In all these examples the work of the artist follows closely on the lines of the carver of cameos—especially of those cameos where advantage is taken of the parallel layers of the natural stone, as in the case of the sardonyx and of the niccolo; it is for this reason that I have described the material of our Barberini and similar vases as onyx glass. But there was another and purer variety of quartz that was coming more and more into favour during the third and fourth centuries. From this time onward all through the early Middle Ages, if we are to judge from the treasures preserved in Christian churches, to nothing was more value attached than to vases and cups of rock crystal, often of imposing dimensions, carved in shallow or deep relief. When once the process of making a clear colourless glass was mastered, this natural crystal could be very closely imitated in a material which was more easily worked. The carvings on the great majority of the examples of rock crystal that have come down to us—for example, the vases in the Louvre from the Abbey of St. Denis, and those still preserved in the treasury of St. Mark’s—are of a distinctly Byzantine, if not rather of a Sassanian or even Saracenic character, and this style is reflected upon much of the ‘crystal’ glass which is so often confused with the harder stone.[[40]]
The Romans of the fourth century were great masters of the art of cutting hard stones. Along with a general decline in taste and artistic invention, there was some advance in the direction of what we should now call applied science, and this is exemplified in the nature of the ‘metal’ and in the method of carving of the later Roman glass.
In the case of this later engraved glass, the lapidary’s wheel was applied at times to produce a rough design by a series of burr-like marks, or again the pattern was built up of a number of shallow, mostly oval depressions; in other examples the glass was deeply undercut, so that the designs appear to float round the vessel, to which indeed they are only attached by small rods not easily visible. Of the last kind is the work that may conveniently be called diatretum, although it is by no means certain that the diatretarii, mentioned by Ulpian and others, were necessarily workers in glass, seeing that carvings of this description, whether in metal, in hard stones, or in our material, were equally in favour at this time.
We have, unfortunately, no complete example of this undercut work easily accessible in our public collections. A fragment, however, in the British Museum throws much light upon the process of manufacture. On this piece there remains a portion of the outer frame in the form of a few letters that have formed part of an inscription; most of these letters, however, have been broken away, and we are thus enabled to see the base of the rods that supported them. The sharp angles of these little rods, and the marks on the surface of the glass, point unmistakably to the use of a cutting-tool, nor is there, I think, any trace of soldering at the base of the rods. We must turn again to the marvellous collection of late classical and mediæval objects that has been so long preserved in the treasury of St. Mark’s at Venice for the most complete specimen of this undercut glass. Here will be found a situla, or bucket-shaped vessel, of slightly greenish glass, about eleven inches in height ([Plate XIV.]). On the upper zone is a hunting scene with two horsemen, treated with a certain energy that calls to mind some of the Byzantine and even Sassanian work of the fourth and fifth centuries. Below we have a raised network, or rather grating—for the motive seems to be taken from a grille of iron or bronze—formed of four rows, each built up of fifteen tangential circles bound together at the points of contact. About half of these circles are more or less broken, and neither on the ground nor on the supporting rods thus disclosed was I able on close examination to discover any of those marks of a cutting-tool so prominent on the British Museum fragment. Indeed it is very possible that this late example may be built up of separately cast pieces soldered on to the base.
The famous cup of diatretum glass found near Strassburg was destroyed during the bombardment of that city in 1870; it bore an imperfect inscription in raised letters, which has been interpreted as referring to the Emperor Maximianus Herculius, the partner of Constantine in the empire, who put an end to his life in 310. In this case a network of red glass and an inscription of green glass were superimposed upon a nearly colourless ground. So in another cup preserved in the Palazzo Trivulzio at Milan, the inscription Bibe Vivas Multos Annos is again in green glass, but the network is here blue. Where the detached decoration is of a different colour from the base, the original vase must have been of an onyx glass formed by a ‘casing’ process and of considerable thickness, unless, indeed, we are to regard the lettering and the network in such cases as formed separately and attached to the base by the little rods. Perhaps the finest example of a vas diatretum is the bowl found in a stone sarcophagus at Worms, of which the fragments are now divided between the museums of Bonn and Mainz. In the former museum may also be seen a tall amphora-shaped vase (some twenty inches in height), with Bacchic scenes carved in low relief, which was found in the same coffin.
BOWL OF OLIVE-GREEN GLASS, ON METAL STAND
LATE ROMAN
The oviform bowl belonging to Lord Rothschild is carved in an olive-green glass, which appears of a deep red by transmitted light. It is surrounded by five figures in what is practically complete relief; the subject represented appears to be the ‘Madness of Lycurgus.’ The arms and the draperies of these figures are connected to the base by little rods as in the previous examples, but to judge from certain cavities in the interior corresponding to the principal external bossages, the glass was originally cast in a mould.[[41]]
The often-quoted expression of Martial, ‘Surrentinæ leve toreumata rotæ,’ written before the end of the first century, can hardly refer to this undercut work, which seems to be all of a much later date, nor is it even certain that the words refer to objects carved in glass rather than in rock crystal and agate. The word toreumata is used in connection with silver and even of earthenware. So the calices and toreumata Nili of the same writer (xi. 12) seem from the context to be rather carved in some precious stone. The following lines, however, are headed ‘Calices Vitrei’: