As a result of this custom, certain names, such as terra di Siena, umbra di Roma, have become generic terms, and their use denotes, not an intention to suggest that the earth colours in question really come from Siena or the vicinity of Rome, but that the properties of the article are equal to those of the old-established colours of Siena or Rome. It would occupy too much space to go into an exhaustive description of all the native varieties of ochre, and would inevitably lead to a good deal of repetition. It will therefore be sufficient, for our purpose, to deal with only a few of them. The best-known ochres are those of Rome and Siena, the latter being frequently called, in commerce, by its Italian name, terra di Siena. Roman ochre forms yellowish-brown masses, of fairly fine texture and composed of ferric hydroxide and clay. They are put on the market both in the raw and calcined state. On calcination, the colour soon changes to red, and if carefully performed, the resulting colours have a very warm, fiery tone. Closely approaching Roman earth is the English ochre, which is worked more particularly in Surrey, and is not infrequently sold as Roman. In many deposits this English ochre occurs in such a high state of purity that the best pieces are picked out and sold without being even crushed or ground. The pieces of lower quality are very carefully ground and levigated, for the purpose of being calcined for the production of different shades, and then furnish highly prized colours. In point of chemical composition, the ochre family also includes terra di Siena, bole, umber and Cassel brown. These minerals, however, are not yellow like ochre, but brown, and will therefore be dealt with along with the brown earth colours. ARTIFICIAL OCHRES Products very similar, both in chemical composition and colour, to the native ochres can also be very simply and cheaply made by artificial means. Their preparation may be particularly recommended to colour-makers who desire to turn out a wider range of iron pigments, but are not in a position to obtain natural ochres at a low price. In the manufacture of artificial ochre, an endeavour is made to imitate the natural processes which have led to the formation of ochre, and, of course, to avoid anything likely to hinder the production of a suitable colour earth, for example the presence of sand or a considerable admixture of extraneous minerals. As already mentioned, the chief impurities in natural ochres are clay and sand, both of which can be easily excluded during the manufacture of artificial ochre, or their amount controlled in such a manner that paler or darker products can be obtained at will, and the tone varied, in any desired manner, by calcination, as in the case of the native article.