and when it has sucked in the oil, dip it 

 again. Repeat this till the oil begins to stagnate 

 upon it ; then take a little cotton wool, hard rolled 

 up, to prevent the oil from sticking to it, and 

 wipe it carefully off. Lay it in a dry place for a, 

 day or two (if longer the better,) and the mould 

 will acquire a very hard surface from the effect of 

 the oil. 



To cast plaster of Paris in this mould, proceed 

 with it in the same manner as above directed for 

 obtaining the mould itself, first oiling the mould 

 with olive oil. If sulphur casts are required, it 

 must be melted in an iron ladle. 



Another viethod with Ishmlass. — Dissolve isin- 

 glass in water over the fire ; then, with a hair 

 pencil, lay the melted isinglass over the medal : 

 and when you have covered it properly, let it dry. 



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