152 CASEIN. 



ing of chalk and driers, coloured grey by the addition of a 

 dark pigment. The use of driers, however, cannot be recom- 

 mended ; and, indeed, it is probably owing to their employ- 

 ment that so many paintings are found to crack within a few 

 years' time. 



When the canvas is primed with a quick-drying varnish 

 the prepared surface appears to be perfectly dry and suitable 

 for painting on at the end of a few days, since good driers 

 set hard very quickly on exposure to the air. This hardness, 

 however, is merely superficial, the under layers of the com- 

 position remaining soft for a very long time and are only 

 resinified very slowly. When this resinification has reached 

 a certain stage the stratum of varnish contracts strongly and 

 invariably cracks. 



The pigments of the painting, which are firmly attached 

 to the priming, are constrained to follow this contraction ; 

 and this phenomenon is first manifested by the disappear- 

 ance of the gloss on the picture. Examination of such a 

 picture with a strong magnifying-glass reveals the presence 

 of innumerable fine cracks all over the surface ; and the end 

 of the picture can be predicted with certainty, namely, that 

 it will eventually crack to such an extent as to become quite 

 indistinct and worthless as a work of art. 



There is, moreover, another very potent reason against 

 the use of varnishes in preparing artists' canvas, namely, 

 that many varnishes are made by the aid of lead prepara- 

 tions. It has already been remarked on several occasions 

 that lead preparations are particularly susceptible to the 

 action of sulphuretted hydrogen, which turns them black; 

 and consequently paintings on such canvases will become 

 darker coloured in course of time. 



Many artists' colours in general use, such as cadmium 

 yellow (Jaune brilliant) and vermilion, are, however, com- 

 pounds of sulphur. When such sulphur compounds come 



