History of glass
The history of glass-making dates back to at least 3600 years ago in Mesopotamia. However, they may have been producing second-rate copies of glass objects from Egypt, where this complex craft originated. Other archaeological evidence suggests that the first true glass was made in coastal north Syria, Mesopotamia or Egypt. The earliest known glass objects, of the mid second millennium BC, were beads, perhaps initially created as the accidental by-products of metal-working (slags) or during the production of faience, a pre-glass vitreous material made by a process similar to glazing. Glass products remained a luxury until the disasters that overtook the late Bronze Age civilizations seemingly brought glass-making to a halt.
Early Christian art and architecture
Gold sandwich glass or gold glass was a technique for fixing a layer of gold leaf with a design between two fused layers of glass, developed in Hellenistic glass and revived in the 3rd century. There are a very fewer larger designs, but the great majority of the around 500 survivals are roundels that are the cut-off bottoms of wine cups or glasses used to mark and decorate graves in the Catacombs of Rome by pressing them into the mortar. The great majority are 4th century, extending into the 5th century. Most are Christian, but many pagan and a few Jewish, and had probably originally been given as gifts on marriage, or festive occasions such as New Year. Their iconography has been much studied, although artistically they are relatively unsophisticated. Their subjects are similar to the catacomb paintings, but with a difference balance including more portraiture of the deceased (usually, it is presumed). The progression to an increased number of images of saints can be seen in them. The same technique began to be used for gold tesserae for mosaics in the mid-1st century in Rome, and by the 5th century these had become the standard background for religious mosaics.