ASSAY 263 



galena occurs when it is heated with iron, tho products being metallic lead and sul- 

 phide of iron ; tho iron in this instance is called tho reducing agent. When a metallic 

 oxide passes from a higher Co a lower state of oxidation, it is also said to bo re- 

 duced. 



Calcination, Roasting. When a powdered substance is exposed to the oxidising 

 action of heat and air, in order to expel tho sulphur, arsenic, carbonic acid, and other 

 matters, the process is called roasting, or calcination. If it is continued until practi- 

 cally complete, it is said to bo roasted sweet or dead. Accidental softening of tho 

 particles, so that they cohere, is called clotting. 



Scarification. A roasting-fusion process, employed in assaying silver ores. 



Cupdlation. An oxidising-fusion process conducted on a porous vessel called a 

 cupel, used in silver assaying. 



Distillation. "When a metal is volatilised by heat, and afterwards condensed in tho 

 liquid form, the process is called distillation. Tho term sublimation is applied when 

 condensation occurs in tho solid state. 



Liquation. When an ore or other substance, part of which is fusiblo and tho other 

 infusible, or fusiblo at a higher temperature, is exposed to a temperature sufficient to 

 melt one portion and not tho other, tho fused product is said to bo liquated, and the 

 operation is named liquation,. For example, when sulphide of antimony is liquated 

 out from associated vein-stuff. 



Flux. The substance which is added to another to render it fusiblo by the applica- 

 tion of heat, is called a flux. Thus protoxide of lead or litharge is a flux for silica, 

 tho melted product being silicate of protoxide of lead. 



Slag. The product resulting from the fusion of an ore or other substance, which 

 floats on the top of tho metal, regulus, or speiss. Those obtained in assay processes 

 are very variable in composition, such as silicates, boro-silicates, metallic alkaline 

 sulphides, metallic oxides, fluo-silicates, &c. 



Eegulus.A. compound of ono or more metals with sulphur. 



Speiss. A compound of one or more metals with arsenic. 



Button of Metal. Tho metal, or alloy, the result of assay, which is found at tho 

 bottom of tho crucible or ingot mould, after cooling. The small more or less rounded 

 metallic particles are called shots, or globules, or pills. 



For an explanation of the terms iised in the wet way, such as filtration, precipita- 

 tion, &c., tho reader is referred to works on chemical analysis. 



FTJBKACES, IMPLEMENTS, APPARATUS, &c. TJSED IN ASSAYING. 



Furnaces. They aro of three kinds. For n description of an air-furnace, see 

 COPPER ; of a cupellation furnace, see SILVEK ; of a small blast-furnace, see IHON. 



Crucibles. Open-mouthed vessels of clay, iron, or black-lead. Clay crucibles 

 should be well dried before use. When a crucible is charged with the assay mixture, 

 it should not bo more than from 5 to rds full. The following may be noticed : 



Cornish Crucibles. These crucibles aro the best for general assay purposes, as 

 they withstand great and sudden alternations of temperature without cracking. They 

 also resist fairly the corrosive action of fluxes, and although they soften when exposed 

 to extreme temperatures, yet iron assays can bo made in them, provided the precau- 

 tion be taken of lowering tho temperature somewhat, so that they become firm before 

 removal from the furnace. They aro greyish-white, coarse in grain, and composed 

 chiefly of silica. They aro generally sold in nests of two, and sometimes three, each 

 as used by copper assayers. Tho larger size are 3 inches diameter at top, and 3J, 

 inches high, outside measure. They may now be obtained of various shapes and 

 sizes. 



Hessian Crucibles. These crucibles, when genuine, have nearly the samo qualities 

 as Cornish pots, to which they approximate in composition. They are reddish-brown 

 in colour, generally triangular at tho top, and aro sold in nests of six crucibles, which 

 fit successively into each other. 



London Crucibles. These pots vary somewhat in shapo and quality. They are 

 light reddish-brown in colour, close in grain, and have a smooth surface. They 

 resist tho corrosive action of litharge and fluxes very well, but require care in using 

 to prevent cracking. They resist high temperatures well enough to be employed in 

 assaying iron ores. Crucibles known as ' skittle pots,' and mado of tho same materials 

 as London crucibles, aro still used by some assayers. They aro deep, drawn in to- 

 wards the top, to allow of plenty of room for effervescence during fusion, especially 

 when undried materials are used. 



French Crucibles. These crucibles have a greyish-white or cream colour, aro 

 thin, firm, very perfect in form, and smooth externally. They withstand the highest 

 temperature of ordinary furnaces, and resist well tho action of fluxes. They 



