Cæruleum1159 is a kind of sand. In former times there were three kinds of it; the Egyptian, which was the most esteemed of all; the Scythian, which is easily dissolved, and which produces four colours when pounded, one of a lighter blue and one of a darker blue, one of a thicker consistency and one comparatively thin;1160 and the Cyprian, which is now preferred as a colour to the preceding. Since then, the kinds imported from Puteoli and Spain have been added to the list, this sand having of late been prepared there. Every kind,1161 however, is submitted to a dyeing process, it being boiled with a plant1162 used particularly for this purpose,1163 and imbibing its juices. In other respects, the mode of preparing it is similar to that of chrysocolla. From cæruleum, too, is prepared the substance known as “lomentum,”1164 it being washed and ground for the purpose. Lomentum is of a paler tint than cæruleum; the price of it is ten denarii per pound, and that of cæruleum but eight. Cæruleum is used upon a surface of clay, for upon lime it will not hold. A more recent invention is the Vestorian1165 cæruleum, so called from the person who first manufactured it: it is prepared from the finer parts of Egyptian cæruleum, and the price of it is eleven denarii per pound. That of Puteoli is used in a similar manner,1166 as also for windows:1167 it is known as “cylon.”

It is not so long since that indicum1168 was first imported to Rome, the price being seventeen1169 denarii per pound. Painters make use of it for incisures, or in other words, the division of shadows from light. There is also a lomentum of very inferior quality, known to us as “ground” lomentum, and valued at only five asses per pound.

The mode of testing the genuineness of cæruleum, is to see whether it emits a flame, on being laid upon burning coals. One method of adulterating it is to boil dried violets in water, and then to strain the liquor through linen into Eretrian1170 clay.