The great merit of lead-glass lies in its absolute transparency and brilliancy, combined with a certain darkness in the shadows. This brilliancy and fire, it is well to point out, are only indirectly dependent upon the refractive power exercised by the glass upon the rays of light that pass through it; in this respect lead-glass differs little from rock crystal or from the Venetian cristallo. But one quality it has which distinguishes it from all other kinds of glass as well as from nearly all transparent natural stones, the diamond, of course, excepted. This is the power possessed by it of dispersing the rays of white light: the elements of which this light is composed in passing through lead-glass are bent aside in different degrees, so that the issuing ray is broken up into its component colours. This it is that gives fire, but this fire is only fully brought out by means of facetted or angular surfaces. On this point—the distinction between refraction and dispersion—a good deal of confusion exists. The following table, which I borrow from a little book on gems by Professor Church, may help to clear up this point:—

Refractive Index.Comparative Dispersing Power.
Diamond,2·7544
Flint-glass,1·5736
Rock-crystal,1·5514
Plate and crown glass,1·5215

We here see that lead-glass or flint-glass has little greater refractive power on light than rock crystal or the ordinary plate and crown glass of commerce which belongs to the same family as the cristallo of the Venetians. In dispersive power, on the other hand, it stands apart from both these substances and rivals the diamond in scattering the component rays of white light.

CHAPTER XX
ENGLISH GLASS OF THE EIGHTEENTH CENTURY

We may probably regard the reign of William III. as the turning-point in the history of our English glass as in so many other of our minor arts. It is to that period that one must assign the first beginnings of our modern industrial life,[[252]] and it is in the Dutch influence, at that time so dominant, that the principal stimulus is to be found.

Of the window and mirror glass of the period a most interesting series is preserved at Hampton Court. Many of the panes of the windows facing the garden façades of the palace are strongly tinged with purple, a result of the process by which the colourless protoxide of manganese is reconverted into the purple bin-oxide under the influence of sunlight. Placed between the windows in William III.’s state bedroom are some curious mirrors with frames ornamented with appliqué plates of deep blue glass carved into patterns and monograms. Observe, too, a charming mirror of the same period over the fireplace in this room.

It is, however, still difficult to point to surviving examples to illustrate the vessels of English glass made about this period. Certain covered bowls (such as that reproduced by Mr. Hartshorne on p. 238 of his great work) may date back to the end of the seventeenth century. The same author gives an illustration of a fine posset-pot with quilled handles, preserved at Chastleton. This bowl, decorated with roses, masks, and berry-like prunts, may be as old as Charles II.’s reign. When one calls to mind the picturesque pottery—the slip-ware—that was made at the time, it would seem not unlikely that in the local glass-houses something similar may have been attempted in glass.

We have, of course, plenty of glass wine-bottles, a few of which may date as far back as the reign of Charles I. These bottles are mostly of a black impure glass and of a globular form, squat and compressed at the sides, reminding one of the leather botel from which our word bottle is derived. Similar bottles are found in the Low Countries, and they may often be seen in Dutch pictures. The introduction of the practice of bottling wine, as far as England is concerned, is generally connected with Sir Kenelm Digby, that universal genius who, in the reign of Charles I., was occupied with so many branches of the arts. Drinking-bottles of this description, dating from the seventeenth and eighteenth centuries, are often dug up while excavating the foundations of houses. An extensive collection, chiefly of local origin, may be seen in the Guildhall Museum, and Mr. Hilton Price has a representative series derived also from excavations in the city. The surface of these bottles is often covered with an iridescent scale giving them an appearance of great age. A circular stamp bearing the maker’s name is sometimes found on the shoulder, but these stamped bottles are in all cases, I think, of later date. There is a small collection of these stamps in the British Museum.

I have already pointed out that during the reign of Charles II. the prevalent form of the drinking-glass was still of the old Venetian type. The stem was almost non-existent; it was at best represented by a spherical bulb connecting the two cones—the upper one often truncated, the lower very shallow—that formed respectively the bowl and the foot. In the Spanish Netherlands, before the end of the century, another form became prevalent: the stem now assumes more or less a baluster form, divided from the bowl by a distinct shoulder; the knop of this stem is often hollow, and generally duplicated. In some cases a silver coin is found lying loose in this hollow bulb. Such a form we may perhaps regard as the starting-point for the vast and varied series of English drinking-glasses which constitutes the principal element in a collection of English glass.