Occasionally, the ferric hydroxide is associated with a certain proportion of clay, and as this increases, the ochre passes over into ferruginous clay. This class can also be used as pigment, in certain circumstances, that is to say when it is sufficiently rich in ferric oxide to furnish a deep red mass on calcination. When, however, the proportion of ferric oxide is low, its pigmentary power is no longer sufficient, and the clay has not the requisite beauty of colour. The ordinary earth used for making tiles is an example of this class, its colour in the raw state being an ugly brownish-yellow, but turning a dull “brick” red when fired. In some deposits the ferric oxide is accompanied by lime. Unless the latter exceeds a certain proportion, such ochres, too, are suitable as pigments, the lime being easily removed by simple levigation; but when the amount of lime is high, it is difficult to obtain certain highly coloured shades of ochre from such material. These shades entail the calcination of the ochre, and the temperature required is oftentimes insufficient to transform the lime into the caustic state. Moreover, the presence of caustic lime would be a drawback in some cases, it being then impossible to mix the ochre with other colours without endangering the shade through the action of the lime on these latter. The following analyses will show the percentage composition of ochres from various deposits: Ochre from-- +-------------+---------+----------+--------------+ | | Comoal | Vierzen. | St. Georges. | | |(Savoy). | | | +-------------+---------+----------+--------------+ |Ferric oxide | 19 | 23·5 | 25 | |Lime | 2 | -- | -- | |Alumina | 20 | 69·5 | 70 | |Magnesia | 1 | 69·5 | 70 | |Silica | 44 | 69·5 | 70 | |Water | 7 | 7 | 5 | +-------------+---------+----------+--------------+ In the majority of cases the mineralogical characteristics of an ochre enable conclusions to be formed as to its suitability as pigment. Good ochre is more or less yellow to dark brown in colour, and can easily be crushed between the fingers to a soft, fine powder which feels like powdered steatite and does not produce a sensation of grittiness, this latter indicating the presence of fine grains of sand in the ferric oxide. The behaviour of the ochre in presence of water is specially important. If it adheres firmly to the tongue, and forms a fairly plastic paste when mixed with a little water, the mineral contains a large percentage of ferric oxide, and as a rule will yield ochre of good colour. In general it may be said that the value of an ochre varies directly with its content of ferric hydroxide or oxide, because when this is large the ochre will furnish a wide range of colours under suitable treatment.