Permanent, Fugitive, And Alterable Pigments

By several different methods, data may be obtained which enable us to classify pigments - roughly, it is true - in accordance with their varying degrees of stability. Such data are derived partly from the known chemical and physical constitution of the various substances; partly from a study of old paintings and drawings in which they have been used; and partly from special experimental tests of permanency to which they have been subjected. Selections from these data are given in Chapters XX., XXIV., and XXVI., of the present work; but much additional information has been furnished by other trials, conducted by the author and other experimenters, for which space could not be found in this volume. Tables constructed from such data must not be regarded as affording exact values, but merely approximations. From some minute and often obscure cause differences of deportment, under exposure to hostile influences, will occasionally be observed in the case of two specimens of the same pigment having the same hue. And, further, the grouping of pigments into a small number of classes is a conventional and convenient arrangement which cannot accurately represent the numerous degrees of stability or instability which characterize the several pigments under discussion.

For when we leave the practically unalterable mineral pigments, we have to deal with a number of preparations which fall by irregular and often barely recog. nisable steps from the almost permanent to the hopelessly fugitive. One example of this difficulty in classification must suffice: aureolin is almost worthy of a place in Class I., Indian yellow scarcely deserves inclusion in Class II. The action of mixed pigments upon one another, though not as frequent as it is supposed to be, creates another difficulty in our classification, so also does the medium employed in painting, which may either protect an alterable pigment from change or aid in its destruction. In fact, each method of painting, if really distinct, requires a special classification of the pigments to be employed in carrying it out.

In the annexed classification, a limit of three orders of stability has been adopted, the first class including the practically permanent pigments; the second class those which, though liable to a variable measure of change, may yet generally be allowed; and the third class those which should be definitely excluded from the palette:

Classified Table Of Pigments For Oil-Painting

In order to adapt the foregoing classified table to water-colours, some changes and additions must be made. Flake white, Naples yellow (true), cadmium (pale), and vermilion (artificial), must be removed from the Class (I.) of permanent pigments and placed in Class III., to which also must be relegated several pigments from Class II., namely, chrome yellow, malachite, and madder brown. Of course, it should be clearly understood that no pigment belonging to Class III. should be employed in artistic painting. One satisfactory addition, and one only, can be made to Class I. in the table. Indian ink is a pigment available for water-colour painting, and when it is free from a brownish hue may be safely used. Bistre and sepia are likewise used only as water-colours, but they are both fugitive, and must be placed in Class III. Almost the same modifications of the table are required in the case of tempera-painting as in water-colour painting. With fresco-painting the exclusion of many more pigments is an absolute necessity, as they are completely ruined by caustic lime.

Not only are all the chromates inadmissible, as well as all the pigments which cannot be trusted as water-colours, but likewise Prussian blue and Antwerp blue, while the madder colours are much altered in hue when used in this process. In stereochromy the number of available pigments is still further reduced.