A broken fragment of glass will indeed often tell us more than a complete vase. We can, for example, see from it whether the pattern passes continuously through the whole thickness of the glass, or whether it has been inlaid, or perhaps pressed into the surface when hot. In one case we have a process that reminds us of mosaic work; in the other there is some approach to a champlevé enamel, only with a base of glass instead of metal. In some rare examples we find the glass inlay surrounded by a fine ribbon of gold, suggesting the cloisonné enamels of the Byzantine jeweller. There is a minute example of this delicate work in the Slade collection (Catalogue, Pl. III. No. 4).
Colours of Roman Glass
It is evident that the Romans had at their command a full gamut of colours, both transparent and opaque, obtained from iron, copper, manganese, and antimony—the same metals, in fact, as the Egyptians made use of. But their deep transparent blue they probably obtained, in most cases, from cobalt, a metal unknown to the latter people.[[26]] There was one great deficiency, however, in their palette. They were never able to obtain a transparent red. The ruby red derived from copper or from gold was known to the early mediæval alchemists, but no undoubted instance of the use of this valuable colour has been observed in glass of the classical period.[[27]] The nearest approach to a transparent red is to be found in the honey and brown-red tints resembling the sard and the hyacinth; colours such as these are derived chiefly from iron, and may pass, on the one hand, into a pale yellow, and on the other into various shades of olive-green. The opaque red glass containing a large percentage of the basic oxide of copper and also some oxide of tin,[[28]] was much admired by the Romans; it was probably the vitrum hæmatinon of Pliny. In the Gréau collection is a head of Neptune in this material, of considerable artistic merit; to this head the oxidation of the surface has given the appearance of a finely patinated bronze.
Wall Decoration of Glass
Before going on to speak of the blown glass of the Romans, it will be well to say something of another application of glass that found favour among them at one time. This consisted in the decoration of the surface of walls, and in a few rare cases of pavements, by slabs of glass of various colours.[[29]] We may, perhaps, trace a double origin for this use of the material. On the one hand, it but carried out more fully the decoration of wall surfaces by rosettes and other patterns, both of glass and of glazed pottery, a plan often adopted by the Egyptians. This style was imitated with the little plaques of glass inlay, of which so many fragments have been found among the vineyards in the neighbourhood of Rome.[[30]] On the other hand, slabs of glass were used to imitate the veneer of porphyry and other marbles, so much in use in Rome in the first and second centuries. The two favourite stones, the red Egyptian porphyry with white spots and the green Serpentino from the Taygetus range with large, whitish crystals of felspar, were admirably imitated in slabs of glass often of large size; of these many important specimens may be seen in the British Museum. This method of decoration must have been introduced at Rome at a comparatively early date, if we are to accept the usual interpretation of the passage where Pliny describes the application of glass to the exterior of the theatre built by Scaurus at the beginning of the first century before Christ.
The best known examples of this glass veneering come from the ruins of a building some four miles to the north of Rome, generally known as the Villa of Lucius Verus; there are many fine pieces from this source in our museums. In private houses this veneering of glass was above all in favour for the bath-chamber. ‘Vitro absconditur camera’ says Seneca, instancing this practice as a sign of the advancing luxury of the age.
In the earlier methods each slab or tile is built up of pieces of glass of geometrical outline; in rarer cases the adjacent pieces have been fused together or again pressed into a base of glass by a plan similar to that formerly used in Egypt. But when the individual pieces of glass have been cut into shapes and then fitted together to form the design, we have the opus sectile of the Romans. We are here dealing with something nearly approaching in character to a true mosaic, and therefore outside the limits we have given ourselves. But it is impossible to pass over without mention the marvellous examples of this class of work which covered the walls of the basilica erected at Rome by Junius Bassus, consul in the year 317. Although this building no longer exists, important remains of the opus sectile which once covered its walls are preserved in a private palace at Rome, and some smaller compartments may be seen in the Church of St. Antonio Abbate on the Esquiline. These have been described in a paper read by the late Mr. Nesbitt before the Society of Antiquaries (Archæologia, vol. xlv.; see especially the coloured plate XVIII.). The main subjects, indeed, and the ground are executed chiefly in coloured marbles, but for us the most interesting part is the band representing embroidery below the large picture of Hylas and the Nymphs. This frieze of small figures is formed entirely of glass, and it will be noticed that in this part both the subject and the treatment are Egyptian. We have here the copy of a wall-hanging—probably of one of the heavy embroidered tapetia Alexandrina. It must be borne in mind that although this work was nearly contemporary with the Christian mosaics of the time of Constantine, the designs must, in part at least, have been copied from some earlier composition. The frieze of figures indeed takes us back to the Egyptian renaissance of Hadrian’s time.
The glass of which the larger plaques of this Roman veneer were made was probably poured out upon an even surface, rolled while hot, and at times, but not always, subsequently polished. It may be regarded as a primitive form of what the French call verre coulé, a term which includes our modern plate-glass. The thick heavy glass that the Romans used for their slit-like windows belongs to the same class; it is well known that slabs of considerable size have been found in position at Pompeii, but we are not concerned here with this purely practical application of the material.[[31]]
The employment of glass for mirrors, although known to the ancients, was, if we may judge from the few specimens that have survived, only practised on a very small scale. Pliny says that the Sidonians had applied glass to this purpose, but he speaks of it rather as a curiosity than as a matter of practical importance. Some little circular mirrors of convex glass, about an inch and a half in diameter, have lately been found in Greek or Greco-Roman tombs at Arsinoe in Egypt. There is one in the Musée Guimet at Paris, set in a silver frame with a ring as if for suspension from a necklace. I do not know the exact nature of the metallic backing (it is merely described as étamé), but this is still quite brilliant. M. Garnier mentions two mirrors mounted in wood from a tomb at Saqqarah; others of watch-glass shape, set in frames of lead, have been found in Roman tombs at Ratisbon.