Ancient Glass.

Scientific American 8, 23.8.1862

In all works published on glass making, twenty years ago, the art of glass making is alleged to be of comparatively modern date. The discoveries of Layard in Niniveh, however, have thrown a new light upon the subject, and have conclusively demonstrated the fact that the ancients six hundred years before the Christian era, were acquainted with the art of glass making, and with the magnifying glass.

Two entire glass bowls, with fragments of others, found by Mr. Layard in one of the palaces at Minroud, are supposed to be 2,600 years old, and are therefore the most ancient known specimens of transparent glass. These glass bowls were covered with pearly scales, the result of long immuration, which on being removed left prismatic opal-like colors of great brilliancy, showing under different lights the most varied and beautiful tints.

With the glass bowls was discovered a rock crystal lens, with opposite, convex and plane faces. Its propoerties could scarcely have been unknown to the Assyrians, and we have consequently the earliest specimen of a magnifying and burning glass. It was buried beneath a heap of fragments of beautiful blue opaque glass, apparently the enamel of some object in ivory or wood which had perished. Of this lens Sir David Brewster observes, "It is plano-convex, and of a slightly oval form, its length being 1 6/10 inches, and its breadth 1 4/10. It is about nine-tenth of an inch thick, and a little thicker at one side than another. Its plane surface is pretty even, though ill polished and scratched. Its convex surface has not been ground, or polished, on a spherical concave disk, but has been fashioned on a lapidary's wheel or by some method equally rude. The convex side is tolerably well polished, and though uneven from the mode in which it has been ground, it gives a tolerably distinct focus at the distance of 4½ inches from the plane side. There are about twelve cavities in the lens that have been opened during the process of grinding it; these cavities doubtless contained either naphtha, or the same fluid which is discovered in topaz, quartz and other minerals. As the lens does not show the polarized rays at great obliquities, its plane surface must ve greatly inclined to the axis of the hexagonal prism of quartz from which it must have been taken. It is obvious from the shape and rude cuttings of the lens, that it could not have been intended as an ornament; we are entitled, therefore, to consider it as intended to be used as a lens, either for magnifying, or for concentrating the rays of the sun, which it does, however, very imperfectly."

Sir David says further of this lens, that it is as sound as it was many thousan years ago when in the for of a crystal in quartz or rock crystal, which is pure silez and other regular crystallizes bodies.

It has been remarked that there is perhaps no material body that ceases to exist with so much grace and beauty as glass when it surrenders itself to time and not to disease. In samp localities, where acids and alkalies prevail in the soil the glass rots, as it were, by a process which it is difficult to study. It may be broken between the fingers of an infant, and in this state we generally find in the middle of it a fragment of a thin fiber of the original glass, which has not yielded to the process of decay. In dry localities, where Roman, Greek and Assyrian glass has been found, the process of decomposition is exceedingly interesting, and its results singularly beautiful.

At one or more points in the surface of glass the decomposition begins. It extends round that point in a spherical surface, so that the first film is a minute hemispherical one of exceeding thinness. Film after film is formed in a similar manner, till perhaps twenty or thirty are crowded into the tenth of an inch. They now resemble the section of a pearl or of an onion. Whenthe decomposition has gone regularly on round a single point, and there is no other change than a division of the glass into a number of hemispherical films, like a number of watch glasses within one another, the group of films exhibits in the polarizing microscope a beautiful circle of polarized light with a black cross. A small glass bottle now in the British Museum, found in the ruins of Nimroud, is said to be of equal age with the glass bowls already described. On this very interesting relic is the name of Sargon, with his title of King of Assyria, in cuneiform characters and the figure of a lion. In the excavations of the mound of Babel, amongst other interesting articles was found a number of small glass bottles, some colored, others ribbed and otherwise ornamented.

A most celebrated unique vase, which was for 200 years the principal ornament of the Barberini palace, and which is now designated the Portland vase, is a rich specimen of early glass manufacture. It was found about the middle of the sixteenth century inclosed in a marble sarcophagus within a sepulchral chamber, under Monte del Grano, about two miles and a half from Rome, supposed to be the tomb of Alexander Severus, who dies in the year 235. It is decorated with white opawue figures in bas relief upon a dark blue transparent ground, the subject of which has not hitherto received a satisfactory elucidation, but the design and arrangement and more particularly the execution, are truly admirable. A part of the blue groud, i. e., all below the handles, was originally covered with white enamel, out of which the figures have been sculptured in the style of a cameo, with most astonishing skill and labor.

Of the several specimens of glass brought to England by Mr. Layard, one, the fragment of a vase, when examined was of a dull green color, as though encrusted with carbonate of copper. This color was quite superficial, and the glass itself was opaque and of a vermilion tint, attributed to suboxide of copper. The outer green covering was due to the action of the atmosphere on the surface of the glass, and the consequent change of the suboxide into green carbonate of copper. This specimen is interesting as showing the early use and knowledhe of suboxide of copper as a stain or coloring agent for glass. The ancients, employed several substances in their glass and colores glazes for bricks and pottery, but of which there remains no published record. But these glasses and other ancient works of art prove that they were familiar with the use of oxide of lead as a flux in their vitreous glasses, and with stannic acid and Naples yellow, as stains or pigments.