Pigments used in artistic work vary much in permanence; not a few of the pictures painted by distinguished artists during the 18th and the first half of the 19th century are already more or less faded by the action of light or otherwise injured by impurities in the atmosphere of rooms. It is imprudent to expose any kind of artistic work in colours to direct sunlight or even, it would seem, to the light of electric arc lamps. The following refers to the durability of pigments employed in oil-painting, when continuously exposed to fairly strong daylight. Among blue colours, ultramarine, both real and artificial, is permanent; while Prussian blue is liable to some change, and indigo is fugitive. Among red colours, vermillion and the red ochres are perfectly durable; while the madder reds and purples can hardly be so thoroughly relied upon, and the carmine and crimson lakes, from cochineal, quickly give way. Among the yellows, raw sienna, yellow ochre, as well as the cadmium and Naples yellows, are quite stable; while Indian, chrome, and lemon yellows, and also aureolin, although less so, are yet fairly durable, but gamboge and yellow lake are not. Among greens, oxide of chromium undergoes no change, terre verte is practically permanent, and so also is emerald green, but it is blackened by contact with cadmium yellow. Among browns, burnt sienna, raw and burnt umber, cappagh brown, and Caledonian brown do not at all fade; but hardly as much can be said of madder brown, Cologne earth, and Vandyke brown, although these are fairly durable. Asphaltum or bitumen has a tendency to move on the canvas unless very carefully prepared, and its rich brown colour is not altogether permanent. Lampblack, ivory black, and charcoal black are quite durable, so also are flake white, zinc white, and baryta white. It is generally the case that a colour produced by a mixture of permanent pigments is also permanent, and it may be added here that flake white (white lead), so much used to mix with other colours for light tints, is liable to discolour when exposed to sulphuretted hydrogen (an occasional impurity in coal-gas), and this colour has also a tendency to tarnish when kept in the dark.
The above remarks on colours made up with oil apply equally to water-colour pigments, with a few exceptions. These are vermillion, especially if artificial, Naples yellow, chrome yellow, and madder brown, the permanency of which cannot be relied upon in Water-colours, under which head some further remarks on this subject will be found.
Fuller information about the pigments named above, as well as others, will be found under the heads ASPHALT, BLACK, BLUE, GREEN PIGMENTS, LAKES, OCHRES, PURPLE COLOURS, RED COLOURS, and YELLOW COLOURS. The oils and varnishes used as media for pigments, as well as the nature of the prepared canvas or paper used for painting upon, have all a bearing on the preservation of the colours of a picture. See the Chemistry of Paints and Painting, by A. H. Church (1890). The history of the introduction of the principal pigments is given at PAINTING, Vol. VII. p. 702.