454 THE ROLE OF CHEMISTRY IN PAINTINGS. 



If we cut from an oil painting a small strip about a millimeter wide 

 and a few millimeters long, we do not injure or detract from the value 

 of the painting, especially if the strip be from the edge, but we have 

 a sample sufficient for many microscopic preparations for observing 

 the layers in cross section. All the touches by the painter then 

 appear in parallel bands laid on in chronological order, and from 

 them we learn the nature, composition, and even the age of the 

 several layers. It will thus be easy, knowing the style of the master, 

 to determine whether a date, or a signature are apocryphal — that is, 

 whether or not they are between two layers of varnish and whether 

 or not the varnishes are of the same composition. 



The chemical questions that have to do with painting have been 

 carefully studied in recent years by a chemist of world-wide repu- 

 tation. Doctor Ostwald, of Leipzig, who is one of the founders of 

 physical chemistry as well as a talented painter. In collaboration 

 with A. Genthe he at first investigated the question of drying, and 

 showed that the oxidation of linseed oil which accompanies the dry- 

 ing process is not a normal chemical reaction. At first the process 

 of absorption of oxygen proceeds very slowly, then it increases, 

 attains a maximum, diminishes, and finally maintains a constant 

 value almost indefinitely. With a normal reaction this rapidity 

 would immediately assume a very great value, then quickly diminish. 

 It was understood that this divergence was due to the formation, 

 from the products of oxidation of the oil, of a catalytic substance — 

 that is to say, acting apparently by its mere presence, and after the 

 manner of a ferment, to produce reactions. The oxidized substances, 

 with which the linseed oil is left in contact or burned, has no other 

 effect than to give birth more rapidly to that substance, and the dry- 

 ing oil that is added in small quantities to colors to make them dry 

 more quickly contains notable proportions of it. 



Other experiments have proved that the autocatalyzation of lin- 

 seed oil is hastened by light ; in other words, that a painting dries 

 more quickly during the day and in the sunlight than during the 

 night and in darkness. It is moreover a fact well known to painters 

 that, other things being equal, the light paints dry more quickly 

 than the dark. This is due to the fact that the black pigment 

 absorbing the light hinders it from hastening the oxidization of the 

 oil. To hasten the drying of a picture it should be put in full sun- 

 light, but if it is to be kept fresh, j^ut it in darkness. 



CAUSES or THE DESTRUCTION OF PAINTINGS. 



To make an unalterable and durable painting, one must necessarily 

 know all the possible causes of alteration and destruction. These are 



