The third explanation previously given as a reason for the superior durability of the colouring of the old over the later oil paintings is thus disposed of by him. He says:—
“We meet very often with the idea that the old masters had been in possession of colours, that is, pigments, the knowledge of which has been lost, and that this accounts principally for the difference between the oil paintings of the 15th and 16th centuries, on the one hand, and that of the 18th and 19th on the other. But this is a great mistake. We know perfectly well the pigments used by the old masters; we possess the same and a considerable number of new ones, good as well as bad, in addition.”
He adds, “In using the expression of good and bad, I am thinking principally of their durability. From this point of view the pigments can be placed under three headings:—
“1. Those that are durable in themselves and also agree well with the other pigments with which they have to be mixed.
“2. Such as when sufficiently isolated remain unaltered, but when in contact with certain other pigments change colour, or alter the others, or produce a reciprocal modification.
“3. Those which are so little durable that, even when isolated from other pigments, the mere contact of the vehicle, the air, or the light, makes them in time fade, darken, or disappear altogether.
“The old masters used without reserve only those belonging to the first of these three categories. For those belonging to the second they imposed on themselves certain limits and precautions. Those belonging to the third they did not use at all.
“That some of the modern masters have not followed these principles is not owing to a lost secret, but to the fact they disregarded those well known principles, and even consciously acted against them. In Sir Joshua Reynolds’ diary, for instance, we read that in order to produce certain tints of flesh, he mixed orpiment, carmine lake, and blue black together.
“Now, orpiment is one of the colours of the
second category, carmine lake one of the third. That is to say, orpiment, as long as it remains isolated, keeps its brilliant yellow or reddish orange colour; but when mixed with white lead it decomposes, because it consists of sulphur and arsenic, and it moreover blackens the white lead, because the sulphur combines with it. Carmine lake, even if left isolated, does not stand as an oil colour, and, therefore, has been superseded by madder lake.