Taken as a whole, the Naples yellows are unsatisfactory pigments, very prone to deteriorate in impure air, and necessitating great care in their preparation to avoid contact with iron, which turns them green. They cover well, are fairly brilliant, and mix readily with water or oil, but their application is declining rapidly. OCHRES.--The large class of mineral pigments known collectively as ochres or sienna earths possess considerable importance, notably on account of their remarkable durability and their reasonable price. They all consist essentially of an earthy base coloured by oxide of iron or of manganese, or of both. Some authorities differentiate between ochres and siennas, and ascribe the latter name only to those earths which contain manganese, but this seems to be an arbitrary proceeding, because the term sienna, or more properly Siena, is derived solely from the name of the Italian province in which these minerals are worked. They are of widespread occurrence, both geographically and geologically, and the methods of mining and preparing them are not subject to much variation. Consul Colnaghi, in his report on the mineral products of the province of Siena, says that Siena earths, known also under the names of ochre, bole, umber, &c., are considered by some mineralogists to be ferruginous clays, by others, minerals of iron. They are chiefly found in large quantities in the communes of Castel del Piano and Arcidosso. The yellow earths and bole found on the western slopes of Monte Amiata are true lacustrine deposits found amid the trachytic rocks, of which it is principally composed. They lie under, and are entirely covered by, the vegetable soil. Varying in compactness and colour, they are termed yellow earths when of a clear ochreous tint, and terra bolare, or bole, when of a dark chestnut colour. Each deposit consists for the greater part of yellow earth, beneath which bole is found in strata or small veins. The mineral being very friable, its excavation is easy, and is generally conducted in open pits.