Gold.. The most marked properties of metallic gold are its ductility, malleability, and insolubility in all menstrua, except aqua regia and aqueous chlorine, and its slight affinity for oxygen. Native gold has a specific gravity of 13.3 to 17.7 ; pure gold, about 19.3; its greatest density is 19.5. Its fusing point is 2016° Fahr. It is characterized by its yellow color, its insolubility in nitric acid, and ready solution in nitromuriatic acid (aqua regia), forming a yellow liquid that stains the skin purple.

3190. Assay of Gold by the Use of Touch-Stones

3190.    Assay of Gold by the Use of Touch-Stones. "When it is desired to ascertain the fineness of small quantities of gold, as in jewelry, etc., touch-needles and stones are employed. The former are made in sets, containing gold of different fineness and differently alloyed with copper and silver. Pieces of black pottery form excellent touch-stones. The mode of using them is to mark the stone with the sample under examination, and to compare its appearance, hard ness, etc., with that produced by one or more of the needles. "When the two are similar, the quality is considered to be the same. They are then further examined by moistening the stroke with aquafortis when red hot, when the appearances resulting from oxida tion, etc., differ according to the nature and quantity of the alloy.

3191. Assay of Gold by Cupellation

3191.    Assay of Gold by Cupellation. This process is divided into five operations.

Cupellation. Either 6 or 12 grains of the alloy is the weight usually taken for the assay, to which is added 16 parts of lead for every 1 part of copper that it is presumed to contain, though considerably more lead may be used when the sample does not contain any silver; but if the reverse be the case, an excess of lead would tend to the loss of the latter metal, which ought not to be separated until the operation of parting. When silver is present an additional allowance of lead, equal to 1/10 of its weight, is made on that account. "When, however, the quantity of silver is small, or is not required to be estimated, it becomes of little consequence what weight of lead is employed, so long as enough be used to carry off the base metals, at the same time that the quantity is not too large for the cupel. The sample is then submitted to cupellation. This process does not require so much care for gold as silver, as none of this metal is absorbed by the cupel, or lost by evaporation, and it will safely bear the highest heat of the furnace without injury. In other respects the operation may be conducted in exactly the same manner as for silver. (See No. 3206 (Assay of Silver by Cupellation).)

Quartation. After gold has passed the cupel, it may still retain either of the other perfect metals, particularly silver. To remove the latter it undergoes the operations of quartation and parting. Quartation is performed by adding 3 parts of silver to one of the cupelled sample, and fusing them together, by which the gold is reduced to one fourth of the mass, or even less; hence the name. In this state nitric acid will dissolve out the silver, which brings us to the next operation. In many cases the operation of quartation is performed conjointly with that of cupellation.

Parting. The alloy of gold and silver formed by quartation is next hammered or rolled out into a thin strip or leaf, curled up into a spiral form, and submitted to the action of nitric acid, specific gravity 1.3, diluted with half its weight of water; this being poured off, another quantity of acid, of about 1.26, and undiluted, may be employed. In each case the acid should be boiled upon the alloy for about a quarter of an hour. In the first case the quantity of fluid should be about 21/2 ounces, and in the second 11/2 ounces. The second part of the operation of parting is called the reprise. If the acid be used too strong it leaves the gold in a state of powder, otherwise the metal preserves its form throughout the process of parting. It is next carefully collected, washed, and dried.

Annealing. The sample of pure gold has now only to be annealed, which is done by putting it into a small porous crucible, and heating it to redness in the muffle.

Weighing. The pure gold is next accurately weighed. This weight doubled (if 12 grains are under assay), or quadrupled (if 6 grains), gives the number of carats fine of the alloy examined, without calculation. The loss of weight by cupellation gives the amount of copper in the sample; that after parting, the amount of silver, deducting, of course, the weight of silver used in the process, which is called the witness. When the sample contains but very little gold, the dry method of assaying cannot be depended on, and chemical analysis must be had recourse to. (Cooley.)