The American process, which is said to be at work under commercial conditions, is not entirely satisfactory in this respect—that it is a mechanical process for the production of cylinders and not of flat sheets, so that the subsidiary processes of splitting and flattening still remain to be carried out as before. In this process an iron ring is lowered into the bath of molten glass through an aperture from above; the glass is allowed to adhere to the ring which is then slowly raised by mechanical means, drawing a cylinder of glass with it. If left to itself, such a cylinder, owing to the effects of surface tension in the glass, would soon contract and break off, but the American invention avoids this action by chilling each bit of the cylinder as soon as it is formed. This is done by the aid of air blasts delivered upon both sides of the glass as it emerges from the bath, and it is claimed that by this means cylinders of any desired length and diameter may be drawn direct from the bath. The obviously great mechanical difficulties connected with these operations have probably been overcome, but not without sacrificing much of the simplicity of the arrangement, and the relative economy of this process as a whole, compared with the hand process, has yet to assert itself.
The inventions of Fourcault, which are at present being developed on the Continent by a syndicate of glass manufacturers, aim at a much more direct process. Here also the glass is drawn direct from the molten bath by the aid of a drawing-iron that is immersed in the glass and then slowly raised, but in this case the piece immersed is simply a straight bar, and the aim is to draw out a flat sheet. In this case the tendency, under surface tension, is to contract the sheet into a thread, and apparently the simple device of chilling the emerging glass is not adequate to prevent this in a satisfactory manner, and subsidiary devices have been added. Those that have been patented include a mechanism of linked metal rods so arranged as to be immersed and drawn out of the glass continuously with the emerging sheet, in such a manner as to support the vertical edges of the glass and so aid in resisting the tendency of the glass to contract laterally. Another device consists in the use of a slit or orifice formed in a large fire-brick that floats on the surface of the glass. Through this orifice the glass is drawn, of the desired thickness and width. The use of this orifice, however, interferes markedly with the perfection of the product, and in fact all the glass produced in this way shows quite plainly a set of longitudinal striations due to the inevitable irregularities in the lips of the drawing slot. Further, it appears to be impracticable to draw thin glass in this way, a thickness of from 2½ to 3 millimetres (about 1/8 inch) being the least that is practicable, on account of the large amount of breakage that occurs with weaker sheets. This process, in its present stage of development, however promising, does not appear to have solved the problem of mechanical manufacture of sheet-glass, since it is just in the thinner, lighter kinds of glass that the advantages of sheet are most pronounced. On the other hand, it is quite possible that this drawing process, or some development arising from it, may shortly supplant the casting process in the production of polished plate-glass, although for the largest sizes of this product also, the difficulty and danger of handling the weights involved may prove a serious obstacle.
Crown Glass.—Although this is a branch of manufacture that is nearly obsolete, it deserves brief notice here, partly because it is still used for the production of special articles, and also because it illustrates some interesting possibilities in the use and manipulation of glass.
The process of blowing crown glass may be briefly described as that of first blowing an approximately spherical hollow ball, then opening this at one side and expanding the glass into a flat disc by the action of centrifugal forces produced by a rapid rotation of the glass in front of a large opening in a special heating furnace. The actual process involves, of course, the preliminary of gathering the proper quantity of glass, much in the manner already described in connection with sheet-glass manufacture. This gathering is then blown out into a hollow spherical vessel. This vessel is now attached to a subsidiary iron rod by means of a small gathering of hot glass, applied at the point opposite the pipe itself, the glass being thus, for a moment, attached to both the pipe and the “pontil” or “punty” (as the rod is called). The pipe is, however, detached by cracking off the neck of the original glass, which now remains attached to the pontil in the shape of an open bowl. This bowl is now re-heated very strongly in front of a special furnace, the open side of the bowl being presented to the fire. The pontil is meanwhile held in a horizontal position and rotated. As the glass softens the rotation spreads it out, until finally the entire mass of glass is formed into a simple flat disc spinning rapidly before the mouth of the furnace. This flat disc or “table” of crown glass is allowed to cool somewhat, is detached from the pontil by a sharp jerk, and is then annealed in a simple kiln in which the glass is stacked, sealed up, and allowed to cool naturally.
It is obvious that by this process no very large sheets of glass can be produced; tables 4 ft. in diameter are already on the large side, and these can only be cut up into much smaller sheets on account of the lump of glass by which the table was originally attached to the pontil, and which remains fixed in the centre of the finished disc. For certain ornamental purposes, where an “antique” appearance is desired, these bullions are valued, but for practical purposes they interfere very seriously with the use of the glass. As a matter of fact, even several inches away from the central bullion itself, crown glass is generally marked with circular wavings, which render it readily recognisable in the windows of older buildings, but which decidedly detract from the perfection of the glass. On the other hand, crown glass is still valued for certain purposes, such as microscope slides and cover glasses, where entire freedom from surface markings, such as those found in sheet glass as a result of the flattening operations, is desirable. While, therefore, the process has merely an historical interest so far as ordinary sheet-glass purposes are concerned, it is still used in special cases.
CHAPTER XI.
COLOURED GLASSES.
In various chapters throughout the foregoing portions of this book we have had occasion to refer to the colour of glass and the causes affecting it, but these references have chiefly been made from the point of view of the production of glasses as nearly colourless as possible under the circumstances. While it is obvious that for the great majority of the purposes for which it is used the absence of all visible coloration is desirable or even essential in the glass employed, there are numerous other uses where a definite coloration is required. Thus we have, as industrial and technical uses of coloured glass, the employment of ruby, green and purple glasses for signalling purposes, as in the signal lamps of our railways, the red tail-lights of motor-cars, or even the red or green sectors of certain harbour lights and lighthouses; again, coloured glasses, ruby, green, and yellow, are extensively employed in connection with photography. Rather less exacting in their demands upon the correctness of the colour employed are the architectural and ornamental uses to which coloured glass is so extensively put in both public and domestic buildings, while, finally, coloured glass is largely the foundation upon which the stained-glass worker builds up his artistic achievements; in another direction, coloured glass is also utilised in the production of ornamental articles and of some table-ware. While it must be admitted that in a great many cases the colour-resources of the glass maker are hopelessly misapplied, yet in really artistic hands few other materials are capable of yielding results of equal beauty.
By the “colour” of a glass is generally understood the tint or colour which is observed when it is viewed, in comparatively thin slices, by transmitted light; the actual colour is thus a property, not so much of the kind or variety of glass as of each individual piece, since thick pieces out of the same melting will show a different tint from that seen in thinner pieces. As we have already pointed out, such glasses as sheet or plate, which appear practically colourless when viewed in the ordinary way, show a very decided green colour when viewed through a considerable thickness. In the same way, a very thin layer of the glass known as “flashing ruby” shows a brilliant red tint, but a thickness of one-sixteenth of an inch is sufficient to render the glass practically opaque, giving it a black appearance by both transmitted and reflected light. Again, cobalt blue glass, when examined with a spectroscope in thin layers, is found to transmit a notable proportion of red rays, but thicker pieces entirely suppress these rays. These phenomena will be readily understood when we recollect that colour in a transparent medium arises from the fact that the medium has different absorbing powers for light of different colours. All transparent substances, and certainly glass, are only partially transparent: all light waves passing through such a substance are gradually absorbed, and the extent to which they are absorbed differs according to the length of these waves. It always happens that for some special wave-lengths the substance has the power of absorbing the energy of the entering waves and converting it into heat-vibrations of its own molecules or atoms. In the most transparent and colourless glasses this process, so far as the waves of ordinary light are concerned, only goes on to a negligibly slight extent; if, however, we extend our view beyond the range of ordinary visible light, and consider the region of shorter waves that lies in the spectrum beyond the violet, we find that ordinary colourless glass becomes strongly absorbent; thus to waves of about half the length of those which produce upon our eyes the impression of yellow light, ordinary glass is as opaque as is a piece of metal to white light. In this wider sense, then, we may fairly say that all glasses are coloured—i.e., all have a power of selective absorption; but in the case of those which are nearly colourless in the ordinary sense, this absorption takes place only for waves which are either decidedly shorter or decidedly longer than those to which our eyes are sensitive. Those glasses which appear coloured in the ordinary sense, on the other hand, owe this property to the fact that the power of absorption for light-waves extends into the region of the visible spectrum; thus a blue or violet glass is practically opaque to red rays, while a red glass is opaque to blue, green or violet rays. This statement may be verified in a striking manner by holding over one another a piece of deep blue or green glass and a similar piece of ruby glass—the combination will be found to be very nearly opaque even when each glass by itself is practically transparent.