This highly renowned pigmentary earth is found in deposits, and, in the crude state, forms dark brown masses which are devoid of lustre, crumble readily between the fingers, have a smooth conchoidal fracture and absorb water with avidity, in consequence of which property they adhere to the tongue. Their chief chemical constituent is ferric hydroxide, with which, however, variable quantities of sand, clay and ferric oxide are admixed. These admixtures cause a considerable divergence in the colour of the earth, ranging from pure brown to reddish-brown, and, in the case of very impure lumps, to an unsightly yellow-brown. Mineralogically, terra di Siena is often regarded as a distinct species which, according to the results of analysis, must be considered, not as ferric hydroxide, but as ferric silicate combined with water. Sometimes, a portion of the ferric oxide is replaced by alumina, so that the percentage composition of the mineral becomes approximately: ferric oxide, 66%; silica, 11%; alumina, 10%; and water, 13%. The hardness of this mineral is 2·5, and the sp. gr. 3·46. The method of formation of terra di Siena was probably on the same lines as that already described in the case of ochre, namely by the breaking down of minerals--in this case brown ironstone--and natural levigation, the powder being deposited in places where the water containing the ferric hydroxide in suspension came to rest and allowed the solid particles to settle down. The best lumps of terra di Siena in point of purity and colour can be used as pigments without any preparation; but in most cases the earth is lightly calcined, in order to improve the colour. This treatment enables a whole series of tones, from pure brown to the brightest red, to be obtained. The stronger the heating, the more water expelled from the hydroxide, and consequently the closer the approximation of the colour to that of ferric oxide. The pigments met with in commerce as terra di Siena can also be prepared artificially, by making ferric hydroxide and heating this, when dried, until the requisite tone is attained. For this purpose, ferrous oxide is precipitated from green vitriol and exposed to the air, under which conditions it is rapidly transformed into ferric oxide, and the greyish-green colour of the mass changes to brown. Lighter tones can be obtained by the addition of inert white substances; and, in other respects, the method of preparation is the same as that of artificial ochre. These pigments are sold under various names, the dark shades, between pure brown and red brown, being usually called terra di Siena or mahogany brown, whilst the paler sorts are sold as satinober--more correctly satin ochre, golden ochre, etc. Other pigments, chemically allied to the ferric oxide or ochre pigments, are sometimes found on the market under various and entirely arbitrary names.