History of glass
Glass remained a luxury material, and the disasters that overtook Late Bronze Age civilizations seem to have brought glass-making to a halt. It picked up again in its former sites, Syria and Cyprus, in the 9th century BC, when the techniques for making colorless glass were discovered. The first glassmaking "manual" dates back to ca. 650 BC. Instructions on how to make glass are contained in cuneiform tablets discovered in the library of the Assyrian king Ashurbanipal. In Egypt glass-making did not revive until it was reintroduced in Ptolemaic Alexandria. Core-formed vessels and beads were still widely produced, but other techniques came to the fore with experimentation and technological advancements. During the Hellenistic period many new techniques of glass production were introduced and glass began to be used to make larger pieces, notably table wares. Techniques developed during this period include 'slumping' viscous (but not fully molten) glass over a mould in order to form a dish and 'millefiori' (meaning 'thousand flowers') technique, where canes of multicolored glass were sliced and the slices arranged together and fused in a mould to create a mosaic-like effect. It was also during this period that colorless or decolored glass began to be prized and methods for achieving this effect were investigated more fully.
Medieval stained glass
Glass comprising multiple layers of clear and (usually) red glass was known to exist in the 12th and 13th centuries. The manufacturing process is not known. Flashing was developed in the 15th century, and refers to the superimposition of a thin layer of coloured glass onto another coloured or uncoloured glass sheet. The procedure may have involved dipping a small sphere of molten glass into a molten uncoloured glass and blowing this into a cylinder form (the cylinder blown sheet process) which was then cut into sections and flattened in an annealing oven. Red, or ruby, copper-based glass, is usually flashed as the colour is too dense to be used alone. Other glass colours may also be flashed. These techniques could be remarkably sophisticated as demonstrated by 15thc. glass from the Carthusian Monastery of Pavia, where layered glasses of blue and violet; green and uncoloured; and red and uncoloured have been identified.