Unfortunately some of the most brilliant colours are perishable to such a degree that they ought never to be used; yet it seems to me that just in one branch of art, in which of late remarkable progress has been made, I mean landscape painting, the artists, in order to obtain certain effects of colour not easy to be realised, do not always resist the temptation to make use of a number of pigments, the non-durability of which is proved beyond doubt.”

Another point which Dr Liebreich regards as of much more importance even than the selection and treatment of their pigments, and in which he says the old masters exercised great discretion, was the more sparing use of the vehicles and liquids they mixed with their colours.

He points out that there are certain pigments which when mixed with the oil impede its drying, whilst others there are which hasten it. “Supposing now,” he says, “we should add to each of the different pigments the same quantity of oil, the drying of it would progress at different rates. But in reality this difference is very greatly increased by the fact that the different pigments require very different quantities of oil, in order to be ground to the consistency requisite for painting.”

Pettenkofer quotes the following figures given to him by one of the colour manufacturers:—

100 parts(weight)White leadrequire12parts of oil.
””Zinc white”14”
””Green chrome”15”
””Chrome yellow”19”
””Vermilion”25”
””Light red”31”
””Madder lake”62”
””Yellow ochre”66”
””Light ochre”72”
””Camel’s brown”75”
””Brown manganese”87”
””Terre verte”100”
””Parisian blue”106”
””Burnt terre verte”112”
””Berlin blue”112”
””Ivory black”112”
””Cobalt”125”
””Florentine brown”150”
””Burnt terra sienna”181”
””Raw terra sienna”140”

According to this table, a hundred parts of the quick drying white lead are ground with twelve parts of oil, and on the other hand, slow-drying ivory black requires one hundred and twelve parts of oil.

It is very important that artists should have an exact knowledge of these matters. But it

seems to me that they are insufficiently known to most of them. All, of course, know perfectly how different the drying quality of different colours is. But that these different colours introduce into the picture so different a quantity of the oil, and how large the quantity is in the colours they buy, and, further, that the oil as well as the mediums or siccatives they add to dry the colours are gradually transformed into a caoutchouc-like opaque substance, which envelopes and darkens the pigments, and, moreover, that the oil undergoes, not in the beginning, but much later on, when it is already completely dry, changes of volume, and so impairs the continuity of the picture—all this is not sufficiently known. Otherwise, the custom of painting with the ordinary oil colours, to be bought at any colourman’s, would not have been going on for nearly a hundred years, in spite of all the clearly shown evil results—results due chiefly to the principal enemy of oil painting, that is to say, the oil.

A close optical examination and accurate study of the pictures of the French and English masters of the last hundred years have revealed to Dr Liebreich their principal defects, which he says are:—

1. Darkening of the opaque bright colours.