gold is commonly found in a quartzose gangue, and nearly always associated with iron and copper pyrites, mispickel or arsenical pyrites, blende, galena, many anthnonial
minerals, and nearly all the primitive rocks.The only artificial alloys of gold which will be specially noticed here are the standard gold of this realm, and alloys of gold with silver or copper used for jewellery. The stan dard of alloys of gold is expressed in fractions of unity. It is assumed that there are 24 carats in unity, and ffnds in the carat. Standard gold, in the 24 carats, contains 22 carats of pure or fine gold, as it is called, and two carats of alloying metal, either silver or copper, or a mixture of the two. Standard gold is, therefore, called 22-carat gold. In a similar way articles of jewellery are denominated 12- carat, IG-carat, 18-carat, <fcc., according to the propor tion of fine gold alloyed with the inferior metal. An ordi nary assay report of gold expresses the variation from the standard, and not the fine metal contained in it, and it is, therefore, marked as either better or worse than standard. The standard of gold being 22 carats fine and 2 alloy, an ingot of gold found to contain only 21 carats pure gold would be reported worse 1 carat. If it contained 23½ carats, it would then be reported better 1½ carat.
The processes by which gold is generally assayed are cupellation, when the alloy consists of copper, and part ing, when the alloy consists of silver. Generally speaking, both operations are necessary. We will describe them as they would be performed in practice. When the stan dard of the alloy to be examined is not approximative^ known, a preliminary assay must be made to ascertain the quantity of lead necessary to fuse with the gold alloy. But in most cases this is unnecessary, as, from the circum stances of the case, the standard of the alloy is generally known within sufficiently close limits.
The process of cupellation is briefly as follows : The gold alloy is fused with a quantity of lead, and a little silver if silver is already present. The resulting alloy, which is called the lead button, is then submitted to fusion on a very porous support, made of bone-ash, and called a cupel. The fusion being effected in a current of air, the lead oxidises. The heat is sufficient to keep the resulting oxide of lead fused, and the porous cupel has the property of absorbing melted oxide of lead without taking up any of the metallic globule, exactly in the same way that blotting-paper will absorb water whilst it will not touch a globule of mercury. The heat being continued, and the current of air always passing over the surface of the melted lead button, and the oxide of lead or litharge being sucked up by the cupel as fast as it is formed, the metallic globule rapidly diminishes in size until at last all the lead has been got rid of. Now, if this were the only action, little good would have been gained, for we should simply have put lead into the gold alloy, and then taken it out again ; but another action goes on whilst the lead is oxidising in the current of air. Other metals, except the silver and gold, also oxidise, and are carried by the melted litharge into the cupel. If the lead is therefore rightly propor tioned to the standard of alloy, the resulting button will consist of only gold and silver, and these are separated by the operation of parting, which consists in boiling the alloy (after rolling it to a thin plate) in strong nitric acid, which dissolves the silver and leaves the gold as a coherent sponge. To effect this parting properly, the proportion of silver to gold should be as 3 to 1. The operation by which the alloy is brought to this standard is termed quartatlon or inquartation, and consists in fusing the alloys in a cupel with lead and the quantity of fine silver or fine gold neces sary to bring it to the desired composition.
What is called the Trial of the Pyx is an ancient ceremony which takes place about once in every three years, at which the standard coin of the realm is carefully assayed. For a description of this see the article Coinage.
It is unnecessary in the present work to describe the various delicate operations which we have briefly alluded to above, but we will describe the implements and furnaces which have been introduced and adopted in the Royal Mint by Mr H. W. Field, the late resident assay-master of that establishment.
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Fig. 1.—Front elevation of Furnace.
Fig. 1 is the front elevation of the furnace ; a, a view of the front iron roller on which it rests ; b, the ash-pit j cc are the dampers moving horizontally from side to side towards each other, meeting exactly in the centre; d, the muffle door by which the as says are introduced ; ee, the door slides. So far, the furnace is similar to that formerly in use, except that the bars on which the muffle stands run from front to back, and are movable, ren dering the removal of the brick-work unneces sary. By this means the muffle stand is easily introduced, and, having steady pins on the under side, it is raised about an inch above the bars. The furnace measures 2 feet 10 inches in height, 1 foot 7 inches in width, and 1 foot 1 1 inches in depth. Instead of the furnace, as formerly, be ing fed at the top, the fuel is charged by the door/t, which also affords the means of regulating the draught, and of throwing a current of air through the muffle by the door d. This door has a bar k, traversing about two-thirds of it, running easily from the top towards the bottom within ii, with a ketch m, on each side to keep it close. These are made on an incline, and about 3i inches long, so as to allow the travers ing bar to slide freely when the door is not required to be closed. In this manner the door may be opened from a quarter of an inch to the extent of three inches. This feeding and regulating door is fixed by hinges II to the front part of the iron frame covering the brick lining on the top of the furnace. On this frame rests the square dome r, the front of which, u<, is removable by two handles nn ; and by taking out the two thumb-screws oo, the door and part of the frame come away, leaving a large opening, so as to enable the furnace to be cleaned, the muffle repaired, (ire. The furnace should be placed in a recess, under a chimney, with a movable iron ceiling t, about 1 foot above the dome, fitting close in every part, so that the draught of air may pass through the furnace. A door, or flap, x, is attached to the iron ceiling by a hinge opening on the side of the recess, with means to fix it at any point required, so that the current of air may be regulated by the operator ; s, a swivel door affords another mode for damping the furnace.
