818 SILVEE ASSAYING 



by a layer of borax, and fused in the assay-furnace, taking care to remove it from tho 

 fire as soon as a perfectly liquid slag has been obtained, since tho unreduced litharge 

 might otherwise cut through the crucible and spoil the assay. When cold, the pot is 

 broken, and tho button of lead cupelled in the ordinary -way, or the liquid-products 

 are poured out into a mould (Jig. 534, see COPPER), and when cold, the slag is de- 

 tached from the lead. 



In this, and all other similar experiments, it is necessary to ascertain the proportion 

 of silver contained in the lead obtained from the litharge used, in order to make the 

 requisite deduction from the results obtained. When other minerals than oxides are 

 to be examined, the addition of charcoal becomes in many cases .unnecessary, since 

 litharge readily attacks all the sulphides, arsenio-sulphides, &c,, and oxidises many of 

 their constituents, whilst a proportionate quantity of metallic lead is set free. The 

 slags thus formed contain the excess of litharge, and the button of alloy obtained is 

 cupelled. The proportion of oxide of lead to be added to ores of this description 

 varies in accordance with the amounts of oxidisable substances present ; but it must 

 always be added in excess in order to prevent any chance of loss of silver from the 

 action of sulphides in the slags. Tho only objection to this method of assay is the 

 large quantity of lead produced for cupellation, since iron pyrites .afford by the re- 

 duction of the litharge 8 parts of lead, whilst sulphide of antimony and grey copper 

 ore yield from 6 to 7 parts. This inconvenience may be obviated by the previous 

 oxidation of the mineral, either by roasting, or by the aid of nitre, by the judicious 

 employment of which, buttons of almost any required weight may be obtained. Should 

 this reagent be employed in excess, it would cause the oxidation of all the metallic 

 and combustible substances present, not even excepting the silver. When, however, 

 the mixture contains at the same time a large excess of litharge, and the quantity of 

 nitre added is not sufficient to decompose the whole of the sulphides, a reaction takes 

 place between the undecomposed sulphide and the oxide. of load added, .which gives 

 rise to the formation of metallic lead, and this combining with the silver, affords a 

 button of alloy, which maybe treated by cupellation. ( The t quantity of nitre to be 

 used for this purpose will depend on the nature and richness of the ores under exami- 

 nation ; but it must be remembered that 2i parts of nitre will decompose and com- 

 pletely oxidise pure iron pyrites, whilst 1 and jfrds of its weight are in the case of 

 sulphide of antimony and galena respectively sufficient. In cases where the excess of 

 sulphur present is very great, a partial' roasting of the ore is preferable to the addition 

 of a large quantity of nitre. Instead 6f operating according to any of the processes 

 above described, it is sometimes found advantageous to' expel the whole of the arsenic 

 and sulphur, by a careful roasting, and then to fuse the residue with a mixture of litharge, 

 carbonate of soda, and borax, taking care -to add a sufficient 'amount of some reducing 

 flux to obtain a button of convenient size. When, in 'addition to silver, the mineral 

 operated on contains gold, the button obtained by 'cupellation will consist of a mixture 

 of these metals, which may be separated by the aid of nitric acid. See GOLD ASSAYING. 

 b. Scorification Method. This process is conducted in a muffle-furnace. A cup- 

 ehaped vessel or scorifier of fire-clay is employed, which varies in size according to the 

 1816 a quantity of ore operated on. The scorifier represented in 



, jig. 1816 is 2 inches in diameter at the top, and the in- 



ternal cavity is J of an inch in depth. Tongs of peculiar 

 construction, and varying from 2 feet 3 inches to 3 feet 

 in length, are used for lifting the scorifiers, tho lower leg 

 being divided near the end into a two-pronged fork, in 

 order to grasp the lower part of the vessel, while the xipper 

 leg is made straight, and holds it firmly on the upper sur- 

 ^ ace ' Scorification is a roasting-fusion process. Tho 

 sulphur and other oxidisable substances present are roasted 

 or decomposed by the oxide of lead produced, and atmospheric air. When these are 

 decomposed, the oxide of lead produces a fusible slag with the silica and other matters 

 present in the ore. The silver alloys with tho lead retained in the lower part of tho 

 ecorifier. The process may bo conducted as follows : 50 or 100 grs. of the finely- 

 divided ore are weighed out and transferred to the scorifier. Tho ore is now mixed with 

 about half of the finely-granulated lead required for the assay, and tho other portion 

 is afterwards placed upon tho top. From 10 to 20 times its weight of lead is usrtl, 

 according to the nature of the vein-stuff, and minerals present ; a small quantity of borax 

 is also added. The scorifier is now placed in a muffle, and the heat increased until 

 fusion occurs. The door of the muffle is now partly removed, in order to admit moro 

 air, and allow the oxidation of the lead and other substances to proceed. When the 

 surface of the lead is covered with slag, tho scorifier is removed, and its contents poured 

 into an iron mould, swfig. 534 (COFFEE). When cold, the slag is detached, the lead sub- 

 mitted to cupellation, and the button of silver weighed. The button of load should bo 



