THE KOLE OF CHEMISTRY IN PAINTINGS. 455 



many. They are, first, darkening of oils and varnishes; second, the 

 chemical action of pigments one on another; third, the diversity of 

 action of exterior agents on the layer of jDaint and on the support. 



In the long run all oils and varnishes resinify and turn brown. 

 The picture then becomes covered with a more or less opaque layer, 

 beneath which the original colors, even though remaining pure, 

 appear as if veiled in black It is to this action that the " museum 

 tone " of almost all old paintings is due. Frescoes are naturally 

 exempt from it. Le Jugeinent dernier^ by Michael Angelo, in the 

 Sistine Chapel of the Vatican is no exception to this rule ; its darken- 

 ing is of exterior origin ; it is due to the incense arising from the 

 alter which was formerly placed l)efore this fresco. As this brown- 

 ing or darkening is due to the oxidization of oil. Doctor Ostwald 

 reconnnends, to prevent this action, that the paintings be sealed be- 

 tween two jjieces of glass cemented at the edges. 



All painters know to-day the disastrous effect wdiich is produced 

 in the end by the use of a color which has a lead base (white lead, 

 for example) when mixed with another color containing sulphur, 

 such as vermilion or cadmium yellow. By double decomposition the 

 lead ultimately returns to its most stable form — that of the blaclc 

 sulphide, because the elements of that transformation have been fur- 

 nished to it. Confined air, which always contains a little hydrogen. 

 sulphate, produces, moreover, this same blackening of all colors with 

 a lead base. 



It is to these kinds of reactions that must be attributed "" les 

 repentirs," apparitions in the light parts of a painting, of subjects 

 which were afterwards covered over and form there a subjacent 

 layer. 



To avoid the difficulties of mixing, which are always to be feared, 

 some i^ainters prefer to obtain the effect of the fusion of colors by 

 their juxtaposition with light touches. Near at hand the effect is 

 often deplorable, but at a distance it is always very good if the 

 painter has a profound knowledge of the laws of optics. This 

 explains how that method, yet little understood, is much superior 

 to the other from a technical point of view, but it does not lend itself 

 easil}^ to all kinds of painting. 



The early painters knew the greater part of these actions, at least 

 m their effects, and took account of them. It is to them that we owe 

 the effects of climvoscuro^ the triumph of Rembrandt. Some people 

 believe that Rembrandt painted with a vivacity of color to shame our 

 most hardy impressionists, but he put on his finished picture a mix- 

 ture called " Rembrandt sauce," w^hich had the effect of darkening 

 all the tones. If we banish lead, we must also banish the use of 

 litharge for making a drying oil, for it always leaves some traces in 



