229. SILVER is a white, brilliant, sonorous, and ductile metal, somewhat more than ten times heavier than water.
It is found in different states. Of these the principal is denominated native silver, from its being nearly in a state of purity. Native silver sometimes occurs in small lumps, sometimes in a crystallized form, and sometimes in leaves, threads, or wire. In many instances the latter are so connected with each other as to resemble the branches of trees, in which case the ore is called dendritic. There are also several ores of silver, in which this metal is combined with lead, antimony, arsenic, sulphur, and other substances.
The silver that is produced from the mines of Potosi, in South America, is of the dendritic kind; and is considered by the Spaniards as the purest that is known. A range of mountains near Potosi, about twenty miles in circumference, is said to be perforated by more than 300 shafts, or openings of mines, and to produce, in the whole, from 30,000 to 40,000 dollars’ worth of ore per week. The annual produce of the silver mines in America has been estimated at near 2,400,000l. sterling.
Silver is also found in several parts of Europe; and, some years ago, there were mines of this metal, worked to a great extent, at Konigsberg in Norway. These were discovered in 1623, and they were found so profitable, that in 1751 forty-one shafts and twelve veins were wrought there; and 3,500 officers, artificers, and labourers, were employed. The perpendicular depth of the principal shaft was more than 750 feet. Specimens of native silver are not uncommon from some of the copper-mines of Cornwall; and, many years ago, a vein of silver ore was, for a short time, wrought with considerable advantage in the parish of Alva, Stirlingshire, Scotland. It is said that from 40,000l. to 50,000l. worth of silver was obtained from it before the repository was exhausted. We are informed that a mass of capillary native silver was found, in veins traversing the blue-coloured limestone of Isla, one of the Western Islands of Scotland. Great quantities of silver are extracted from lead. There was lately melted in one refining house in London 50,000l. worth of this metal, from lead of the Beralston mines in Devonshire.
Different methods are employed, in different countries, to extract silver from its ore. In Mexico and Peru the mineral is pounded, roasted, washed, and then mixed with mercury in vessels filled with water; a mill being employed for the more perfectly agitating and mingling them. By this process the silver combines with the mercury. The alloy thus obtained, after undergoing some further processes, is submitted to the action of heat, by which the mercury passes off in a state of vapour, leaving the silver behind. The silver is then melted and cast into bars or ingots. In other countries, after the earthy matters are cleared from the silver ore by pounding and washing, the remainder is melted with lead: which, by a subsequent process, is separated, and leaves the silver alone and pure.
This metal ranks next in value to gold. Like gold, it is coined into money, and is manufactured into various kinds of utensils, such as goblets, vases, spoons, and dishes, which have the general appellation of silver plate. For all these purposes it is alloyed with copper, which does not affect its whiteness, and is not easily detected, unless it be in too great proportion: the intention of this is to render it harder than it would otherwise be, and thereby the better to adapt it to receive fine and sharp impressions on being cast. Our standard silver is composed of somewhat more than 12¼ parts of pure metal and one part of copper; and the metal of this standard is used, both for silver plate, and in the coinage. The mark or stamp which is given to it at Goldsmiths’ Hall is similar to that which has been explained for sterling gold.
After platina ([226]) and gold ([227]), silver is considered the most unchangeable of all metals. The air does not easily act upon its surface in such manner as to injure it; but, when long exposed to the atmosphere, especially in frequented or smoky places, it acquires a covering or rust of dark brown colour, which, on examination, is found to be what chemists denominate sulphuret of silver. The fumes of sulphur and other inflammable substances blacken silver. Various powders have been contrived with a view to restore to plate its original lustre; but these should be used with caution, as some of them are very injurious.
Silver is nearly as ductile as gold. It may be beaten into leaves so thin that a single grain in weight will cover a space of more than fifty-one inches; and it may be drawn into wire much finer than a human hair, indeed so fine that a single grain of silver has, in this form, been extended nearly to the length of 400 feet. It is this wire gilded that has the name of gold wire; and what is denominated gold lace ([227]) is but flatted silver thread gilt, twisted round silk, and woven.
The plating of copper with silver is a very useful operation, and is thus performed. Plates of silver are bound with iron wire, upon small ingots of copper. The quantity generally allowed is one ounce of silver to twelve ounces of copper. The surface of the plate of silver is made not quite so large as that of the copper; and upon the edges of the copper, which are not covered by the silver, a little borax ([204]) is put. By exposing the whole to a strong heat, the borax melts; and, in melting, contributes to fuse that part of the silver to which it is contiguous, and to attach it, in that state, to the copper. The ingot, with its silver plate, is then rolled between steel rollers moved by machinery, till it is of proper thickness. It is afterwards cut into such sizes and to such shapes as may be required for use. An ounce of silver is thus often rolled out into a surface of three square feet, having its thickness, upon the copper, not more than the three-thousandth part of an inch. Hence we ought not to be surprised at the silver being soon worn from the sharp edges of plated goods. To prevent this, it is customary, with the best articles, to have all the edges, and the parts liable to be worn, formed, to a considerable thickness, of silver.
What is called French plate is made by heating copper, or more frequently, brass, to a certain degree, then applying leaf-silver to the surface, and strongly rubbing it with a burnisher. The durability of this plating depends of course on the number of leaves which are applied on a given surface. For ornaments that are not much used ten leaves may be sufficient; but a hundred will not last long, if the metal be exposed to frequent handling or washing.