ASSAYING. (FR., Docimasia ; GER., PrObirkUnSt.)—The term "assaying," in its strictest sense, means the process by which the precious metals are separated from their alloys, or ex tracted from their ores, and subsequently purified. Of late, however, the word has come to be used in a much wider sense, namely, the examination of ores and minerals, by any method whatever, with a. view to ascertaining, first, what metal or metals they contain ; and secondly, with as much accuracy as possible, how much of these metals, We wish, at the outset, to make a very broad distinction between the province of the assayer aud that of the analytical chemist. The assay of a mineral is a mere mechanical process, depending to a very large extent upon manipula tion and practice, by which the operator can quickly and easily estimate the character and value of the mineral brought under his observation ; whereas in order to conduct an exact chemical analysis, it is requisite that the operator should have much elaborate and costly apparatus, as well as a complete knowledge of chemistry, at his disposal. Having made this distinction, we shall endeavour as far as possible to avoid the use of all technical expressions which are familiar only to the pro fessional analyst, aud to confine ourselves to such information as will be of use to tho miner, tho mechanic, or the explorer, and to those methods which admit of being easily understood and employed with only a slight previous knowledge of chemistry, and which do not require any appa ratus that cannot be easily made or readily procured by the assayer. Although the assayer who knows little or nothing of chemistry may attain a degree of accuracy hardly surpassed by an experienced analyst, at the same time we must remind the reader that so-called "rule-of-thumb" methods are not by any means to be recommended in preference to the more accurate methods of chemical analysis, and we must impress upon all who are engaged in mining or metallurgical operations the inestimable advantages to he derived from a thorough knowledge of chemical science, and the help afforded by understanding the chemical nature of the ores dealt with, and the various changes undergone by these in the process of assaying.
The assay of a mineral may be divided iuto two parts : the qualitative assay, by which the composition of the mineral, or the nature of the metals which it contains, is ascertained ; and the quantitative assay, by which we determine the amount of valuable matter contained therein. In dealing with the former part of the subject, we shall describe the methods usually employed to estimate the character of the most commonly occurring ores. A practised assayer can frequently judge of the nature of different minerals by a mere inspection of them—by the colour, weight, fracture, &o., which they possess or exhibit ; but as it takes years of experience to do this with certainty, it is indispensable that those who have not gained this experience should possess some rough but sure means of discovering whether any ore brought under their notice is valuable for smelting purposes, and if so, how much metal they may expect to extract from it.
The system of examining minerals by means of what is termed "blowpipe analysis " is extremely valuable to the mineralogist, since it is readily performed aud gives sufficiently reliable results. Though not, of course, so accurate as the system known as " analysis by wet methods," blowpipe analysis occupies far less time, aud necessitates the use of very few reagents. Indeed, the operator
can pack up all that he is likely to need in a small box or case, to be carried iu his pocket, so that he may examine on the spot any mineral met with iu the course of his explorations.
Blowpipe .Analysis.—A brief description of the blowpipe, and its auxiliary apparatus and reagents, will here be of use. The best form of blowpipe is that shown iu Fig. 277. It consists of a tube made of brass or German silver, and having a horn mouthpiece ; a second tube fitted with a platinum point is fixed into the end at right angles. The air-chamber serves to partially regulate the blast, and to contain the condensed moisture, thereby preventing the narrow air-passage from becoming obstructed. The whole is made to unscrew into three pieces, for the purpose of cleaning. In using the blowpipe, the lips are pressed against the mouthpiece, aud the stem is firmly held ; the cheeks are inflated with air, which is expelled from the mouth through the pipe by contracting the muscles of the cheeks, care being taken to inhale only through the nostrils ; by this means a continuous blast is kept up. When gas can be had, the best flame for the blowpipe is that of the Bunsen lamp, Figs. 27s and 279. In this lamp, the gas issues from a small central burner a, and passing 'subunit up the tube e, draws air np with it through the holes d; the blowpipe flame, Fig. 279, may be divided into two distiuct parts: the oxidizing flame a, where there is excess of oxygen, and the reducing flame b, where there is excess of carbon. The flame of an ordinary small oil lamp is, however, sufficient for ordinary purposes ; this lamp should have a flat wick, and refined rapeseed or olive oil should be used. When a flame is propelled by a current of air blown into or upon it, the flame produced may be divided into two parts, possessing respectively the properties of reduction and oxidation. The reducing flame is produced by a weak current of air acting upon the flame of a lamp or candle; the carbon contained iu the flame is thus brought into contact with the substance to be examined, which is reduced. The oxidizing flame is formed by blowing strongly into the interior of the flame. Combustion is thus thoroughly effected ; and if a small piece of an oxidizable substance be held at the point of the flame, the former speedily acquires an intense heat, and combines freely with the oxygen of the surrounding air. The substance to ho analyzed should, when exposed to the flame of the blowpipe, be supported on some infusible, and in many cases, incombustible, material. Other articles commonly used in assaying are crucible tongs, agate mortars, platinum wire and fell, forceps, test-tubes, &c.; and for the preliminary examination of minerals, a strong pocket lens will often be found of use. The reagents most commonly employed are the following ;—Carbonate of soda, which must be anhydrous and perfectly pure, is used to reduce metallic oxides and sulphides, and to flux silicates ; borax and microcosmic salt are largely used for dissolving metallic) oxides, in a manner to be described hereafter ; nitrate and cyanide of potash, nitrate of cobalt, and strong solutions of hydrochloric, sulphuric, and nitric acids are also used by the assayer, together with a variety of other reagents. These should all be kept in stoppered bottles.