It has been suggested that the idea of glazing first came from slags of the early metal smelters. Many of the beautiful Egyptian glazes in fact owed their richest colours to inclusions of powdered copper. Greenish-blue glazed ware was made even by the Pre-Dynastic people and during the next 1500 years this art reached a high degree of excellence, culminating in fine blue glazed ware for inlay and decoration. There were also glazed tiles, likewise coloured glass stones for finger-rings, earrings and other trinkets.
In some cases the blue hues were obtained by a mixture of copper and soda, whilst the brilliant ruby red was derived, as it is still, from cuprous oxide; specimens from Amarna contained as much as 12 per cent of this compound.
According to the eminent Egyptologist, Sir Flinders Petrie, Egyptian glassmaking techniques were as follows: 'The manufacture of glass is shown by examples in the XVIIIth Dynasty. The blue or green colour was made by fritting together silica, lime, alkaline carbonate and copper carbonate; the latter varied from 3 per cent in delicate blues to 20 per cent in deep purple blues. The silica was needed quite free from iron in order to get the rich blues, and was obtained from calcined quartz pebbles; ordinary sand will only make a green frit. These materials were heated in pans in the furnace so as to combine in a pasty, half-fused condition. The coloured frit thus formed was used as paint in a wet state, and also used to dissolve in glass or to fuse over a surface in glazing. An entirely clear, colourless glass was made in the XVIIIth Dynasty, but coloured glass was mainly used. After fusing a pan full of coloured glass, it was sampled by taking pinches out with tongs; when perfectly combined it was left to cool in the pan, as with modern optical glass. When cold, the pan was chipped away and the cake of glass broken up into convenient pieces. A broken lump would then be heated to softness in the furnace, rolled out under a bar of metal held diagonally across the roll, and when reduced to a rod of 1/4 in. thickness, it was heated and pulled out into even rods of about 1/8 in. thick. These were used to wind round glass vases, to form lips, handles, etc., and to twist together for spiral patterns. Glass tube was similarly drawn out. Beads were made by winding thin threads of glass on copper wires and the greater contraction of the copper freed the bead when cold.' (5)
Again, 'Large tiles a foot in length were glazed completely all over and used to glaze the walls of a room; they were retained in place by deep dovetails and ties of copper wire... A kind of visiting card was also made in glaze, with the figure of a man and his titles to present in temples which he visited; and glazed ornaments and toggles for fastening dresses were common... A piece of a glazed tile and part of a glazed vase exist which have the royal titles and name of Menes, originally in violet inlay in green glaze (3200 B.C.).' (6)
Long afterwards the Assyrians also used copper silicate to give a blue colour to their glazed bricks.
6 Ibid. p. 73.