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KNOWLEDGE & SCIENTIFIC NEWS. 



[Jan., 1905. 



should be mentioned that the back of the article is 

 coated with some non-conducting material, such as solid 

 paraffin wax, otherwise the whole article would be- 

 come covered with copper, and it would then be im- 

 possible to remove the deposited metal. In depositing 

 the metal certain precautions have to be taken. Thus, 

 for example, the regulation of the current strength 

 (current density) is a matter of great importance. If 

 a heavy current is employed, the copper is very apt to 



be deposited in a rough and irregular form, and may be 

 so powdery as to actually rub off. The colour of the 

 copper is bright, and the appearance smor)th and regu- 

 lar when low currents are employed, but it is rough 

 and brown (burnt) with currents of too great intensity. 

 When a sufficiently thick deposit has been obtained 

 the article is removed frf)m the bath, well washed with 

 water, and dried. The point of a pen-knife or other 

 sharp instrument is then inserted under the edge of the 

 deposited metal and the metallic coating carefully 



stripped from the article upon which it has been de- 

 posited. Sometimes it is rather difficult to strip it with- 

 out bending and injuring the thin metallic shell, and 

 when this takes place it is not by any means an easy 

 matter to properly smooth it out again. The thin shell 

 thus obtained is backed up with lead or with an allov 

 of lead, which melts at a lower temperature than the 

 lead itself. In order that the backing metal may adhere 

 satisfactorily, the back of the shell must first be tinned; 

 a satisfactory tinning mixture consists of an alloy of 

 50 parts lead and 50 parts tin. The hacking metal is 

 then run in; a useful allov for this purpose consists of 

 90 parts lead, 6 parts antimony, and 4 parts of tin. 

 Wood's alloy is sometimes used, but is too expensive 

 for ordinary practice. It consists of an alloy of lead, 

 tin, cadmium, and bismuth, and melts below the tem- 

 perature of boiling water. 



A complete copy of a modal can be obtained by de- 

 positing the metal first on one side and then on the 

 other. The two shells thus obtained are, after tinning. 



placed back to back and the fusible alloy run in between 

 them. After filing and polishing the edges, copper can 

 be deposited on the rim when the whole — reproduced — 

 medal appears to be composed of copper. Fig. 2, A 

 and B, shows a photograph of two sides of a medal 

 commemorating the French revolution, and reproduced 

 in the above manner. The original medal was in this 

 case coated with sulphide. It is seen that by the 

 above method even the faintest lines are reproduced, and 

 we are thus able to obtain an absolutely exact replica 

 of medallions or engravings. 



Another and more commonly employed method is to 

 make a cast or matrix of the object which it is desired 

 to reproduce. This may be done in a variety of ways. 

 Sometimes a metallic cast is made directly from the die, 

 and upon this cast a thin film of copper is deposited. 

 Fig. 3 shows such a repioduction which was cast in 

 soft metal, then thinly coated with copper and treated 

 so as to give it the appearance of bronze. The medal 

 appears, in fact, exactly as if it was made of bronze. 



