Stability according to Winsor and Newton

In closing this chapter it may be useful to state that the pigments to which a place in our Class I. has been assigned have stood the very severe test of long exposure to direct sunlight. On a subsequent page it will be shown that this method of determining the stability of pigments is not in all cases a fair one, because changes brought about by such exposure may not occur at all when the temperature does not rise beyond a particular point, and when the radiant energy of light and actinism does not exceed a moderate measure of intensity. So far, then, as exposure to light is concerned, it may happen that some of the pigments in Class II. really deserve a higher position than that assigned to them in our table. In this connexion we may give some of the conclusions which Messrs. Winsor and Newton have published as to the stability of oil colours when exposed, not to sunshine, but to a strong north light. It will be noticed that the class of permanent pigments has been greatly enlarged as the result of the milder ordeal through which the materials have passed:

Class I. - Permanent

Class II - Moderately Permanent

Class III - Fugitive