Gold glass

Gold glass or gold sandwich glass is a luxury form of glass where a decorative design in gold leaf is fused between two layers of glass. First found in Hellenistic Greece, it is especially characteristic of the Roman glass of the Late Empire in the 3rd and 4th century AD, where the gold decorated roundels of cups and other vessels were often cut out of the piece they had originally decorated and cemented to the walls of the catacombs of Rome as grave markers for the small recesses where bodies were buried. About 500 pieces of gold glass used in this way have been recovered. Complete vessels are far rarer. Many show religious imagery from Christianity, traditional Greco-Roman religion and its various cultic developments, and in a few examples Judaism. Others show portraits of their owners, and the finest are "among the most vivid portraits to survive from Early Christian times. They stare out at us with an extraordinary stern and melancholy intensity". From the 1st century AD the technique was also used for the gold colour in mosaics. Various different techniques may sometimes also be described as "gold glass". Zwischengoldglas is very similar but the two layers of glass are cemented, not fused. It mostly comes from Germany and Bohemia from the 18th and 19th centuries. Verre églomisé properly covers a single layer of glass which is gilded (or coated with other types of metal leaf) on the back, as used in 19th century shop signs and the like. One process was revived by Jean-Baptise Glomy (1711–1786), hence the name. Both of these processes were also used in ancient times, and the German and French languages often use their native terms for what is called "gold glass" in English. Gold ruby glass or "cranberry glass" is actually red, coloured by the addition of gold oxide. Gold-band glass is another ancient technique covered below.

Technique The manufacturing process for gold glass was difficult and required great skill. For a Late Roman glass, first a small round flat disc, typically about three to five inches (7.6 to 12.7 centimetres) across, was cut away from a blown sphere with a flattened bottom, either made of coloured or plain glass. A piece of gold leaf was then glued to this with gum arabic. The design was created by scraping away gold leaf. The main vessel, a cup or bowl, was formed by blowing and cutting, with a flat bottom the same size as the first disc. This was then heated again and carefully lowered onto the disc with the design, superimposing the flat bottom with the disc with the design so that they fused together. The complete vessel was then heated a final time to complete the fusing. Different accounts of different periods vary somewhat as to the precise sequence of stages and other details, but the process is essentially the same. The larger Hellenistic glass bowls are thought to have been formed using moulding rather than blown, as the whole bowl is doubled and the inner and outer vessels must fit together exactly. Some of the finer later medallions seem to have been made as such from the start, and some contain pigments other than the gold. These smoother-edged medallions exploited the medium of glass as a matrix for portrait miniatures, and it has proved to be a very effective one, outlasting all alternatives except for precious metal and engraved gems. They were probably initially made to be hung for display, or set in jewellery in smaller examples like that of Gennadios, but were also used for funerary purposes, and often use a base of blue glass. They are a few Roman examples of vessels from Cologne of a different style where several of what have been called "sidewall blobs", small gold glass medallions about 2–3 cm (0.8–1.2 in) across, with images, are fused into the walls of a vessel.

Apart from roundels with figurative images the fused sandwich technique was used to create the tesserae for gold in mosaics, and for beads and the like. Gold glass tesserae, at least by Byzantine times, had a very thin top layer of glass, which was probably poured onto the lower glass with the gold leaf glued to it. Tesserae were made in blocks or "cakes" and then cut into cubes, which are relatively large in the case of gold backgrounds. Gold backgrounds were laid over earth red or yellow ochre backgrounds which enhanced their visual effect. Most colours of tesserae seem to have been made locally to the mosaic, but there is some discussion as to whether this was true for gold glass ones. In the 11th century the relatively new Christian centre of Kiev seems to have used gold tesserae made in Constantinople. Roman gold glass beads were made by using an inner tube or rod to which the gold leaf was stuck. A larger tube was slid over that and the beads crimped off. Easily transported and very attractive, Roman gold glass beads have been found as far outside the Empire as the Wari-Bateshwar ruins in Bangladesh, and sites in China, Korea, Thailand and Malaysia. Gold-band glass is a related Hellenistic and Roman technique, where strips of gold leaf, sandwiched between colourless glass, are used as part of the marbling effect in onyx glass. It is mostly found in small perfume bottles and the like.

Grave-markers The most common form of vessel in late Roman examples was a bowl or drinking cup, which are thought to have been originally family gifts for weddings, anniversaries, New Year, the various religious festivals and the like, in some cases perhaps presented at birth or Christian baptism. None of the type of gold glass vessels cut down as grave-markers has survived complete, though about 500 of the cut-off bottoms are known, but since so many have inscriptions encouraging the owner to drink, they are usually referred to as "cups" or "glasses". However Roman drinking cups and glasses were often very wide and shallow, though tall straight-sided or slightly flared shapes like modern tumblers are also found. A mosaic in the North African ruins of Dougga shows two hefty slaves pouring wine from amphorae into two shallow bowls held by slaves waiting on the banquet. The two amphorae are inscribed with "ΠΙΕ" and "ΣΗϹΗϹ" the Greek originals of the toasting formulae "pie zeses" ("Drink, may you live", discussed below) so common on Roman glasses, and it has been suggested that the mosaic shows the shape a complete cup would have had.

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