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.