546 SEC. 13. CHEMISTRY. 



7. Mould for making cupels, each of which holds four assays. 



8. Cupel tongs. 



9. Assay tongs. Master of the Mint. 



The process of gold assaying comprises six distinct operations. 



1st Process. The portion of metal to be assayed is adjusted to an exact 

 weight by cutting and filing (see specimen, Case A.). Such accurately 

 weighed portions of alloy are wrapped, together with definite weights of pure 

 silver (B), in capsules of lead foil (C C 1 ), and placed in order on a tray, 1. 



2nd Process. The packets are transferred to porous cups or " cupels " of 

 phosphate of lime, which are arranged in rows, corresponding to those on the 

 tray, in a mufiler or small oven, 2, which is fixed in a suitable furnace and 

 maintained at a bright red heat. The lead oxidizes and is absorbed by the 

 cupel, together with the copper and other oxidizable metals present, and the 

 silver and gold remain behind in the form of a button (D), which may also 

 contain platinum, iridium, or other metals possessing similar properties. 



3rd Process. These buttons are hammered out into discs (E), which, after 

 being annealed, are rolled into thin strips (F), and these are again annealed 

 and bent into loose coils or " cornets " (G). The annealing takes place in an 

 iron tray, 3. 



4th Process. The cornets are placed in small perforated cups of platinum 

 arranged in a perforated tray of the same metal, 4. The whole is then intro- 

 duced into a platinum boiler, 5, which contains boiling nitric acid of specific 

 gravity 1-25, in which.it is allowed to remain for 15 minutes; it is then 

 transferred to a similar boiler with acid of specific gravity 1 4. The silver is 

 removed by the action of the acid, and the gold remains in a spongy state (H). 



5th Process. The gold sponge (which retains the original form of the 

 cornet) is rendered coherent by annealing at a dull red heat. This is accom- 

 plished by introducing the platinum tray into the muffle. The cornets then 

 assume the appearance of (I). 



6th Process. The final operation consists in weighing the cornets. This 

 is done in a specially constructed " assay " balance, 6, which is capable of 

 indicating about ^oVoth of a grain when loaded with 7 5 grains in each pan. 

 The weights employed bear a decimal relation to the original weight of the 

 piece of metal operated upon. The percentage of gold, therefore, present in 

 the alloy, is at once indicated without calculation. The weighing in the first 

 process was of course conducted on the same or a similar balance. 



Other appliances are 



7. A mould for making cupels of an improved form, in each of which four 

 assays can be worked. 



8. A pair of tongs for introducing such cupels into the muffle. 



9. A pair of tongs for placing the assays in the cupels. 



2771. Appliances used in the Assay of Silver* 



1. Pipette for " standard " salt solution used in silver assaying. 



2. Pipette for " decimal " salt solution. 



Riemsdijk's apparatus for adding drops of decimal solution. 



The Master of the Mint. 



The assay of silver can be conducted by cupellation, an operation similar 

 to that already described in the 2nd process of the assay of gold, the only 

 differences being that no pure silver is added to the assay piece, and the opera- 

 tion terminates when the button (D) has been obtained and weighed. The 

 wet method of Gay Lussac is, however, now usually employed for the assay 

 of silver when the " standard " is approximately known, and the alloy contains 

 not less than 50 per cent, of silver. It consists in precipitating the precious 

 metal from the solutions of a known weight of the alloy to be assayed, the 



