ASSAYING, the art of testing ores or loys for the purpose of determining the amount of some particular metal that is present in the material analyzed. Assays may be made by °wet° or °dry° methods, and will vary greatly in detail, according to the metal to be termined. The present article will be chiefly devoted to the usual process of estimating gold and silver in the °dry* way. The mode of cedure is substantially the same, whether the assay is made upon ore or upon bullion, except as to the method of obtaining the sample to be examined. If the material proposed for the assay is bullion, or any metallic alloy, the sample for examination is obtained by drilling into the specimen in different places, and mixing the borings. In the case of an ore, the usual method of obtaining a sample for assaying is by °quartering.° If this is done by hand, every tenth shovelful of the ore to be examined is thrown upon the floor, until a conical heap containing perhaps i 10 tons has been accumulated. This heap is next flattened somewhat and divided into four quarters, as nearly equal as possible. Two of the diagonally opposite quarters are thrown back into the main body of ore, and the re maining two quarters are thoroughly mixed, spread out into a second pile and °quartered° again in the same manner. The process is con tinued (the ore being crushed in the mean time as often as appears necessary) until the original sample has been reduced to from one to three pounds, after which it is ground fine and the specimens desired for examination are made up by random selections from the final pulverized product. More commonly, ores are sampled by mechanical or semi-mechanical methods and the sampling is not done by the miner, but by a *sampling mill,* which acts as the agent both of the miner and of the smelt ing works. In such cases the ore is first shipped to the sampling mill, where it is unloaded, weighed, crushed and passed through a chute in which one quarter is mechanically selected and passed into a separate bin. The quarter thus mechanically reserved is next thoroughly mixed, after which the °cutting down° is com menced. This consists in removing the ore from the floor by means of a specially con structed sampling shovel which catches about half of it, and lets the remainder fall into a barrow. The ore retained by the shovel is thrown into three buckets, in rotation, and the contents of the buckets are then coned up in one pile and divided again in the same man ner. The ore is then further crushed, and the process is continued until, finally, three samples, weighing about 10 pounds each, are obtained Part of each of these is sent separately to the assayer, who assays all three. If the results
are not adjudged to be sufficiently accordant, the sampler concludes that the mixing was not well done, and the operations described are re peated. But if the three samples agree fairly well, their average is taken as representing the value of the ore; and on this basis the sampler settles with the miner and afterward with the smelter, thus acting as a middle-man in the sale of the ore.
The specimen of ore received by the assayer is ground fine enough to pass through a 60 mesh or a 100-mesh screen, any ametallics° (or particles of metal that will not pass through the screen being carefully collected and re served for a separate assay). If the ore is new to the assayer, his next step is to examine it microscopically and to apply various prelimi nary tests so that the general nature of the ore may be known before the quantitative work begins. If assays of the same material have been made before, and the only object is to as certain the richness of this particular lot of ore, he may proceed directly to the process of Iscorification,° by which the gold and silver present in the ore are obtained in the form of a metallic button. The scorification process de pends for its success upon the fact that when an ore of gold and silver is strongly heated with metallic lead in the presence of air, the baser metals that are present will oxidize and the lead oxide that is also formed will dissolve the silica (or quartz) that is present; while the gold and silver will not oxidize, but will be left in the metallic state, alloyed with that por tion of the lead which remains unoxidized. To apply this principle, 50 grains or so of the ore are mixed with some 500 grains of granulated lead and placed in a sort of crucible, called a Another charge of 500 grains of lead is spread evenly over the mass, and a few grains of borax are sprinkled upon the top. The crucible and its contents are next heated for about three-quarters of an hour in a muffle to which a small amount of air is admitted, after which the melted mass in the crucible is poured into a mold to or cool. When the mold is cold it will be found to contain a button of metallic lead (in which the gold and silver originally in the ore are concentrated), and also a considerable amount of slag, consisting of oxide of lead, oxide of the base metals that are present in the ore and silicates of lead, derived from the combination of the melted lead oxide with the quartz of the ore. The slag is readily detached from the metallic button by taps with a hammer.