VII. METALLURGY. 665 



VII. METALLURGY. 



2772. Old Cupellation Furnace, .supposed to have been the 

 one used by Sir Isaac Newton, when Master of the Mint, in some 

 experiments on the cupellation of silver. The Master of the Mint. 



In general construction it is precisely similar to those now in use, the only 

 difference being that, in modern forms, more perfect means are adopted for 

 regulating the draught. 



2773. Touchstone for the Assay of Gold, formerly used 

 in the Royal Mint. The Master of the Mint. 



The method is based on the fact that the greater the amount of gold con- 

 tained in an alloy, the brighter is the gold yellow colour of a streak drawn 

 with it on a black ground, and the less is it attacked by pure nitric acid or by 

 a " test " acid. In ascertaining the richness of the alloy under examination 

 its streak is compared with marks drawn with alloys whose richness is 

 accurately known. 



277O. Case showing successive processes of Gold 

 Assaying. 



1. Tray on which assays are placed when ready for the furnace. 



2. Muffle containing cupels. 



3. Tray for annealing the buttons after being rolled or flatted. 



4. Tray of platinum cups for " parting " the assays. 



5. Platinum boiler in which the assays are treated with strong 

 nitric acid. 



6. Assay balance, capable of indicating the ^oVirtii of a grain 

 when loaded with 7*5 grains in each pan. 



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



8. Cupel tongs. 



9. Assay tongs. The 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 A. in Case). Such accurately 

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

 silver (B), in capsules of lead foil (C C l \ 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 muffle 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 (^E), 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 1-26, in which it is allowed to remain for 15 minutes; it is then 



