These are exclusively applied on glass, for making mirrors, but the method is almost obsolete, giving place to silver, sec p. 341. The substance employed to make the mercury or quicksilver adhere to the surface of the glass is tin-foil, as thin as paper, and which has a strong attraction for mercury. A drop of mercury combines with the tin-foil, and they become one substance, which adheres pretty firmly to glass. The glass is made perfectly clean on both sides, particularly on that which is to be silvered. If the slightest speck of dirt be allowed to remain on the surface, it will appear very conspicuous when the glass is silvered. The tin-foil is generally made in sheets about 6 ft. long and of various widths, varying from 10 up to 40 in., the diversity of widths being to enable the silverer to cut out small pieces suitable to various-sized glasses. For larger sizes, the foil is generally made to order, and of greater thickness than for smaller glasses. A sheet of tin-foil being unrolled, it is laid down flat, and cut to the same shape as the glass, but an inch larger each way. It is then laid down as smoothly as possible on the silvering stone, which is a very large and carefully-prepared slab of slate, porphyry, or marble, perfectly flat and smooth.
The foil is worked out level and smooth on the silvering stone by means of a smooth wooden roller, which is worked over it in every direction. The silverer pours some mercury into a wooden bowl, and then, by means of an iron ladle, pours the mercury over the whole surface of the foil till every part is covered. The glass plate is then laid upon the liquid mercury; but it is not laid at once flat down on it, being made to slide on, the edge of the glass first coming in contact with the mercurv. As it is slid along, it pushes before it the greater part of the mercury, because the edge of the glass almost scrapes along the foil as it passes, that all air-bubbles and impurities may be pushed off, allowing only a thin film of very pure mercury to remain between the glass and the foil. In this much care and delicacy arc required. It is a matter of some difficulty to clean the glass so perfectly as not to show any marks or streaks after it is silvered. It is often necessary to remove it from the foil two or three times after it has been laid down, to wipe off specks of dirt which are visible when the glass is silvered, however difficult of detection they may previously be; this is especially the case in damp weather.
This renders it necessary that the foils for large glasses, which necessarily require a longer time than small ones to perform the different processes, should be thicker than those for smaller; for such is the attraction between the mercury and the foil, that if a glass, after having been removed for further cleaning, is not speedily replaced on the mercury, the latter will combine with the foils and give it a rottenness which will prevent its adhesion to the glass; the thicker the foil, the less this is likely to occur.
When the glass is properly placed on the tin-foil, and it is ascertained that all specks and air-bubbles are removed, it is covered almost in every part by heavy iron or leaden weights; so that a large glass will have several hundredweight pressing upon it. This pressure is to force out from between the glass and the foil as much mercury as possible, so that the thinnest film only shall remain between them. To effect this more completely, the silvering stone is made to rest on a swivel underneath, by which it can be made either perfectly horizontal, or thrown into an inclined position. While the glass is being laid on the foil, the silvering stone is horizontal, to prevent the mercury from flowing off; but when the superfluous mercury is to be drained off, the stone is made to assume an inclined position, so as to ensure one general direction for the flow of the mercury., A hollow groove runs round the sides of the stone, into which the mercury flows as it is forced out from between the glass and the foil. A pipe, descending from one corner of this trough, conveys the mercury into a bottle placed beneath to receive it.
Although an immense weight of mercury must be poured on the foil for the silvering of a large glass, yet the quantity which actually remains between the glass and the foil is extremely small. The glass, with the weights upon it, is allowed to remain in the inclined position for several hours, or, if the glass is large, it is allowed to remain until the next day, in order that as much as possible of the mercury may be pressed out before the weights are removed. On the removal of the weights, one end of the glass is tilted up and supported by blocks, the other end still remaining on the stone. A piece of foil is then laid on the lowest corner, to draw off the mercury which collects in a little pool at the bottom of the glass. In this state the glass remains from a few hours to 3 or 4 days, according to its size. When as much of the mercury as possible has drained from the glass in this way, the glass is taken up, when it ii found that the two metals have combined together, and in the combined state adhere to the glass, which neither the one nor the other would have done separately. The removal of the glass from the stone is effected in different ways, according to its size.
If it is not too wide for the arm-span of the silverer, he takes it by the two edges, lifts it from the stone, and places it edgeways on a shelf or on the floor of the silvering room, resting its' upper edge against the wall, and allowing one corner to be lower than the rest, so as to facilitate the draining towards that corner. If the glass is long and narrow, two men take it up instead of one, but in the same manner. If, however, the glass is very large, the following mode is sometimes adopted. The draining room is situated beneath the silvering room, and an opening in the floor of the latter is so arranged that a portion of the silvering table can be let down through it, on account of its facility of motion round the swivel. By a gradual turning of the silvering table, the stone and the glass upon it can be brought into a nearly perpendicular position. In this position of the glass, several men in the lower room grasp it by the edges, and place it against the wall of the room, where it is left to drain.
When the plate is thus placed against the wall of the room, it is left to drain for a time, varying from one day to several days, according to its size, in order that any remaining superfluous mercury may leave it, and that the foil may become still better attached to the surface of the glass. When the draining appears to be complete, the glass is ready to be applied to its intended purpose. The above is the process for silvering plate glass. But there is an important reason why common glass, used for cheaper purposes, such as the inferior sort of dressing-glasses, cannot be silvered in this way; for any heavy pressure on such glass breaks it at once, on account of its thinness and crookedness. These common glasses, which are always small in size, are not silvered on a stone, but on a board or flat box. The foil is cut to the requisite size, and laid on the board and covered with mercury, as in the former instance. But instead of sliding the glass on to the mercury, a piece of clean paper is laid on the mercury, and the glass is laid on the paper.
The silverer now, laying one hand pretty firmly on the glass, takes hold of the edge of the paper with the other, and by a quick motion, draws out the paper from between the glass and the foil, and with it the greater part of the mercury, together with air-bubbles and impurities - leaving the glass resting on a thin but brilliant film of mercury; this is a process requiring much manual dexterity. The common glass employed for these purposes is always irregularly bent at its surfaces; it is a general-rule to silver the concave side, when one side is more concave than the other. The crown glass now made is better than that which was produced a few years ago, and although it is always curved, yet the curvature is pretty nearly the same in different tables from the same crate. This circumstance assists the silverer, for each silvered glass acts as a weight to another of the same size. It is usual to silver a great number of the same size at the same time; and as each one is silvered, it is placed fiat down on a shelf, or in a shallow box; and on it the others are successively laid as they are silvered. The concave side of each is silvered, and as the concavity is nearly equal in all, each one helps to press out the superfluous mercury from the one beneath it.
The silvering in common glasses is seldom found to be so perfect as on plate glass, from the impossibility of giving equal pressure in every part.