On "Surcharge" of the Bullion Assay. 89 



Art. XXI. — On "Surcharge" of the Bullion Assay. 

 By Robert Barton, Esq. 



[Read 6th September, 1875.] 



It is generally understood by assayers that the surcharge 

 of the gold cornet does not exactly represent the residue of 

 silver which the parting has failed to boil out, and that it is 

 on the contrary a resultant error, in fact the difference 

 between this silver residue and the loss which the gold 

 suffers by volatilization in the muffle and absorption by the 

 cupel. 



As these two opposite sources of error vary according to 

 circumstances of temperature of the muffle, porosity of the 

 cupel, thickness of the ribbon forming the cornet, quantity 

 of lead and proportion of silver employed, strength of acid, 

 time of the operations, &c, it becomes necessary to control 

 the work by the use of "proof" assays of gold of known 

 fineness, which, passed with the work under exactly the 

 same conditions throughout, show what correction for 

 " surcharge " is to be made in each case. 



The more closely a routine is adhered to— the same from 

 time to time in all its minor details of temperature, &c, 

 using cupels of the same make, acid from the same bulk — 

 the more uniform will the surcharge remain, from day to 

 day. But even when this approximate uniformity is secured, 

 there still remains an influencing cause, the neo-lect of which 

 will lead to reports of a comparatively inaccurate character ; 

 the loss on the cupel depends upon the quantity of gold in 

 the assay, so that the surcharge will be greater in samples 

 of gold of high fineness, than in those of low gold, passed 

 in the same fire and parted in the same acids. 



The loss of gold may be so great as exactly to neutralize 

 the excess weight due to silver left in the cornet, when a 

 surcharge of will result ; or the volatility may exceed what 

 is required for compensating the excess weight cine to 

 remanent silver, when the surcharge will be represented by 

 a negative sign. Something must in such instances be 

 added to the weight of the cornet, in order to represent the 

 exact fineness of the sample assayed. 



In practice, the variation of surcharge in agreement with 

 the varying fineness of the samples may be compensated by 

 passing in the same fire proofs of fine gold of weights 



