The Useful Arts Employed in the Construction of Dwelling Houses. Second Edition

by Anonymous

In the preparation of plate glass the materials are selected with greater care than in any other branch of the glass manufacture. The materials employed are sand of the finest and whitest kind, soda, and lime. Manganese and oxide of cobalt are also used for the purpose of destroying colour, which they do by the curious, and at first view, paradoxical property each has of imparting colour. The manganese has the effect of a slight tinge of red, the cobalt of blue; while the sand and alkali produce a slight yellow tinge; and thus these three colours (being those which naturally produce white light) by proper combination in the glass neutralise each other, and the result is an almost perfectly transparent material.

The process of filling the pots and fusing the materials is similar to that already described for crown glass. The crucibles are of two kinds; the larger ones wherein the glass is melted, are called pots, and because these when full of glass are too bulky and heavy to be moved, smaller ones, called cuvettes, are employed. These are kept empty in the furnaces, exposed to the full degree of heat, so that when the glass is ready for casting and is transferred to them, they may not greatly lower its temperature.

The subsequent operations are very well described in an abstract of Mr. Parkes’s essay, given by the writer of the volume on Glass and Porcelain, in the Cabinet Cyclopædia.

“When the glass is thoroughly refined, the cuvette—which must be perfectly clean, and, as already mentioned, of a temperature equal with that of the glass—is filled in the following manner:—A copper ladle, ten to twelve inches in diameter, fixed to an iron handle seven feet long, is plunged into the glass pot, and brought up filled with melted glass, which is transferred to the cuvette; the ladle during this transference is supported upon a strong iron rest, placed under its bottom, and held by two other workmen. This precaution is necessary to prevent the bending and giving way of the red-hot copper under the weight of fluid glass which it contains. When by successive ladlings the cuvette is filled, it is suffered to remain during some hours in the furnace, that the air bubbles formed by this disturbance may have time to rise and disperse; an effect which is ascertained to have ensued by the inspection of samples withdrawn from time to time for the purpose.

“Another essential part of the apparatus consists in flat tables whereon the plates of glass are cast. These tables have perfectly smooth and level metallic surfaces, of suitable dimensions and solidity, supported by masonry. At St. Gobain, and formerly also at Ravenhead, these tables were made of copper; the reason assigned for preferring this metal being, that it does not discolour the hot melted glass, while the use of iron was thought to be accompanied by this disadvantage. These copper tables were very costly, both from the nature of their material, and the labour bestowed in grinding and polishing their surfaces; and as the sudden access of heat that accompanied the pouring over them of such a torrent of melted glass occasioned the metal frequently to crack, the tables were by such an accident rendered useless. The British Plate Glass Company having experienced several disasters of this nature, its directors determined upon making trial of iron; and they accordingly procured a plate to be cast, fifteen feet long, nine feet wide, and six inches thick, which has fully answered the intended purpose—having, during several years of constant use, stood uninjured through all the sudden, and violent alternations of temperature to which it has been exposed. This table is so massive, weighing nearly fourteen tons, that it became necessary to construct a carriage purposely for its conveyance from the iron foundry to the glasshouse. It is supported on castors, for the convenience of readily removing it towards the mouths of the different annealing ovens.

“The foundry at Ravenhead wherein this table is used is said to be the largest room under one roof that has ever yet been erected in this kingdom; it is 339 feet long, 155 feet wide, and proportionately lofty. Westminster Hall, to which the superiority in this respect is so commonly ascribed, is smaller—its length being 300 and its breadth only 100 feet. The melting furnaces, which are ranged down the centre, occupy about one-third of the whole area of this apartment. 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. Their floors being level with the surface of the casting table, the plates of glass may be deposited in them immediately after they are cast, with little difficulty and without delay.

“When the melted glass in the cuvette is found to be in the exact state that experience has pointed out as being most favourable for its flowing readily and equably, this vessel is withdrawn from the furnace by means of a crane, and is placed upon a low carriage, in order to its removal to the casting table, which, as it is previously placed contiguous to the annealing oven that is to be filled, may therefore be at a considerable distance from the melting furnace. Measures are then taken for cleaning the exterior of the crucible, and for carefully removing with a broad copper sabre any scum that may have formed upon the surface of the glass, as the mixture of any of these foreign matters would infallibly spoil the beauty of the plate. These done, the cuvette is wound up to a sufficient height by a crane; and then, by means of another simple piece of mechanism, is swung over the upper end of the casting table; and being thrown into an inclined position, a torrent of melted glass is suddenly poured out on the surface of the table, which must previously have been heated, and wiped perfectly clean.

“The glass is prevented from running off the sides of the table by ribs of metal, one of which is placed along the whole length of each side, their depth being the exact measure which it is desired to give to the thickness of the glass. A similar rib, attached to a cross piece, is temporarily held, during the casting, at the lower end of the table. When the whole contents of the crucible have been delivered, a large hollow copper cylinder, which has been made perfectly true and smooth in a turning lathe, and which extends entirely across the table, resting on the side ribs, is set in motion; and the glass, during its progress, is spread out into a sheet of uniform breadth and thickness. Its length depends upon the quantity of melted glass contained in the cuvette: should this be more than is needed for the formation of a plate having the full dimensions of the table, the metal rib is removed from its lower part, and the surplus glass is received in a vessel of water placed under the extreme end for the purpose.

“Mr. Parkes, in speaking of this operation, remarks—‘The spectacle of such a vast body of melted glass poured at once from an immense crucible, on a metallic table of great magnitude, is truly grand; and the variety of colours which the plate exhibits immediately after the roller has passed over it, renders this an operation far more splendid and interesting than can possibly be described.’

“At least twenty workmen are busily employed during this process of casting. From the time that the cuvette is removed from the furnace, to the completion of the casting by the hardening of the glass, the apartment must be kept as free as possible from disturbance; even the opening and shutting of a door might, by setting the air in motion, disturb the surface of the glass, and thus impair the value of the plate. So soon as it is completely set, the plate is carefully inspected; and should any flaws or bubbles appear upon any part of its surface, it is immediately divided by cutting through them.”