The former of these properties imparts brightness of colour; and such products will furnish, on calcination, a wide range of colour shades. When, as is the case with the finer qualities of ochre, the mineral contains only a very small proportion of impurities, there is no difficulty in bringing it, by simple grinding or levigation, into a condition in which it is at once fit for use as a pigment.
The Italian ochres have, for long ages, enjoyed a high reputation for their beauty of colour and permanence. This category includes, for example, the renowned Siena earth, Roman earth, Italian umber, and other ochre colours. This high renown is probably due less to the inherent properties of the mineral than to the circumstance that the art of painting attained a high state of development at an early period, and that the artists paid special attention to the use of bright and permanent colours for their work.
Although, at present, many deposits of ochre are known that are quite able to compete, on the score of beauty, with the best Italian products, the good name of these latter has nevertheless been maintained. It is true that the name of Italian ochre is often merely borrowed, for application to a product originating in some other country, varieties of terra di Siena, for instance, being put on the market that have actually been derived from deposits in Germany.
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.