_Smalto blues._ Next in importance to the real ultramarine come the artificial _smalto_ or "_enamel_" blues, which were used largely in Egypt at a very early date under the name of artificial _cyanus_; see Pliny, _Hist. Nat._ XXXVII. 119. Among the Greeks and Romans too this was a pigment of great importance, and when skilfully made is but little inferior in beauty to the natural ultramarine. _Vitreous pigment._ _Smalto blue_ is simply a powdered blue glass or vitreous enamel, coloured with an oxide or carbonate of copper. Vitruvius (VII. xi. 1) describes the method of making it by fusing in a crucible the materials for ordinary bottle-glass, mixed with a quantity of copper filings. The alcaline silicate of the glass frit acts upon the copper, which slowly combines with the glass, giving it a deep blue colour. The addition of a little oxide of tin turns it into an opaque blue enamel, which when cold was broken up with a hammer, and then powdered, but not too finely, in a mortar. Smalto blue is largely used for the simple blue initials which alternate with red ones in an immense number of manuscripts. The glittering particles of the powdered glass can easily be distinguished by a minute examination. Like the ultramarine, the smalto blue is always applied in a thick layer. The monk Theophilus (II. 12), who wrote during a period of some artistic and technical decadence, the eleventh century, advises the glass-painter who wants a good blue to search among some ancient Roman ruins for the fine coloured _tesserae_ of glass mosaics, which were so largely used by the Romans to decorate their walls and vaults, and then to pound them for use. _German blue._ _Azzurro Tedesco_ or _Azzurro della Magna_, German blue, was much used by the illuminators as a cheap substitute for ultramarine. This appears to have been a native compound of carbonate of copper of a brilliant blue colour. It was occasionally used to adulterate the costly ultramarine, but this fraud was easily discovered by heating a small quantity of the pigment on the blade of a knife; it underwent no change if it was pure; but if adulterated with _Azzurro della Magna_ it showed signs of blackening[271]. _Indigo._ _Indigo blue._ The above-mentioned blues are all of a mineral character, and are durable under almost any circumstances. To some extent however the vegetable _indigo_ blue was also used for manuscript illuminations, both alone and also to make a compound purple colour. _Method of using dyes._