In the preceding pages, a large number of white pigments has been enumerated. It would be easily possible to increase the list, but the result would be of no practical interest, for other pigments, neither in respect of quality nor price, can compete with the cheaper white pigments. Although white lead has at the present time an enormous use, it is to be hoped that this pigment, of good colour but little permanence, may be replaced entirely in the course of time by zinc white, and for some purposes by enamel white.
White pigments, in addition to their use alone, are employed to produce tints, which are obtained by mixing deeper colours with the white pigment. By adding the proper quantity of white to a colour it is possible to produce all paler shades of that colour. For example, the different varieties of the red lakes which are found in commerce are obtained by mixing white pigments with the deep red lakes.
The particular white pigment to be employed in these mixtures depends on the nature of the colour and on its specific gravity. It should always be remembered that white lead will not increase the permanence of a colour, since it will be discoloured in a short time by the action of the atmosphere. In the manufacture of fine colours for artists lead pigments should be absolutely excluded.
To produce paler shades of colours which contain lead and therefore have a high specific gravity, e.g., chrome yellow, white lead may be used; for other colours, barytes or zinc white should be employed. In the case of lakes enamel white is too heavy; zinc white or magnesia is suitable.
CHAPTER XI.
YELLOW MINERAL PIGMENTS.
As was the case with white pigments, so with yellow: of the large number known very few are in extensive use. In former times the number of yellow pigments employed in painting was far greater than at present; several, formerly in general use, have dropped out, partially or entirely, owing either to their poisonous character or to their replacement by others, deeper and more handsome. Especially since the discovery of cadmium yellow and the development of the manufacture of chrome yellows, many colours once in general use have properly fallen into disuse.
Again, unfortunately, the most important of the yellow mineral pigments contain lead, and have little stability; but a series of yellow colours free from lead is known, and though some of them are inferior in shade to the lead pigments they surpass the latter in permanence.
In addition to lead compounds, pigments derived from barium, zinc, antimony and cadmium are in general use. The yellow lead pigments were formerly preferred, and at present, so far as concerns beauty, they must be regarded as superior to other mineral yellows. The endeavour to provide the artist with permanent colours has resulted in the use of others, perhaps less brilliant, but very lasting. Under the name of chrome yellow a single pigment consisting of lead chromate was formerly understood; to-day, under this title is comprised a series of pigments containing zinc or barium in place of lead, but all commercially known as chrome yellow.