It is not generally realized that the roller-moulded type of window-pane, so common in Italy and the western provinces of the Roman Empire from the first century A.D. onwards, was unknown, or at least extremely rare, in the East. Its place was taken, at least from the fourth century A.D. onwards, by circular panes of blown glass resembling very shallow dishes without a foot-ring. The rim is not plain, but is folded over outwards (a curious feature, presumably for added strength); the sides, normally about ½ to 1 mm. thick, are straight, and the bottom, usually 3 to 5 mm. thick, is bulbous, being flat or slightly concave externally. The average diameter is about 16 to 20 cm.