Apart from a few remarkable specimens found beneath some of the old churches of Cologne, as at St. Ursula and St. Severinus, these gilt glasses come almost exclusively from the catacombs of Rome. The Roman collections naturally contain the most numerous specimens; in the British Museum, however, may be seen an important and typical series, illustrating most of the points of interest. In the preparation of these _vetri a fondi d’oro_, the gold leaf was laid down upon the glass with some gum or varnish; the superfluous gold was then scraped away, and the internal lines of the draperies accentuated with a sharp metallic point; a covering of glass was then superimposed. So far all are agreed; but as to the actual process by which the two sheets of glass were united, there is some difference of opinion. The problem had already appealed to Heraclius, the writer of some barbarous hexameters treating _De Coloribus et Artibus Romanorum_. Heraclius was probably a monk living at Rome, perhaps about the end of the tenth century. The fifth of his little didactic poems is inscribed ‘_De fialis auro decoratis_.’ In this he tells us how he produced some small cups of pure glass, smeared them with gum with a brush, and then proceeded to lay down on them leaves of gold. On the gold leaf, when dry, he inscribed birds, men, lions, as it pleased his fancy. ‘Finally,’ says Heraclius, ‘I fitted over the surface, glass rendered thin by a skilful blast of the fire; but when the glass had yielded equally to the heat, it united itself admirably to the phials as a thin sheet.’[61] Theophilus, writing a few generations later, probably in Germany, knew nothing of this cemetery glass. He describes, however, the process by which the Byzantine Greeks made their gold mosaics by sprinkling a layer of powdered glass over the gold leaf covering the surface of the tesseræ; this coating was then fused on. But this was an enameller’s process, and the coating must have consisted of a somewhat fusible glass, perhaps containing lead. The Greeks employed, he tells us, a similar process in decorating their glass cups. Signor Andrea Rioda, the art director of the _Impresa Venezia-Murano_, tells me that in the case of some clever imitations of _fondi d’oro_ made by his firm, the gold leaf was fixed upon a thickish sheet of glass, a thinner sheet was then placed over it, and the whole heated to the softening-point. A third method has been adopted in the preparation of some experimental imitations made by Mr. Westlake: that gentleman soldered together the two sheets of glass round the edges only, by means of a flux.