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three hundred and thirty-nine feet long, and one hundred and fifty feet broad, being considerably larger than Westminster Hall.
The melting furnaces range down the centre of this room; the annealing ovens are placed in two rows, one on each side of the foundry, and occupy the greatest proportion of the side walls. Each of these ovens is sixteen feet wide and forty feet deep.
When the melted glass in the cuvette is ready for casting, it is first skimmed with a broad copper sabre, the table is heated and made perfectly clean, the cuvette is wound up to a sufficient height with a crane, and then, by means of another simple piece of mechanism, is swung over the upper end of the casting-table, and being tilted, a torrent of melted glass is suddenly poured over the surface of the table. It is prevented from running over the side by a rim of metal, and when the glass is thoroughly distributed, a large copper roller is passed over it, and we have a beautiful plate of large, clear glass. During this process at least twenty men are employed, and the greatest caution is necessary, as even the opening and shutting of a door, by setting the air in motion, would disturb the surface of the glass, and thus injure the plate. When the plate has been sufficiently cooled, it is slipped from the table into the annealing oven, where it remains fourteen or fifteen days to cool.
The plate has now to be polished. The edge is first cut off with a diamond square, and the plate thoroughly examined ; next, the plate, in union with several others of the same thickness, is cemented on a large stone table with plaster-ofparis; a piece of plate, about one-quarter of the size of the cast plate is now fixed on the base of a pyramid of stone in an
even frame by cement, and, by proper machinery, a motion of rotation is given to it, with great uniformity of friction; while the process is facilitated by the employment in the first instance of water and sand, as in the polishing of marble, and afterwards with smelt, emery, and putty, to give it its final lustre. The last part of the process is performed by hand, and often by females, who slide one plate over another with a little moistened putty of tin between them.
SILVERING PLATE GLASS FOR MIRRORS.
This process consists in laying on the back of the plate a surface of tinfoil, alloyed with mercury.
For this process a smooth table is prepared of freestone or marble, perfectly level, having a ledge round the sides, and a groove or gutter in connexion with it. By machines beneath, this table can be inclined to an angle with the horizon of 12 or 13 degrees. When the table has been thoroughly cleaned and wiped, the workman, taking a sheet of tinfoil adapted to his purpose, spreads it on the table and applies it closely with a brush, which perfectly smooths it from any folds or wrinkles. He next pours over the tinfoil, a small quantity of quicksilver, and spreads it with a roll of woollen stuff, so that the tinfoil is penetrated and apparently dissolved by the mercury.
He next pours on it a quantity of mercury, sufficient to form every where a layer about the thickness of a crown piece; then removing with a linen rag the oxide and other impurities, he applies to the surface of the mercury as it lies thus evenly distributed the edge of a sheet of paper, which he advances about half an inch. He now takes the glass, and laying it flat, passes it over the slip of paper, and slides it gradually along over the whole metallic surface, taking care that neither air nor any coat of oxide can be between the two bodies. When the plate reaches its position, it is fixed by a weight applied to its top; the table is then raised a little at one end, and the loose quicksilver runs off by the gutter and spout into a receptacle to receive it. At the end of five minutes he covers the mirror with a piece of flannel, and loads it with a great many weights, that each part may press uniformly on the metal. These weights are left on it for one or two days, during which time the table is more and more inclined. At the end of this time the glass is removed to a wooden table; the inclination goes on daily till it reaches a vertical position; and, in about a month, the whole of the superfluous mercury is completely drained off, and the mirror is complete.
By the operation of cutting, the surface of glass may be fashioned into almost any ornamental or useful form. It is generally believed that Casper Lehmann, a cutter of steel in the service of Rodolphus I., was the original inventor of this mode of embellishing glass, which has proceeded to great perfection in this country, and which exercises a considerable degree of taste in the arrangement of forms and figures.
In a glass-cutting room attached to a factory, is seen a double bench, extended length-ways, which is divided into several compartments for an equal number of men. In front of each workman is a thin wheel, revolving on a horizontal
axis, and above some of the wheels are vessels containing sand and water, which drop through a small orifice at the bottom, and fall on the edge of the wheel. All the wheels are set in motion by steam power, and each workman has the means of unfixing his wheel, and putting on another of a different kind. These wheels are very numerous, of various sizes, and are made of various substances, such as cast-iron, wrought-iron, Yorkshire stone, and willow wood. The edge of the wheel is that part by which the grinding is effected, and different shapes and thicknesses are given to these edges in order to produce different results.
In the manipulation, the workman takes the glass decanter, or other manufactured article, and holds it against the edge of the revolving wheel, by which the substance of the glass is ground down, and flat or curved surfaces produced. The vessel is held in various positions, according to the pattern required. A tub of water is on the right hand of the workman, with which, from time to time, he washes away the particles of sand or powder which adhere to the glass. The iron wheels, with sand and water, are used for grinding away the substance of the glass; the stone wheels, with clean water, for smoothing the scratched surfaces; and the wooden wheel, with rotten stone and putty-powder, for polishing.
The grinding of glass, or frosting it, in order to lessen its transparency, also forms a branch of the glass-cutter's art.
The objects to which, in the present day, this grinding process is most commonly applied, are lamp shades. The process is simple : the shades are fixed in a lathe, and the workman holds in his hand a piece of wood, which he covers with wet sand, and holds it close to the inner surface during the progress of the revolution of the shade.
There have been various improvements in ornamental glass during these last few years; one, called glass incrustation, was patented by Mr. Pellatt, which consists of an opaque substance embedded in a mass of colourless glass. A medallion, or bas-relief, is moulded in a kind of clay which resists intense heat, and this is enclosed between two masses of soft glass, and when finished produces a very chaste and elegant appearance. There is also a mode of encrusting opaque ornaments or devices on the surface instead of within the substance of the glass. This is effected by moulding the glass to the model, by means of a brace mould.