Assaying

quantitative analysis for the determination of metals (mainly noble metals) in ores and various metallurgical products, by-products, alloys, and manufactured articles.

Assaying methods were known in 2000 B.C. in Egypt and later in Babylonia, Greece, and Rome. They were used in conjunction with the extraction and commercial circulation of gold. In Russia, the reforms introduced by Peter I—reforms on expanding trade, increasing the minting of gold and silver coins, and developing gold and silver mining—aided the development of assaying. Assaying was perfected by M. V. Lomonosov and D. I. Mendeleev, as well as by the Soviet scientists N. K. Pshenitsyn, O. E. Zviagintsev, V. Ia. Mostovich, V. G. Ageenkov, and I. N. Plaksin.

Assaying methods are precise and are used to determine, for example, the gold content in ores up to 0.1-0.05 g/ton. This makes it possible to use assaying as a means of control and arbitration. Assaying facilitates the calculation of the content of noble metal in mineral deposits and the control of the technology of extracting, refining, and electroplating. It also facilitates the recording of noble-metal consumption and the production of jewelry and domestic goods of a given purity.

Assaying is carried out by pyrometallurgical and chemical methods. The former include the crucible process, scorification, and cupellation. The process is used for analyzing ores and their conversional products with a noble-metal content ranging from tenths of a gram to tens (and more) of grams per ton of initial product; samples weighing 25‣100 g are required for analysis. Scorification serves as an additional operation during the analysis of ores and their conversional products. It is also an independent method of analyzing ore products rich in noble metal (hundreds of grams of gold or platinum and kilograms of silver per ton of product); samples weighing less than 5 g are required for analysis. Cupellation in a muffle is an auxiliary process employed during the crucible and scorification processes. It is also an independent method of analyzing alloys, ingots, semifinished products, and manufactured articles.

Chemical assaying methods are used for all materials containing noble metal, except those products designated for the crucible process. Sometimes chemical methods are used in combination with pyrometallurgical techniques. A popular method of chemical assaying is assaying on a touchstone, when an approximate determination of the purity of jewelry and domestic goods is made without damaging them. A uniform line, 10‣12 mm long and 2‣3 mm wide, is scratched on the surface of the touchstone using the article to be tested. At a distance of 0.5‣1 mm, an identical line is drawn with a scratch needle of a known fineness that is suited to the fineness of the tested article; if gold is being assayed, the needle should be of the same color. The scratches are then moistened with the proper assay reagent, and the effect is observed over a period of several seconds. Upon completion of the reaction, the reagent is carefully removed from the stone with filter paper, and the effect of the reagent on the scratches is observed. The purity of the article made of noble metal is determined by the intensity of the stain’s color on the scratch made by the needle.