A BKIEF ACCOUNT OF THE PROCESSES EMPLOYED IN THE ASSAY OF GOLD 

 AND SILVER COINS AT THE MINT OF THE UNITED STATES. 



FROM THE ANNUAL REPORT OF JAMES POLLOCK, DIRECTOR. 



PRINCIPLES OF THE OPERATION. 



Accordiiig to law, the standard gold of the United States is so constituted 

 that in 1,000 parts by weight 900 shall be of pure gold, and 100 of an alloy 

 composed of copper and silver. 



The process of assay requires that the copper and silver be both entirely 

 removed from the gold ; and to effect this, two separate operations are necessary. 



The first is for the removal of the copper ; and this is done by a method called 

 cupellation, which is conducted in an assay furnace in a cupel composed of cal- 

 cined bones. To the other metals, lead is added ; this metal possesses the prop- 

 erties of oxidizing and vitrifying under the action of heat, of promoting at the 

 same time the oxidation of the copper and other base metals, and of drawing 

 with it into the pores of the cupel the whole of these metals, so as to separate 

 entirely this part of the alloy, and to leave behind the gold and silver only. 



The separation of the silver from the gold is effected by a process founded on 

 the property possessed by nitric acid of dissolving silver without acting upon 

 gold. But that the gold may not protect the silver from this action, sufficient 

 silver must first be added to make it at least two-thirds of the mass. The pro- 

 cess to be described is based upon the rule of quartatlon, in Avhich the propor- 

 tion of silver is three-fom-ths. 



PROCESS OF ASSAY. 



The reserved gold coins are placed in a black-lead crucible, and covered with 

 borax to assist the fluxing and to prevent oxidation of the copper alloy. They 

 are thus melted down and stirred ; by which a complete mixture is effected, so 

 that an assay piece may be taken from any part of the bar cast out. The piece 

 taken for this purpose is rolled out for convenience of cutting. It is then taken 

 to an assay balance (sensible to the ten-thousandth of a half gram or less,) 

 and from it is weighed a half giam,, which is the normal assay weight for gold 

 being about 7.7 grains troy. This weight is stamped 1000 ; and all the lesser 

 weights, (afterwards brought into requisition,) are decimal divisions of this 

 weight, down to one ten-thousandth part. 



Silver is next weighed out for the quartation ; and as the assay-piece, if stand- 

 ard, should contain 900 thousandths of gold, there must be three times this 

 weight, or 2700 thousandths of silver; and this is accordingly the quantity used. 

 It is true that there is already some silver in the alloy, but a little excess over 

 the quantity required for the quartation does no injury to the process. 



The lead used for the cupellation is kept prepared in thin sheets, cut into 

 square pieces, which should each weigh about ten times as much as the gold 

 under assay. 



The lead is now rolled into the form of a hollow cone ; and into this are in- 

 troduced the assay gold and the quartation silver, when the lead is closed round 

 them and pressed into a ball. 



The furnace having been properly heated, and the cupels placed in it and 



