The Manufacture of Window Glass.
The glass with which windows are generally glazed, is called Crown glass. It is formed of different materials in different manufactories. In some instances the materials consist of fine white sand, carbonate of lime, carbonate of soda, and clippings or waste pieces of old glass; while in other cases they consist of white sand, pearl-ash, saltpetre, borax, and arsenic, in certain proportions. On this point we shall not dwell, for almost every manufacturer has a favourite receipt of his own. Whatever substances are employed, they are intimately mixed before being melted. The melting takes place in large crucibles or melting pots, made of a particular kind of clay capable of enduring intense heat. Several such crucibles are placed in a furnace, a little door being situate in the furnace opposite to each crucible. Through this door the materials are introduced and are suffered to melt; and as soon as these become melted, other portions of the materials are added, until the crucible contains a given amount of melted material. A curious effect is then observable. Although most or all of the materials are nearly opaque in their separate states, it is found that when they are all melted together, they form a transparent liquid, which is glass.
It requires about forty-eight hours of intense heat to bring the whole contents of the crucible to a liquid state. During this period, a quantity of dross or impurity, called sandiver or glass gall, collects at the surface, and is carefully removed; it is afterwards sold to refiners of metals, who use it as a flux. The temperature of the furnace is then gradually lowered, by which means the glass loses sufficient heat to assume a pasty consistence, which is more convenient for the workman than if it were perfectly fluid.
The glass maker then stands before the door of the furnace, exposed to an intensity of heat such as few persons can adequately conceive, and dips into the pasty mass of glass the end of a hollow iron tube about five feet long. On withdrawing the tube, a portion of glass is found adhering to it, and this is made to equalize itself round the circumference of the tube by turning the latter rapidly round. The workman then applies his mouth to the other end, and blows through the tube, whereby the pasty mass is made to assume a hollow globular form at the remote end of the tube. This process is continued for some time and with great dexterity, until the globe has attained a considerable diameter and a proportionably small thickness. The globe is then somewhat flattened at the side opposite to the tube by pressing it upon a hard plane surface; and a solid iron rod, called a punt, having a small quantity of melted glass at the end, is applied to the centre of the flattened side opposite to the tube, to which it adheres; the tube is then removed by wetting the glass near the point of union with the tube, leaving a small circular hole. During these processes the glass is repeatedly heated by holding it for a few minutes at the door of the furnace, in order that it may retain the requisite degree of softness.
The punt, with the flattened globe of glass at its end, is then rapidly whirled round in a manner nearly resembling that in which a mop is twirled. By this motion, the globe becomes more and more flattened and extended in diameter, until at length, not being able longer to retain its shape, it bursts open, and spreads out in the form of a flat circular sheet of glass three or four feet in diameter. There is perhaps nothing in the whole range of the mechanical arts more astonishing to a spectator than this process, and there are few that require, from the workman, more of that dexterity of hand which can only be acquired by long practice. The workman continues to whirl the sheet of glass round,—gradually receding from the furnace,—until it is sufficiently set or solidified to retain its form. The punt is then, by a dextrous movement, detached from the centre of the sheet, leaving that bulb which is known as the “bull’s eye,” or the “knot.” The sheet is placed in an annealing oven, the temperature of which is lowered by slow degrees until cold; for it is found that glass is less brittle when it has been allowed to cool gradually than when the cooling has been rapid. Considerable care is required to regulate the temperature of the annealing oven; if the heat be too great the softened glass will bend: if the heat be insufficient the plates are liable to crack, or they prove so brittle that when they come to be used, the glazier will not be able to divide the glass so as to suit his purposes. Indeed, the management of the heat in the manufacture of crown glass requires so much care and skill that few workmen produce an article of the same value, even though working at the same furnace; hence crown glass is known in the market as firsts, seconds, thirds, and fourths; the fourth quality producing less than one-half of the price of the first.
We have not interrupted this description, to refer to engravings; but we may now illustrate it by the following cuts representing the glass in eight different stages of its formation.
1st. The melted glass attached to the tube, and worked on a board.
2nd. The workman blowing through the tube, to expand the glass.
3rd. Whirling it rapidly at the mouth of the furnace.
4th. Transferring it from the hollow tube to the solid punt.
5th and 6th. Successive stages of expansion, by constant and rapid rotation.
7th. Final expansion into a flat circular sheet.
8th. The sheet of glass, held on a kind of fork, being placed into the annealing oven.
When cold, the sheets of glass are cut into two unequal pieces, one of which contains the knot, and are packed with straw in wooden crates, in which they are forwarded to the warehouses, and from thence to the glaziers.
As plate glass is sometimes used for windows, a slight notice of it seems to be necessary in this place, in order that the reader may have a clear idea of the difference between these two descriptions of window glass.
The manufacture of plate glass is confined to very few hands, and great reluctance is manifested by the proprietors to permit visitors to inspect their works. The late Mr. Parkes, however, was permitted to visit the works of the British Plate Glass Company, at Ravenhead, and has recorded his observations in one of his valuable chemical essays, from which the following details are taken.
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.”
“When the plate of glass thus formed has been sufficiently fixed by cooling, it is slipped from the table gradually and carefully into one of the annealing ovens, where it remains in a horizontal position; its treatment differing in this respect from that pursued with crown and broad glass, which stand on edge during the annealing process. As each oven in this manner becomes filled, it is closed up by an iron door, the crevices of which are carefully stopped with mortar or clay, to prevent an access of external air to the oven; and thus to provide as far as possible for the gradual cooling of the plates, the necessity for which has already been sufficiently explained. When the glass has remained during about fifteen days in these ovens, they are opened, and the contents withdrawn.”
The plates have then to undergo the operations of squaring, grinding, and polishing, which need not be described in this place.
The various kinds of glass manufactured in Great Britain amount every year to the enormous quantity of 300,000 cwt., which is valued at two millions sterling.