262 ASSAY 



substance produced when gum, sugar, starch, or broad aro roasted until they turn 

 brown. See CARAMEL. 



ASSAY. (Etsai, Fr., Probe, Ger.) ASSAYING. (Docimaxie, Fr., Probirkunst, 

 Ger.) Assaying is tho art of estimating the proportion of metals in minerals, ores, 

 metallurgical products, and various alloys used for monetary and other purposes. 

 It is generally regarded as a branch of metallurgy, and should occupy an important 

 place in metallurgical instruction. A knowledge of chemistry is essential to a correct 

 understanding of the nature of tho operations, and of the reactions which occur in 

 the various assay-processes. To the practical metallurgist it is essential that the ores 

 under treatment at smelting works should be submitted to assay, to afford a clear 

 indication of tho quantity of metal obtainable, and that tho products of furnaces, &c., 

 obtained at various stages of the operations should also bo tested, so as to exercise a 

 salutary control over the working of the various processes. To the miner, also, it is 

 indispensable to have the various parcels of ore submitted to assay, to ascertain their 

 economical value. Assaying may likewise be advantageously employed to test tho 

 working capabilities of dressing machinery, or to control the dressing processes. It 

 is also of the utmost importance that the alloys employed for monetary and other 

 purposes should be submitted to careful and exact' assay, to ascertain that they aro 

 of the correct standard, and to prevent fraud. To the colonist and explorer, a know- 

 ledge of assaying would often prove of great service, in enabling them to. ascer- 

 tain the nature and economical value of metalliferous minerals, or ores which they 

 may discover. Assaying has been practised for many generations; formerly tho 

 assays wero made by the agency of fire, and were, more or less, but miniature 

 smelting processes. Since tho study of chemistry has advanced, and within com- 

 paratively recent times, other methods by the use of liquid rengents have been intro- 

 duced. The various methods of assaying, exclusive of those made by the blowpipe, 

 are, therefore, conveniently divided into (1) assays by the dry way, (2) assays by 

 the wet way. 



1. Assays by the Dry Way. These assays are made by the agency of fire, by 

 fusion of the ores or other substances, with or without the addition of appropriate 

 fluxes and reagents in crucibles or other suitable vessels of iron, clay, or black-lead ; 

 the requisite temperature being obtained in an air-furnace, muffle-furnace, or small 

 blast-furnace. 



2. Assays by the Wet Way. These assays aro niado by the agency of water, 

 or liquid reagents. 



1. By volumetric methods. 



2. By analysis. 



3. By mechanical means. 



1. The Volumetric Methods of assay are of comparatively recent introduction. 

 These assays are made by the use of standard solutions of known strength, supplied 

 from graduated vessels, as burettes, alkalimeters, or pipettes ; unfortunately only 

 a few of the numerous volumetric methods which are known can be applied practi- 

 cally for the purposes of assaying, as they are not sufficiently reliable, consume too 

 much time, or do not work with the requisite degree of accuracy. 



2. Analysis. Tho methods of assay by analysis are only used when other methods 

 are not available, or in special cases. 



3. Mechanical Methods. These assays consist in the separation, by the agency of 

 water, of the substances associated or occurring with metalliferous minerals and 

 ores, and are performed by hand-washing on a vanning-shovet, or in a washing-boivl 

 of wood or metal. A known quantity of the pulverised ore is put on a shovel, or in 

 a bowl with water, and by certain rotary and other movements, to bo acquired only 

 by practice, the lighter substances aro separated from those of greater specific gravity. 

 The vanning-shovel is usually employed for tin ores, and the washing-bowl, or tin 

 dish, for gold ores. Washing may also be practised to ascertain the nature and pro- 

 portion of certain minerals, as galena, iron pyrites, &c. present in samples of ores. 

 It may likewise be had recourse to for ascertaining the proportion of metallic grains 

 present in slags or other furnace-products, and which may be afterwards extracted 

 from them by stamping and washing on the largo scale. 



EXPLANATION OP TEEMS USED IN ASSAYING (BY THE DEY METHOD). 



Fusion. Rendering fluid by tho aid of heat. The term fritting, or semi-fusion, is 

 applied to materials which have softened sufficiently by heat to become cemented 

 together. 



Reduction. When a metal is separated from its compounds by another agent, 

 it is said to bo reduced, and tho operation is called reduction. Thus tho reduction of 



