Page:The American Cyclopædia (1879) Volume II.djvu/39

 ASSAYING 27 to cool and then broken open, a button or globule of lead will be found at the bottom, covered by a dark glassy slag and a layer of salt. This button contains the gold and most of the silver of the original 50 grms. of ore. The oxide of lead, the quartz, and carbonate of potash form a fusible glassy slag which ab- sorbs earthy impurities. The oxide of lead and nitre unite to drive off the sulphur as sul- phurous acid. The coal dust reduces a portion of oxide of lead to a fine spray of metallic lead, which m settling alloys the gold and silver, carrying them to the bottom of the crucible. The button usually contains, besides lead, gold, and silver, some copper, nickel, antimony, and sulphur, if these substances were present in the ore. The process of separating gold and silver from the other metals with which they are alloyed depends on the principle that they cannot be converted into oxides when heated in the air, while the other metals with which they are generally alloyed can be oxidized at a high temperature, especially when a large quantity of lead is present. The lead button is placed in an earthenware dish made of fire clay, called a scorifier (scoria, slag). A wind furnace containing a muffle is used for heating the assay in this and in the succeeding opera- tion. The fuel generally employed is coke or anthracite; charcoal is sometimes used when the other cannot be obtained. The muffle is a flat-bottomed earthen vessel, 8 or 10 in. long, 3 or 4 in. wide, and 2J or 3 in. high, its top arched over, one end open, the other closed ; in fact it is half a cylinder open only at one end. In its roof and sides are little apertures through which the air drawn in at the open end can pass. It is set in the furnace, in the front of which is an opening corresponding to the open end of the muffle. Coals are heaped around and upon it to expose it to the full heat of the furnace. In the scorifier, when heated to a bright red heat, the so-called baser metals are oxidized and form a slag, leaving a small quantity of pure lead alloyed with silver and gold. This alloy while in the molten state is poured into a cooling mould, hammered to free it from slag, and is then ready for the next operation, which is called cupellation, and is performed in a little cup called a cupel. The cupels should be prepared of bone ashes well burnt, ground, and washed, and then shaped into cylindrical forms an inch or so high and 2 in. in diameter, their tops having a shallow depression to hold the metal. These cupels have the property of absorbing the oxides of metals and of holding those that will not oxi- dize ; but as they cannot absorb a greater weight than their own of oxide of lead or litharge, not quite so much of this metal should be put into any one cupel as its own weight. At the mints the assayer is mostly called upon to practise his art upon coin and bullion, alloys of copper, lead, gold, and silver, or containing two or more of these metals. In this case the previous operations of fusion in the crucible and slagging in the scorifier are omitted, and the assay begins at this point. The alloy to be assayed is carefully weighed in a delicate balance. It may be from 2 or 3 grammes, or even less, if already considerably alloyed. A proper quantity of lead, known to contain no silver, is put with it, and the two are placed by means of small tongs in the cupel, which with the muffle has been brought to a full red heat in the furnace. It is convenient to carry on several of these operations at once, and therefore a number of the cupels are usually introduced together on the floor of the muffle. The metals when placed in the hot cupel im- mediately melt and form a bright globule, which spins around and keeps in continual mo- tion. The air drawing in through the muffle oxidizes its surface, and fumes of the oxide of lead are carried off by the draft. At the same time a floating scum of the oxide is constantly flowing down the sides of the globule and sinking into the cupel, while freshly formed oxide replaces it. Any copper that is present is oxidized with the lead and absorbed into the cupel. Thus the operation goes on till it ter- minates by all the lead being oxidized, which is indicated by a sudden brightening up and subsequent darkening of the little globule, and the cessation of the appearance of the fumes and scum of oxide. This little globule, which is pure silver, pure gold, or an alloy of the two metals, shows by its weight the quantity that was in the sample. Care should be taken to avoid too intense heat, as this may volatilize a portion of the silver; and the globule should not be cooled suddenly, as the pure metal ab- sorbs oxygen when melted, and gives it out in cooling. If the change is sudden, some silver is apt to be ejected with the gas. By a little experience and care this operation is made so perfect that no sensible difference should be detected in the weight of two buttons obtained from two assays of equal weights, when tested by a balance that turns with ^ of a milli- gramme. The quantity of lead that should bo added is a matter that can only be determined by experience. Too little lead for the oxi- dation of impuVities prevents the formation of a clean button of silver, free of oxide, and too much lead is apt to carry down with it into the cupel a small quantity of silver. This operation is often performed with the blowpipe, and small cupels adapted to its uses. The weight of the little button is ascertained by the size of the round hole, of a graduated series of such holes in a brass plate, which it fits, the weight of a button of silver or one of gold for each hole having been previously ascertained. In skilful hands this is conducted very expeditionsly, and with considerable accu- racy. It is especially adapted to the testing of argentiferous lead ores, to determine ap- proximately their percentage in silver. The lead also may be quantitatively determined by the reducing process with the blowpipe, that must precede the cupelling. If the button