130 Laboratory Arts 



Every corner having been blacked, the blacklead is polished 

 with the tip of the third finger until every portion shines brightly, 

 taking care to include the edges of the mould, and a margin of 

 about half an inch all round the actual mould of the medal. 

 A hole is now to be pricked with a bradawl inside this margin, 

 and a piece of copper wire threaded through and coiled round 

 the gutta-percha, blacklead being well rubbed into the hole in 

 order to make good contact between the copper and the black- 

 lead film. Too much attention cannot be paid to the securing 

 of a perfectly contin uous, smooth, and bright surface of black- 

 lead, as upon this the whole success of the process depends. 



It may be well to note here that bronze powder is frequently 

 recommended in preference to blacklead, but this material is 

 very liable to be put on too thickly, with the result that upon 

 immersion into the plating solution the whole film strips. In 

 addition, even when this does not destroy the film completely, 

 there is a tendency for a connective column of fragments to 

 form upon the surface of the plating liquid, and " short " the 

 leads. It is found, however, that if bronze powder be mixed 

 with a little benzene, and the mixture fainted on the mould 

 it must be done rapidly, or the gutta-percha will become partly 

 dissolved, and ks sharpness entirely spoiled success may be 

 achieved. The surface is, however, liable to be lumpy, and, 

 all things considered, the blacklead process is to be preferred. 

 The wire attached to the mould is now connected to the 

 negative pole of a (pint) bichromate cell, 1 and the positive pole 

 connected to a sheet of copper, the mould and copper being 

 then dipped under the surface of a slightly acidified solution of 

 copper sulphate in water (100 grams cryst. CuSO 4 , 250 c.c. water, 

 2 c.c. H 2 SO 4 ) mould and copper plate facing each other about 

 an inch apart, and parallel to each other. The zinc of the cell 

 is now gra dually dropped into the exciting liquid, when a current 



1 Chromic acid should be used as the exciting liquid in the "bichromate " 

 cell rather than bichromate of potash and sulphuric acid. The best 

 formula is 5 oz. (avdp.) chromic acid, 3 fl. oz. cone, sulphuric acid, water 

 to 40 fl. ozs. To this 5 grams of mercury dissolved in the minimum 

 quantity of cone, nitric acid, evaporated to dryness, and redissolved in 

 water, may be added, this serving to keep the zinc amalgamated. 



