Pigments And Bases Are Subject To Deoxidation Or To A Loss Of Oxygen


in which case the colour is apt to deepen. Pigments generally are more

affected by oxidation and fading in a water vehicle, and by deoxidation

and darkening in one of oil.

A principal test of permanency in pigments is the impunity with which

they bear exposure to light and air, an artistic proof of their

stability the mere chemist is apt to neglect. Provided the colour remain

unaffected by sulphuretted hy
rogen, &c., he seldom hesitates to

pronounce it safe. But a pigment may be fast in one sense and fugitive

in another, believed in by the laboratory, and found wanting by the

studio. It has happened before now that the same colour has been dubbed

durable and the reverse, by the man of science and the man of art. The

former, we take it, looks upon a pigment as a coloured substance of a

certain composition, possessing maybe an acid and a base, either, or

neither, or both of which, gases and other reagents may injure or

destroy. The latter views a colour chiefly as part and parcel of his