The famous blue of the Egyptians, of which we hear from Vitruvius and other later writers, was essentially a silicate of soda, lime, and copper. It should be borne in mind that without the presence of the first two bases—the lime and the soda—a good copper blue in glass or glaze cannot be obtained. Indeed in the case of porcelain and fayence, the blues obtained from copper have always been confined to various shades of turquoise, as in the well-known glazes and enamels of the Chinese and the French, and even these turquoise blues, always, as we have said, containing lime and soda as well as copper, have only been produced with great difficulty. The mastery of a complete series of copper blues, ranging through every shade from a blue-black to a pale greenish turquoise, we may thus regard as a special triumph of the old Egyptians. At one period a darker shade has been in favour, at another a paler hue, according as the _lapis lazuli_ on the one hand, or the turquoise or green felspar on the other, was taken as the standard of excellence, so that the shade of colour of the glaze on a scarab or a bead may at times throw some light on its date. Distinct shades of green, apart from greenish blue, were much less in favour with the Egyptians, nor did they ever attain to the brilliant tints of the malachite. A green glass, generally comparatively transparent, was indeed at times obtained when a certain amount of iron was present in the materials employed; but this was merely an accidental modification of the blue. The pale tint of the green felspar was also imitated in an opaque glass used for inlaying.