Specific Gravity of Gold
Since gold prospecting is based in the characteristics and properties of the gold bearing minerals and gangue, it is important to know the know the importance of the specific gravity of gold. Pure gold, on account of its very rich color and non-liability to tarnish in air or water, either by oxidation or from the action of hydrogen sulphide, it is used for a variety of commercial and ornamental purposes. Being very solid it has a high specific gravity, one other metal only being superior; this metal (platinum) being the heaviest known. The specific gravity of gold is 19.2, but by hammering it becomes 19.5.
It has been established that the term specific gravity is the key indication of a gold deposit. The specific gravity of a substance is the proportion it bears in weight to that of an equal bulk of water. Specific gravity is, then, the difference subsisting between the weights of equal bulk, or given dimensions, of liquids or solids. For example, if we take two pieces of gold wire, each of exactly the same size and length, and place them separately in the opposite pans of a pair of scales, the scales will exactly balance, the gold wire being identical in weight, and therefore of the same specific gravity. If we now remove one of the pieces of gold wire and substitute one of silver of exactly the same size and length, by balancing the scales again we shall at once see that the gold wire is much the heavier; and the difference between the two metals shows their relative specific gravity.
It is important to mention that water, in consequence of its lightness in comparison with the metals, and the ease with which it is obtained pure, is adopted as the standard of specific gravity. Taking water as the unit, we write after it the figure 1 or 1.000; these numerals may represent grains, pennyweights, or ounces. The weight of gold is nineteen and a half times that of water of the same bulk, silver ten and a half times heavier, and copper nearly nine times. Perfectly pure gold, or fine gold, as it is more generally called, cannot be obtained easily in consequence of the long chemical refining process which would make it too expensive for manufacturers of jewellery.
The different qualities are expressed in carats; the finest gold, which should be quite free from any alloy, is commonly expressed as 24, but the fine gold of commerce consists only of about 23.1 to 23.5 carats, and this is quite good enough for ordinary practical purposes. Fine gold consists of irregular minute grains of a dull yellow color, but it can be made bright by heating and boiling in hydrochloric acid; however, this is only a matter of taste, and does not make the slightest difference to the working of the metal; in fact, some masters give a preference to the dead color. The melting point of fine gold is 1063° C, and it appears of a greenish shade when fused in the pot; when heated to the above degree it reflects like a mirror. Fluxes will change the color of gold; borax makes the color rather paler, whilst potassium nitrate deepens it.