In other cases the surface of a piece of clear white glass will become gradually filled with a series of minute intersecting fissures, which in time may penetrate the whole mass. When this change has been fully developed we get a true crackle-glass, not to be confounded with the frosted glass of Venice mentioned in [Chapter XIII]. This fissuring of the glass-mass in its various stages may be traced in many of the specimens of Venetian, Netherlandish, and English glass at South Kensington. When fully developed the effect is at times very beautiful.

The tints of coloured glass may, it would seem, change in the course of time. Colourless glass also, from which the greenish shades derived from protoxide of iron have been removed by the addition of binoxide of manganese, is above all liable to assume in the course of time a purple tint under the action of sunlight. Again, if sulphur be present in glass, as is the case where sulphate of soda has been employed as a source of the alkali, the soda salt may be reduced by any protoxide of iron that is present. The sulphide of sodium and the sesqui-oxide of iron thus gradually formed will both of them tend to give a yellowish tint to the glass.[[6]]

Changes of this nature may occasionally have come about in the stained glass of the windows of our Gothic churches—the flesh-tints, which we know were produced in early days by manganese, may in the course of time have become of a more pronounced purple hue.

CHAPTER II
THE PRIMITIVE GLASS OF THE EGYPTIANS AND SYRIANS

From a technical point of view the history of glass might be divided into three periods—periods, it is true, of very unequal length and relative importance.

The first of these, one more especially of archæological interest, would include all the glass made before the discovery of the process of forming a vesicle by blowing through a hollow tube. Nearly all the glass that finds its way into our collections would be classed in the second period; this would extend from the beginning of our era to the end of the eighteenth century. In the course of these long centuries, the work of the glass-maker has of course been influenced by the varying schools and fashions of different ages and countries, but technically there is no great advance to be noted in the work of the seventeenth and eighteenth centuries when compared with that of the early days of the Roman Empire; and this is still more true if we consider merely the materials employed, their preparation, and the methods of their fusion. But before the end of the eighteenth century a great change had set in. The manufacture of glass in England and France had become an important industry, and we enter upon the third or industrial period. With the general advance in mechanical processes that is so characteristic of the time, the old methods of the working of glass were swept aside, so that before the middle of the last century, whatever of interest was to be found in the manufacture and in its results depended upon anything rather than upon the artistic qualities of the glass made.

Now, as I have said, the characteristic and dominant quality of glass is to be found in its capability of being blown into vessels of varying shape when in a viscous and semi-fluid state. All glass then, made at a time when advantage had not yet been taken of that essential property of the material, we may class together in a primitive group. This line of demarcation is as important, to return to a comparison I have already made, as that between hand-moulded pottery and that thrown on the potter’s wheel. The objects made in the earlier period by primitive processes were mostly small, and their merit depended chiefly upon the brilliancy and the skilful juxtaposition of a few simple colours—they may for the most part be classed as verroterie.

It has long been acknowledged that it is from Egypt that our earliest specimens of glass have come. But until quite recently the greatest misconceptions have prevailed as to the age and the methods of preparation of Egyptian glass. Misled by an erroneous interpretation of what are probably representations of metallurgical processes, on the walls of Twelfth Dynasty tombs at Beni Hassan and elsewhere, it was inferred that the art of blowing glass was known to the Egyptians at least as long ago as the days of the Middle Empire; by others the art was carried back to a still earlier period. We now have almost full assurance that glass in a true sense was practically unknown to the Egyptians before the time of the Eighteenth Dynasty (say between 1600 and 1500 B.C.),[[7]] and that for at least a thousand years after that period all that was made was produced by a primitive process in which the blowing-iron found no part. We have, unfortunately, up to the present time absolutely no evidence to show in what country or at what date this new process—I mean the blowing of a vesicle of glass—first came into use. There is, as we shall see, some reason to look for it rather in Western Asia than in Egypt, but the important point to bear in mind is that it was only after the introduction of this process of blowing, first to Alexandria and then to the Rome of the early empire, that the employment of glass for objects of daily use became in any way general.

Glass, indeed, in these early days, whether in Egypt or in the Greek world of the Mycenæan age, was something very different from what we now understand by the term. We must ‘think away’ a great deal of the modern connotation of the word. We must, above all, think of the material in connection with the native precious or semi-precious stones that it more or less resembled, and which were used along with it for decorative purposes. We do not know the Egyptian name for glass, but probably, like the Greeks, they divided all the hard stony bodies used in the arts into such as were ‘dug up’—natural products, that is, which they found ready at hand—and such as had been artificially prepared, and above all previously melted (the Λίθος όρωρυγμένη on the one hand, and the Λίθος χυτή on the other).