720 GOLD AND SILVER REFINING 



for coining. This requires modification, for although those metals, as well as lead, 

 render gold so brittle that it will readily break between the fingers, yet it is not true 

 to say that it renders gold so brittle as to bo incapable of being coined. In June and 

 July 1859, some brittle gold, to the extent of about 64,000 ounces, passed through 

 the Mint. The bars were so brittle that they broke with the slightest blow from 

 a hammer, but by special treatment the gold was coined into the toughest coins ever 

 produced. It may now be stated that if the system of manufacture bo changed to 

 suit the requirements of the case, gold cannot bo found too brittle for the purpose of 

 coining. This is simply a matter of fact, but the expense of coining brittle gold is 

 undoubtedly very great ; it is therefore wise that Mr. Warrington's plan should be 

 adopted for all gold containing the volatile metals or tin. Osmium-iridium does not 

 render gold brittle. Dr. Percy and Mr. Smith have demonstrated that all metallic 

 substances found in commerce contain traces of gold, which can be separated by 

 carefully-conducted chemical processes, and it is found that silver is peculiarly liable 

 to be in alloy with gold, and gold with silver ; hence a process of refining which shall 

 effect the separation of as little as one five-hundredth part of gold from its mass of 

 silver, is a matter of the utmost commercial importance. 



It is with regret that it is stated that the refineries of London are conducted with 

 such secrecy as to render a full description of any one of them impossible, while the 

 ignorance which will induce the proprietors of these establishments to attempt such 

 quietude is much to bo pitied, for, except so far as regards details of interior arrange- 

 ment, their processes are as well known and understood as it is possible for any 

 manufacture to be. 



In Paris (the London refiners are known to use the ' French process '), the plan 

 adopted is founded on the fact, that at a high temperature sulphuric acid parts with 

 one equivalent of its oxygen to oxidise an atom of a metal, while the atom of 

 oxide so formed at once combines with another atom of sulphuric acid to form a 

 sulphate. The atom of sulphuric acid which has parted with its atom of oxygen 

 passes off as gaseous sulphurous' acid. 



If mercury be boiled with sulphuric acid (commonly called oil of vitriol), it is found 

 that it entirely loses its metallic existence, and assumes the form of a dense white salt. 

 This change takes place at the expense of the sulphuric acid, and is shown by the 

 following equation. For explanation sake, call mercury Hg, and sulphuric acid SO 3 ; 

 if now it is assumed that one part or atom of Hg be boiled with two parts or atoms of 

 SO 3 , we have Hg + SO 3 + SO 3 , and for elucidation we may write SO 3 as equal to 

 SO 1 + ; then we have Hg + + SO 3 + SO 2 , which, under the influence of heat, 

 .become HgOSO 8 + SO 2 . 



a white salt. gas. 



If now the mind substitutes silver for mercury, and so writes Ag instead of Hg, 

 the whole matter will be understood. The silver is dissolved in sulphuric acid just 

 as sugar would be in water, and in this fact we have a valuable means of separating 

 it from gold. If for a moment one imagines a mass of silver alloyed with gold to Im 

 represented by a piece of sponge filled with water and frozen, it is well known that if 

 the mass be warmed the ice is melted, and in the form of water filters from the sponge ; 

 just so, if a mass of the alloy of the precious metals be boiled in sulphuric acid, the 

 silver is dissolved or washed away, leaving the gold in the form of a sponge, which, 

 as it becomes exposed to the bubbling of the acid, is detached and falls to the bottom 

 of the vessel in which it is boiled. 



If by assay the silver to bo refined is found to be very rich in gold, it is bettor to 

 fuse the mass with more silver, so as to produce a mass containing at least 3 of silver 

 to 1 of gold, and this alloy, in its fluid state, should be poured into cold water, by 

 which the falling stream is suddenly chilled, and the particles become what is tech- 

 nically called ' granulated.' The stream should fall some distance (not less than 2 

 feet) through the air before it reaches the water, that the copper (if any be present) 

 may be as much as possible oxidised, with a view to saving sulphuric acid. 



In all cases the alloyed metals should be granulated, because the extended surface 

 of metal presented to the hot acid saves much time. 



Silver containing less than -^ part of its weight of gold is found not to pay for 

 separation, but any which contains this amount or more is treated as follows : 



Vessels of platinum were formerly used and were deemed indispensable, but oxp-ri- 

 ment has proved that these may bo safely replaced by cast-iron vessels; in both rases 

 the boilers or retorts are provided with tubes passing from the top into chambers which 

 receive the acid gases and vapours. 



The platinum vessels used by Mr. Mathisonand subsequently by Messrs. "Rothschild 

 for many years are now out of use; but as sketches of the vessels actual! 

 cannot be obtained, it is deemed wise to give a sketch of the platinum ^ 



