GOLD AND SILVER REFINING 723 



8 ft. long, 3 ft. deep, and 3 ft. wide, that the crystals may deposit slowly, as slow 

 formation produces large crystals, which are more easily collected. The sulphate of 

 copper is represented by CuO,S0 3 ,5HO (CuSO 4 ,5H 2 O). The mother-liquors are 

 evaporated and returned to the works, being in fact-free sulphuric acid, with a small 

 amount of sulphate of copper in solution. The parts of the hydraulic presses which 

 come in contact with the silver at the time of pressing are coated with a compound of 

 tin and lead, hardened by mixture with antimony. Cast iron is very little attacked by 

 concentrated sulphuric acid, but it is necessary to avoid wrought iron in any shape, and 

 copper vessels would of course be rapidly destroyed. 



The floors should be covered with lead of tolerable thickness. The melting-pots 

 used in France are made of Picardy clay, and hold from 2,200 to 2,600 troy ounces of 

 silver. The pots cost from 4d. to 6d. each, and if dried and used with care, very 

 seldom crack or break. i. & 



The total cost of refining silver in Paris, inclusive of the loss by melting, is stated 

 to be 15 centimes for 32 troy ounces ; but it must be understood that the loss of 

 silver by melting is absolutely very minute, because the flues are swept, and the sweep- 

 ings so obtained are made to yield the silver which has been volatilised, while the 

 pots, &c., are ground and made to yield their absorbed silver. 



In the event of the mass containing much copper and little silver, it is usual to 

 granulate the mass and roast the granulated particles to oxidise the copper ; the oxide 

 of copper is then dissolved out by diluted sulphuric acid, and the remaining mass of 

 silver, with a smaller amount of copper, is treated in the ordinary way. 



If the gold contains platinum, it is found that it is apt to retain from 4 to 5 per 

 cent, of silver, which must be separated by mixing the precipitated gold with about 

 a fourth of its weight of anhydrous sulphate of soda (which is preferred to sulphate 

 of potassa, on account of its greater solubility in water), and to moisten this mass 

 with concentrated sulphuric acid, using about 6 or 7 parts of acid to every 10 parts 

 of sulphate of soda. The moistened mass is then heated till sulphuric acid ceases to 

 distil off, and the heat is then raised till the whole mass melts ; and by extracting 

 the sulphate of silver and sulphate of soda the gold will be found to contain 99 '40 per 

 cent, of gold ; but if the process be repeated, the gold is obtained of a purity of 99 '90. 



When the silver has been removed, the gold is fused with nitre, which oxidises 

 and removes the platinum ; but the potash salt formed is found to contain gold, so 

 that the gold and platinum are obtained from the potash salt mixed with fused nitre 

 by the process of cupellation, for which see SILVER. G.F.A. 



A simple process of refining gold by means of chlorine gas was introduced a few 

 years ago by Mr. F. B. Miller, of the Sydney branch of the Koyal Mint. It consists 

 in passing chlorine through the melted gold, by thrusting into the molten metal a small 

 clay tube connected with the stoneware vessel in which the chlorine is generated. The 

 evolved chlorine combines with the silver in the alloy, and the chloride of silver thus 

 formed floats on the surface of the purified gold. Miller's processs is now extensively 

 used, and may always be advantageously applied when the proportion of silver in the 

 gold does not much exceed ten per cent. 



The following remarks on the ' Hall-Marking of Gold ' is from the pen of Mr. Wm. 

 Chaffers, the author of ' Hall-Marks on Plate.' 



The guarantee of the quality of gold and silver as stamped by the Government 

 Assay Offices is a matter of such great importance to the public that any doubt thrown 

 upon it ought immediately to be thoroughly investigated. 



The standard of the precious metals has been insisted on by the laws of the land 

 for more than 600 years, and the assaying of them was a privilege conferred, as early 

 as the year 1300, by Edward I., upon the Goldsmiths' Company, who stamped them 

 with the Leopard's Head, as a sign to the public that a piece of gold or silver was 

 actually of the quality thereby signified. It was ordained in 1327 that in all cities 

 and towns in England where goldsmiths reside, one or two of the craft for the rest of 

 the trade should come to London and have the stamp of a puncheon of a Leopard's 

 Head marked upon their work, as of ancient time it has been- ordained, and at that 

 early date it was enjoined that three stamps should be put upon the plate: 1. The 

 goldsmith's mark who made it. 2. The mark of the assayer (a letter of the alphabet 

 denoting the year). 3. The mark of the Goldsmiths' Hall (a Leopard's Head crowned). 

 Another mark of a lion passant, denoting that the piece was standard, was added about 

 1545. ... In 1423 other towns were privileged to mark plate ; but as many had dis- 

 continued, another act was passed in 1700, by which York, Exeter, Bristol, Chester, 

 and Norwich were formally appointed. In 1702 Newcastle was added to the list, and 

 in 1773 Birmingham and Sheffield were also privileged to assay and stamp silver 

 plate, each town placing a stamp of their arms for the purpose of identification. 



These two standards of plate gold of 22 carats, and silver of 11 oz. 2 dwts. or 

 sterling have remained unchanged, except for a short period of 25 years, when a purer 



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