2. Fading of the transparent brilliant colours.
3. Darkening, and above all, cracking of the transparent dark colours. He states that these cracks are so characteristic and distinctive of the pictures of this period that they might be used as a test as to whether or not a picture really belonged to this school, or was only a copy.
This peculiar cracking in the paint is, according to Dr Liebreich, particularly observable in Guericault’s ‘Wreck of the Medusa’ in the Louvre, and also in Ingres’ ‘Portrait of Cherubini,’ and as the same defect is not to be seen in the works of the Dutch and Italian artists, the very rational inference to be drawn is that the methods followed by these schools were sounder than those adopted by their English and French successors. Dr Liebreich believes the cracks were owing to the practice of painting over one colour with another before the first was perfectly dry.
“The study of the alterations,” says Dr Liebreich “already fully developed within the last hundred years only, and their comparison with the works of the old masters would suggest the following rules for the process of painting:—
“1. That the oil should in all colours be reduced to a minimum, and under no form should more of it than absolutely necessary be introduced into a picture.
“2. All transparent colours which dry very slowly should be ground, not with oil at all, but with a resinous vehicle.
“3. No colour should be put on any part of a picture which is not yet perfectly dry, and above all, never a quick-drying colour upon a
slowly-drying one which is not yet perfectly dry.
“4. White and other quick-drying opaque colours may be put on thickly. On the contrary, transparent and slowly-drying colours should always be put on in thin layers. If the effect of a thick layer of these latter is required, it must be produced by laying one thin layer over another, taking care to have one completely dry before the next is laid on. If transparent colours are mixed with sufficient quantity of white lead they may be treated like opaque ones.”
Dr Liebreich concludes his interesting lecture with some judicious advice on the subject of picture cleaning, and points out that, since different pictures require to be differently operated upon, all universal agents and methods suggested for the purpose are open to suspicion and should be discarded.