Assuming that assaying be defined as “the estimation of the commercially important elements in their ores, alloys, or products,” the following classification of methods, though not exhaustive, may be adopted for convenience.
METHODS OF ASSAYING.
As examples of these methods repeatedly come under the student’s notice, only a few words of explanation will be given here.
I. Dry (OF Fire) Methods.—These methods involve the direct application of heat by one or more of the following operations:—
Fusion and Reduction, Scorification, Cupellation, Roasting, Distillation. Many of these operations are followed by Inquartation and Parting. These operations have not been included in the foregoing tabulation of methods, as in most cases each operation by itself does not constitute a complete method of assaying, but requires the help of one or more of the other operations mentioned. The operations themselves will be described in detail further on.
II. (a). Gravimetric Methods.—These methods are distinguished in that the element sought for is isolated in such a form (generally as a chemical compound) as to be estimated by weighing on the balance (see Quantitative Analysis, Part II.).
II. (b). Electrolytic Methods.—These methods are distinguished in that the element sought for is deposited in a weighable form (generally as the uncombined element) by the agency of the electric current. (See Electrolytic Analysis, Part II.)
II. (c). Volumetric Methods.—These methods are distinguished in that the element sought for is obtained in such a state that it may be estimated by the direct measurement of the volume of a liquid. (See Volumetric Analysis, Part II.)
II. (d). Gasometric Methods.—These methods are distinguished in that the element sought for is obtained (free or combined) in the gaseous state, and is then estimated by measurement of its volume. (See Gasometric Analysis, Part II.)
II. (e). Colorimetric Methods.—These methods are distinguished in that the element sought for is estimated by a combination of volumetric methods (II. c.) and the determination of colour tints. (See Colorimetric Analysis, Part II.)
In recent literature the term ‘assay’ is of very wide application. Certain works, for instance, treat of the ‘assay’ of drugs and other vegetable substances. It seems more fitting that the definition of the term be restricted to the determination of the commercially important elements in their ores, alloys, or similar products, and that the term ‘analysis’ be applied to other determinations. Reference to the literature on the subject will show how varied is the definition of this term — a variation of definition ranging from pure ‘fire work’ to ‘general analytical work,’ involving, with some extremists, the estimation of certain organic compounds in drugs.
In modern metallurgical practice the demands made on the assayer are gradually increasing. Besides being expert in the fire-assays of gold, silver, lead, tin, etc., he must have a thorough knowledge of ‘ wet methods ’ as applied to several of the foregoing elements, and more especially as applied to the estimation of copper, iron, zinc, and other elements. He must be competent in volumetric, gravimetric, and electrolytic methods, and in certain technical methods of gas, water, and other analyses. The demand thus made on the resources of the assayer is gradually becoming more severe, and necessitates a more thorough and a wider grasp of general inorganic chemistry than was hitherto considered essential.
As metallurgical science is ever growing, it is but natural to expect that its demands on its ally, assaying, will likewise increase ; and that the supply should meet the demand, it is absolutely essential that the modern assayer should have a thorough grounding in theoretical and practical chemistry, else he will be a mere rule-of-thumb worker, incapable of the least original work, and therefore of little use to his employers when they most need advice and assistance.