KNOWLEDGE. 



[June, 1901. 



standard have an advantage over those of higher 

 standard, inasmuch as the abrasion suffered by an alloy 

 with 90 per cent, of silver would be less than that 

 suffered by an alloy containing 92.5 per cent. As 

 silver and copper are capable of uniting in all pro- 

 portions by direct fusion, standard silver is prepared 

 by melting together the requisite quantities of the 

 two metals. The fusion is usually effected in plumbago 

 crucibles, those in use at the Royal Mint having a 

 capacity of 4000 ounces. 



Alloys of these metals possess the characteristic piu'e 

 metallic whiteness of silver until the copper amounts 

 to 50 per cent, of the alloy, and the tint becomes more 

 and more red with the increase in the amount of copper 

 above this limit. The effect of copper is to increase 

 the hardness and elasticity of the alloy, hence silver is 

 always alloyed with copper to obtain the hardness 

 required to enable it to withstand the wear to which 

 coins, etc., are subjected ; pure silver, as previously 

 stated, being too soft for the purpose. The hardest 

 alloy of these metals consists of about 31J^ per cent, of 

 silver and 68f per cent, of copper, or a ratio of 5 to 11. 



Although the products of the fusion of silver-copper 

 alloys in any proportions are comparatively homo- 

 geneous, yet it has long been remarked that ingots of 

 alloys of these metals are not absolutely identical in 

 composition throughout. 



Considering that silver-copper alloys of various com- 

 position have long been used universally for the pur- 

 poses of coinage and plate, it is not surprising that this 

 series of alloys has been the subject of careful experi- 

 ment, and much is now known respecting them. Recent 

 research has shown that one of the most interesting facts 

 connected with them is the remarkable molecular re- 

 arrangement they undergo during solidification. 



Levol, who is one of the chief authorities on the 

 subject, concluded that the only homogeneous alloy of 

 the series contains 71.89 per cent, of silver and 28.11 

 per cent, of copjier ; and he considers this alloy to be 

 a definite compound of the two m.otals having the 

 formula AgjCu.j. All other alloys of silver and copper 

 Levol regarded as mixtures of this definite alloy with 

 excess of either of the metals. 



His experiments showed that in alloys containing 

 more than 71.89 per cent, of silver the centre of the 

 solidified mass is richer in silver than the exterior, 

 while in alloys of lower fineness than this the centre 

 contains less silver than the external portions. J 



Sir Roberts-Austen repeated many of Level's experi- 

 ments, and concluded§ that uniformity in composition 

 of the series of silver-copper alloys depends greatly on 

 the method of cooling. By slow cooling many alloys 

 other than the one mentioned above may be made as 

 uniform as it, its peculiarity consisting in the fact that 

 its composition is iiniform whether it is cooled slowly 

 or rapidly. 



With regard to the homogeneity of English standaed 

 silver (925) cast under ordinary conditions, it appears 

 from a long series of experiments made some years ago 

 by Col. Smith for the Indian Mints|| that the tendency 

 of the silver and copper to separate depends upon the 

 inequality of the rate of cooling in the different parts 

 of the ingot. The act of cooling causes a partial 



+ Ann. de Chim. et de P/;».t., Vol. 30 (18.52), p. IQ.^i ; Vol. 39 

 (1853), p. 1(53. 



§ Pror. Soil. Soc , Vol. XXIII. (1875), p. 481. 



11 Idem, p. 433. 



separation or liquation of the copper towards the sur- 

 faces which cool first, those parts of the bar being richest 

 in silver which solidify the last. 



The researches of Roberts-Austen have shown that 

 the maximum difference in the composition of an alloy 

 containing 925 parts of silver and 75 parts of copper 

 is only 1.40 parts per thousand when the alloy is slowly 

 cooled, while it is as much as 13 parts per thousand 

 when the alloy is rapidly cooled. 



The irregularity in the composition of standard silver 

 due to liquation is a matter of great importance in mints 

 where the production of alloys of uniform composition, 

 is very desirable. 



With suitable moulds and uniformity of cooling 

 liquation may be almost entirely prevented, but unless 

 great care is exercised in the preparation and casting 

 of the alloy the irregularities in the composition will 

 be much greater than those quoted above. 



In establishments whei-e standard silver is prepai-ed 

 for trade purposes and is subsequently manufactured 

 into goods to be Hall marked it is the common practice 

 when prepaxing the alloy to add a little extra pure silver 

 beyond the quantity actually required. This is done 

 in order to overcome the irregularities in composition 

 due to liquation, and to obtain an alloy which will be a 

 little above the standard in all parts and thus satis- 

 factorily pass the Hall. 



Standard silver, apart from its attractive appearance, 

 possesses many properties which render it valuable in 

 industrial art. It is very malleable and ductile, and 

 can be readily rolled into thin sheets and drawn into 

 very fine wire. It possesses a brilliant metallic lustre 

 and is capable of taking a high polish. One of its 

 most valuable properties is the readiness with which 

 it may be made to " flow " tinder pressure. It is well 

 known that " when a malleable metal or alloy is ex- 

 tended by mechanical processes, such as rolling, stamp- 

 ing, or hammering, a true flow of the particles of the 

 metal occiu's, analogous in all resjjects to the flow of 

 viscous fluids. The i^ressure exerted upon the surface 

 of the metal is transmitted in the interior of the mass 

 from pai-ticle to particle, and tends to produce a flow in 

 the direction where the resistance is least."1I 



The application of this fact, that solid metals and 

 alloys flow like viscous fluids, is of great importance in 

 industrial art, and the production of various complicated 

 and artistic forms with standard silver by stamping, 

 sjjinning, chasing, etc., entirely depends on the flow ,pf 

 the metal when suitably guided by the artificer. 



The striking of coins presents a familiar instance of 

 this valuable property of standard silver, the metal, 

 under pressure, being made to flow into the sunken 

 portions of the die without fracturing, thus producing 

 a true impression with every detail clearly defined. The 

 stamping of knife handles and small trays also affords 

 a familiar example of this property. The art of stamping 

 and shaping ai'ticles of jewellery, etc., in a press from 

 sheets of standard silver came into general use just 

 previous to the first Exhibition of 1851, and a very 

 large quantity of work is now produced by this means. 



During the mechanical processes, such as hammering 

 and stamjiing, the metal becomes more or less haixl and 

 brittle, and requires to be annealed at frequent intervals. 

 This is effected by heating the metal to a dull red 

 heat in a non-oxidising atmosphere, whereby its original 

 softness is restored. 



IT Percy. "Metallurgy," Vol. I., page 22. 



