TEST TUBE.
Dry reagents, such as litharge, borax (crystallized), silica, cyanide of potassium, yellow prussiate of potash, argol, charcoal, starch, metallic iron, pure lead, nitre, powdered lime, sulphur, carbonate of ammonia and common salt are necessary. As solvents and precipitants, distilled water, sulphuric, nitric and hydrochloric acids, chloride of sodium, nitrate of silver and sulphuretted hydrogen are also indispensable.
This will seem rather a formidable list, and so, under certain conditions, it may be; indeed, where means of transport is limited, all regular assay work must be postponed until the return to civilization. Assaying is not, however, difficult, being mostly a matter of rule of thumb, and correct results may be arrived at without a deep knowledge of chemistry, although such knowledge will never come amiss.
A preliminary examination will show what the ore probably is. The blow-pipe is especially useful, though to the skilled assayer often unnecessary. The ore is first powdered, and any metallic flakes picked out and tested separately. A fair sample must be selected, otherwise all the work will be thrown away and the result be valueless.
The next step is weighing the ore and the reagents. Moisture is drawn off by heating in a crucible, a low heat being sufficient. Roasting will eliminate sulphur, antimony, arsenic, etc., and must take place in a flat dish, so that the air may have free access. The powder should be stirred frequently.
Reduction is the operation of removing oxygen, and it takes place usually in a crucible or scorifier.
Scorification consists in placing the ore in an open dish with proper reagents, and collecting all the volatile ingredients in the slag. Cupellation, on the other hand, collects them in the bone ash, of which the cupel is composed.
When silver must be separated from gold, it is sometimes convenient to increase its proportion by the addition of some known weight of the inferior metal. After fusing, the globule is placed in nitric acid, and the silver parted from the gold, which may then be weighed. This result subtracted from the weight of the original globule gives the amount of silver.
To test an ore for gold, take a pound of it, crush in mortar and pass through a fine sieve. Take one-fourth ounce Troy of the powder. Place in scorifier with an equal amount of litharge. Cover with borax that has been melted and powdered, and put the scorifier in the muffle of the furnace. A blacksmith's forge might do at a pinch. Heat until the mass has become a fluid, possibly twenty or thirty minutes. Next pour into the scorification mould, and, after the slag has set, remove it with a hammer. Hammer the button into a cube and place it in the cupel, which must first have been thoroughly heated. Heat until all the base metal has been absorbed by the cupel and the button has "brightened," or flashed; when this occurs, remove the cupel to the front of the muffle, cool, and remove the button with pincers. Weigh it, and you have the amount of gold and silver in ¼-ounce Troy. A simple sum in proportion gives the amount in a ton.
All ores containing sulphur, arsenic, antimony, or zinc, should be roasted.