Page:The New International Encyclopædia 1st ed. v. 02.djvu/177

ASSAYING. ore, and the final measuring of the quantity of a solution necessary lo complete a certain chemical reaction; or, as in the case of colorimetric determinations, by measuring the color imparted to a definite quantity of the liquid by the constituent sought, in comparison with the color imparted to the same quantity of water, or other suitable fluid, by a known quantity of the constituent sought. In fire assaying, generally but one constituent of the ore is determined from the assay of a single sample; in gravimetric analysis, frequently several or all of the constituents are determined from one sample: and in volumetric analysis a separate sample is taken for each determination.

The first operation in making an assay is to select the sample. This work sh(mld be fairly done, no discrimination against any portion of the lot from which the sample is taken being allowable. There are two kinds of sam- pling: (1) ore sampling, and (2) sampling of metallurgical products. There are various methods of taking a sample of ore, it being done both by hand and by machine. One form of hand sampling only will be described here, which is generally termed quartering. This method is as follows: As the ore-cars are un- loaded every tenth shovelful taken indiscrimi- nately is thrown into a wheelbarrow as a sample. When about ten tons have thus been selected, it is shoveled into a conical heap, which is then flattened out into a circular cake and divided diametrically into quarters, as one would cut a pie. Two opposite quarters are then thrown aside, and the other two made into a second cone, which is flattened and quartered as before, two quarters being thrown aside and two shoveled into a third cone. This process is repeated, the ore being occasionally crushed smaller as the amount decreases, until the samples here remain- ing are a mass of powder, weighing from one to three pounds. This is ground until it will pass through a very fine sieve. and is spread out and filled into two sample bottles by taking por- tions here and there on the point of a steel spat- ula. In sampling metallurgical products, drill- ings are taken from the bars of metal, or the hot metal may be ladled directly from the fur- nace.

When an assayer receives a sample for determination, he proceeds first with a preliminary examination by means of microscopical studies and blowpipe tests to familiarize himself with the general character and composition of the substance. The formal analysis is then undertaken. The laboratory apparatus necessary for general assaying is quite extensive, consisting of a large variety of chemicals, balances, crucibles, cupels, furnaces, beakers, etc. A different mode of procedure is adopted in assaying different metals, and for illustration the method of assaying gold and silver by the fire-assay, and of silver by gravimetric analysis, will be described, the reader being referred to special treatises on assaying, metallurgy, and chemistry for details of the processes employed for other metals.

Gold and silver are generally associated together in ores, and such ores are universally determined by fire-assay. The fire-assay consists essentially in the collection of the gold and silver in a button of metallic lead, either by scorification or by fusion. The lead button is then freed from the

adhering slag by hammering on an anvil, and is finally hammered into the form of a cube, when it is ready for cupellation. Both scorifica- tion and fusion consist in melting the ore with lead and certain fluxes; the fluxes used, the size of the ore sample, and the size and shape of the cricible differing in the two cases. The cube of lead, gold, and silver resulting from the scorification or fusion process is subjected to cupellation as follows: The cube is placed in a cupel, a small basin-shaped vessel, made of bone ash, and the cupel is placed in a small furnace of special construction, called a muffle fur- nace, which, as used in the United States assay offices, is heated by gas, but which may be con- structed for use with either coal or coke. Ow- ing to the heat, the lead in the cube is oxidized, part being absorbed by the cupel and part run- ning off in fumes. Just before the last traces of the lead are removed, the melted button exhibits a play of colors, when the heat is increased to dispel the last of the lead, this being indicated by the 'brightening' or 'winking' of the button. The button is then cooled, cleared of adhering dirt, and carefully weighed. It now consists of silver and gold mixed. To determine the amount of each, the button is submitted to the process of parting. In order that the button will part it must contain at least 2½ times as much silver as gold, and should this not be the case sufficient silver is added and fused with the button to secure the proper proportions. The button is now hammered into a flat ribbon or strip and boiled in nitric acid, which removes all the silver and leaves the gold, which is fused and weighed.

Silver bullion may be assayed by cupellation with lead, as explained above, the process being exactly as described for gold and silver ore, except for the preliminary scorification or fusing. The method of assay for silver bullion adopted by the United States mints and assay offices, and in the mints of most foreign countries, is volumetric analysis. Brief- ly described, this consists in dissolving the silver compound in nitric acid and then adding sodium chloride (common salt) solution, which insures the precipitation of the chloride of silver. The salt solution is made of definite strength, and is poured out of a graduated vessel until further precipitation of the silver ceases, when the amount of salt solution used is read off the graduated vessel, and by a simple calculation its equivalent in pure silver is determined.

The assay of gold bullion requires the removal first of the base metals, and second of the contained silver. The base metal is removed by cupellation with lead, as described above, leaving a button of gold and silver. The silver is then removed from the gold by solution in nitric acid, the gold remain- ing beliind in an insoluble state, when it is fused and the button weighed. The mode of assaying gold described cannot always be fol- lowed out in the examination of jewelry and other manufactured articles, as, though only a few grains are required for the assay, yet the removal of such might entail the destruction of the article, and in such circumstances the touch- stone is resorted to. This stone was originally brought from Lydia, in Asia Minor, and consist- ed of a cross-grained quartz saturated with bituminous matter, but black basalt and other stones are now employed for the same purpose.