METHODS OF ANALYSIS 55 



to pass 'the desired screen aperture. Assay for gold and silver, 

 then weigh up 50 to 100 grams of the mixed ore and salt, place 

 in a roasting dish and set in a cool muffle.* Gradually raise the 

 temperature to cherry-red. When the fumes cease coming off, 

 remove, cool and weigh. Note the loss of weight, then assay for 

 gold and silver and calculate the loss of precious metals. 



In practice the salt is often added toward the end of the 

 roasting furnaces to prevent gold losses in the earlier stages of 

 roasting. Chloridizing roasting is only necessary for silver ores, 

 and, where the amount of the silver is considerable, it may be 

 preferable to leach the roasted ore first with sodium thiosulphate 

 to remove the silver chloride; wash well, and extract the residue 

 of the gold and some of the silver compounds by cyanide solutions. 

 Except for the fact that gold is soluble in thiosulphate solutions, 

 a combination of the former, with the cyanide process, could be 

 used on chloridized ores; the former to remove the silver, the 

 latter the gold. I have found that thiosulphate would extract, in 

 some cases, 50$ of the gold present in chloridized ores, while sub- 

 sequent cyanide treatment extracted not only a large proportion 

 of the remaining gold, but also, in cases, as much as 10$ of the 

 remaining silver values. The Patera process is cheaper than 

 cyanide in the treatment of these ores, but it never gives a satis- 

 factory gold extraction. Therefore, when the cyanicides can be 

 removed by preliminary water washes, followed by an alkaline 

 wash, the cyanide process will extract both the gold and silver 

 in one operation, greatly shortening and simplifying the treat- 

 ment of the ore. In testing ores, as above outlined, the chemist 

 should clearly keep in mind that the solution of his problem is, 

 that process or combination of processes that will give the highest 

 results at the least expense of operation, and, if possible, the 

 smallest investment of capital in a plant. 



MODIFICATION OF BOTTLE TEST. 



In the foregoing tests it is assumed the ore treated in the 

 bottles is fine enough for assaying. Should this not be the case 

 the extraction can be determined by assaying the solutions by the 



* Chloridizing roasts must be started at a very low temperature and gradually raised to 

 cherry-red at the end, moreover, when the sulphur exceeds i$ a preliminary roast is 

 desirable, then cool, add the salt and complete the roast. Gold is easily volatilized as 

 chloride, but is, to a large extent, recoverable from the dust collected in a proper sys- 

 tem of condensing flues. 



