The cameo engraved glass was produced by placing a layer of white opaque glass on a ground of transparent blue, the design being formed by cutting away the white surface to the blue ground, leaving the blue as the background; the remaining white which formed the design was then carefully finished by engraving. Light and shade was produced according to the thicknesses of the cameo left by the engraver. [Illustration: Fig. 297.—Ancient Roman Glass Bottle. (S.K.M.)] The celebrated Barberini or Portland Vase, in the British Museum, is made in a blue and white cameo. This splendid work of art was discovered in a marble sarcophagus near Rome, which is supposed to have been the tomb of the Emperor Alexander Severus. The vase is ten inches in height, is two-handled, and has for the subject of one side a figure decoration representing Thetis consenting to be the bride of Peleus, attended by Poseidon and Eros; on the other side is Peleus and Thetis on Mount Pelion, and on the bottom is a bust of Paris. The ground is transparent blue glass, and the subjects are beautifully engraved in cameo out of the superimposed white layer. [Illustration: Fig. 298.—Roman Glass Tablet in relief. (S.K.M.)] A similar kind of vase, but smaller, was found at Pompeii, and is now in the Museum at Naples, and the remains of the Auldjo Vase in the British Museum is also in a similar style, the cameo decoration of it consisting of vine-leaves. Intaglios and cameos, sometimes of a large size, were copied in glass from gems; these were usually cast in moulds, and many of them are of high value as works of art. (Fig. 298). The Romans made window glass of small squares or oblongs, which was manufactured by rolling it on a plate. In the early Christian period gold leaf was used as a means of decoration on glass: sometimes the gold was annealed to the surface, and sometimes it was placed, as the making of the gold smalto for mosaics, between two layers of thin glass, and afterwards fired. Patterns and figure subjects were executed in gold foil, and formed the decoration of glass dishes and bowls, the broken remains of which have been found in the Christian tombs of the Roman Catacombs. Though extensive glass works are known to have existed at Constantinople and at Thessalonica between the ninth and thirteenth centuries, there are scarcely any remains of Byzantine glass in existence that can with certainty be ascribed to the Eastern empire, unless we except the five cups and two shallow basins of thick green glass that are decorated with Byzantine ornament, and which form part of the treasure of St. Mark’s at Venice. Glass was used in the windows of Byzantine churches and, of course, in the making of the mosaic tesseræ.