In passing from a liquid to a solid state there intervenes a viscous stage when the glass may be gathered at the end of an iron rod; the ductile, tenacious mass may now be drawn out into long threads, whose length and fineness are only limited by the difficulty of maintaining the requisite temperature. Again, if the rod upon which the mass is gathered is hollow, the glass may be blown out into a vesicle or bulb, the starting-point from which an endless variety of objects, bottles, cups, tubes, or even flat sheets of glass, may be subsequently formed. Until advantage was taken of this remarkable property of glass—its capability, I mean, of being blown out into a hollow vesicle when in a viscid condition—the art of the glass-maker was in a primitive stage. We may compare the glass prepared without the aid of the blowing-tube—that of the ancient Egyptians, for instance—to the pottery made by hand before the invention of the potter’s wheel.
In dealing with the practical side of our subject—the materials from which glass is made, how these materials are first fritted and then fused together, and how the fused mass is subsequently dealt with—the best plan will be to approach the questions in each case from the point of view of the time and country. But as, on the one hand, for classical times, our sources of information for these practical details are but scanty, and as, on the other, I am not concerned with the industrial developments of the nineteenth century, it will be well to postpone any fuller treatment of such matters until I come to speak of the glass of late Mediæval and Renaissance times. I shall then be able to make use of contemporary accounts which will throw light on the processes of manufacture.
A few preliminary notes on the chemical and physical properties of glass may, however, not be out of place.
Glass, Merret tells us, is ‘a concrete of salt and sand or stones.’ This, in modern scientific language, we should express by saying that it is a combination of silica with an alkali. But these substances alone are not enough. You cannot make a glass fit for practical use from a pure quartz sand with the addition of nothing else than a salt of potash or soda. Such a glass—a simple alkaline silicate—would indeed be transparent, but it would be difficult to work and very fragile. In all cases there is need of a second base, and this, to speak generally, should be either lime or oxide of lead. The latter base we may for the present neglect; speaking generally, it is the presence of lime that gives the working qualities and the requisite toughness. These, then, are the essential materials for the preparation of glass. Other substances may be present; alumina, for example, or one or other of the oxides of iron, but as a rule the presence of these latter bases is not desired—the glass would be better without them.
Putting aside, then, for the present the glass in which lead is a constituent, as well as that in which the soda is replaced by potash, it is remarkable how little difference of composition we find in examples of glass of the most divergent origin. Let us compare the composition of a Roman ‘lachrymatory’ with that of a piece of modern English plate-glass. In a hundred parts we find—
| Silica. | Soda. | Lime. | Iron Oxide. | Alumina. | |
| Roman lachrymatory | 71·5 | 16·5 | 8 | 1 | 2 |
| English Plate-glass | 72 | 17 | 6 | 2 | 2 |
These examples are indeed two extreme terms of a long but continuous series. A sample of Saracenic glass of the fourteenth or of Venetian glass of the sixteenth century, would yield on analysis much the same result.[[1]]
This, then, may be regarded as the normal composition of such glass as I shall have to deal with in this history. The main question has generally been—How can the sand or silica, the premier element in glass, be best converted into a substance which shall in external aspect resemble as closely as possible the native rock crystal (itself pure silica), but which at the same time shall be not only fusible, but after fusing pass on cooling through a plastic condition when it may be expanded into a vesicle and otherwise worked up into various shapes? Long practical experience has shown that this can be best effected by adding to the sand materials containing both soda and lime, and as far as possible nothing beyond these bases. A glass thus compounded we may take as our normal type, but, as I have said, the soda may in certain cases be replaced by potash and the lime by lead oxide.
Silica in any case is the essential element in glass, and in any normal glass there may be present from 60 to 75 per cent. If, however, the bases with which it is combined have a high combining number—and this is especially the case with lead—the percentage of silica may fall below the former figure. Thus, in a bottle glass with 12 per cent. of iron oxide and alumina[[2]] the proportion is reduced to 54 per cent., and in a flint glass with 43 per cent. of lead oxide there is only 45 per cent. of silica.
It was once the fashion among English writers on glass to classify the substance under the heads of crown-glass, bottle-glass, broad-glass, plate-glass, flint-glass, etc.; but such a classification, not very logical in itself, would be of no use to us.[[3]]