Glass

by Edward Dillon

Glass, of course, varies in optical properties, in hardness, and in fusibility, but I do not think that any useful classification could be based directly on these properties. But there is one distinction of the greatest importance technically and geographically, and this is between the glass of maritime countries in which the alkali is soda, and that of inland and forest districts where the soda is replaced by potash. In the first group, by far the most important—I have indeed regarded such glass as the normal type—may, it would seem, be placed not only the ‘primitive’ glass of the Eastern Mediterranean, but probably all the glass of the Romans. To it belongs also the glass of the Saracens and the greater part of the artistic glass of the Renaissance, including the Venetian glass, although in this last the soda is often in part replaced by an appreciable quantity of potash. The potash group, on the other hand, includes the old voirre à fougère of the French and the wald-glas of the Germans. In addition, almost the whole of the glass of higher quality made in later days in Germany and in the Bohemian borderlands belongs essentially to this last class. Finally, it may be mentioned that in the case of the abnormal family where the lime is replaced by oxide of lead, the alkali is invariably potash. Of this family our English flint-glass is the most important member.

With regard to the hardness of glass, Merret mentions as the thirteenth property possessed by that substance, ‘that it only receives sculpture or cutting from a Diamond or Emery stone.’ But such a statement would be likely to give an exaggerated idea of the hardness of glass. If we take the scale of hardness used by the mineralogist, it will be found that there are few kinds of glass that do not fall between the fifth and sixth divisions of that scale. In other words, it would be difficult to find a specimen of glass on which a crystal of apatite (phosphate of lime) would make any impression, whereas all glass in ordinary use is readily scratched by felspar. It is possible, however, that some kinds of Bohemian glass may equal the latter mineral in hardness; it is indeed a common statement that certain Bohemian or German ‘combustion-tubes’ will strike fire with steel. On the other hand, the presence of lead tends to make a soft glass; our cut flint is perceptibly softer than common window-glass, and perhaps the most important defect of the paste used to imitate precious stones—such paste may contain as much as 50 per cent. of lead oxide—is to be found in its comparative softness.

At the same time, the greater the amount of lead in a glass, the greater its dispersive power on the light that passes through it. Hence the brilliancy and fire of flint-glass, and still more of artificial gems.

Apart from the varieties containing lead, samples of glass differ little in weight; the specific gravity may range between 2·4 and 2·8. That of flint-glass, on the other hand, varies from 3 to 3·8; indeed in some optical glasses containing a large percentage of lead, and again in the paste used for false jewellery, the specific gravity may be as high as 4·5 or even 5.

The high melting-point, or more definitely the high softening-point, of certain kinds of Bohemian and German glass, makes them invaluable in the laboratory of the chemist. On the other hand, the ready fusibility of glass containing lead was, as we shall see, one of the causes that promoted the adoption of such a glass in our furnaces.

Thus we find that the potash-lime glass of Bohemia, containing a high percentage of silica, excels in hardness and resistance to heat; on the other hand, the various kinds of glass containing lead are soft and easily fusible, and at the same time they combine a high specific gravity with a wide dispersive power. What we may call the maritime or soda-lime glass takes an intermediate place in all these respects. This is indeed an additional reason for regarding this great family of ‘Mediterranean’ glass as the normal type.

The two essential elements, then, required by the glass-maker are, in the first place, silica, and secondly an alkali, in each case as pure as possible, and in a convenient form for mixing and fusing together. I do not propose here to do more than indicate the source of these materials.

The silica has at all times been derived either from solid quartz, whether in the form of rock crystal or of the white pebbles from the beds of Alpine rivers, or more often from sand obtained either by excavation or from the seashore.

In the case of the alkali, the maritime people of the South extracted their soda, for the most part, from the ashes of certain plants growing in salt marshes near the sea. Most of these maritime plants belong to the natural order of the Chenopodiaceæ, the goose-foot or spinach tribe, and we find among them various species of Salsola, Chenopodium, Salicornia, etc. These plants were all included in old days under the vague name of kali. The roughly lixiviated ashes exported from Spain were known in the trade as barilla; those from the Levant as roquetta.[^[4]] In other instances the impure alkaline carbonates were found ready at hand—as in the case of the natron deposits not far from Cairo. In the North the principal source of soda was till recent days the varech or kelp, cast up on the west coast of France and of Scotland.

The inland folk, on the other hand, had to find the alkali for their glass in the ashes of plants. This ‘potash’ was obtained by lixiviating the ashes of various trees and bushes—in Germany the ashes of beechwood, in France those of the bracken or fougère, were most in favour.