CHAPTER V
YELLOW EARTH COLOURS
All the yellow earth colours, without exception, have ferric oxide as their colouring principle, the differences in shade being entirely due to the varying proportion in which that oxide is present. The various names under which they are known date back to a period when the chemical nature of these colours was still unknown, and have been mostly derived from the locality of origin.
The yellow earths can therefore be divided into two groups, according to their chemical character. The first group, in which the ferric oxide is present as hydroxide, comprises all the ochres, Siena earth, and a number of others which are obtained from native ochre by special treatment. In the colours of the second group, ferric oxide is still the colouring principle, but is combined with other substances in place of water.
It is, as a matter of fact, incorrect to rank the ochres in general as yellow earths, because they can be made to yield nearly every variety of colour from the palest yellow to the deepest red, brown and violet. These colours merit the particular attention of the colour-maker and the painter, being distinguished by very low cost of production, unusual permanence and beauty of tone. In the interests of that highly important matter to the artist, namely the production of colours of unlimited permanence, it is desirable that colour manufacturers should bestow greater care on the manufacture of these colours than has hitherto been the case. An extremely favourable point about nearly all these pigments is that they can be very cheaply prepared by artificial means, so that the manufacturer is in a position to turn out a large number of the handsomest and most durable colours with a small amount of expense and labour.
The Ochres
Ochres are found in many localities, most frequently in stratified rock and rubble. The deposits are rarely extensive, mostly occurring in pockets or beds. Wherever found, ochre may be termed a secondary product, that is to say, one that has been formed through the destruction of other minerals. The analysis of ochres from different deposits shows great divergence in composition; and some consist almost entirely of pure ferric hydroxide, that has already undergone natural levigation and can be used as a pigment as soon as dug.
Such a form is, however, rare, and most ochres are intermixed with smaller or larger amounts of extraneous minerals, the contamination being sometimes so great as to preclude the use of the ochre as pigment by reason of the high outlay required for extracting the colouring constituents.
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: