468 OILS, ETC., USED IN THE PAINTING OF PICTUEES. 



previously described, which had twelve months before been coated 

 with yellow ocher and then, when only just dry on the surface, coated 

 with umber. After twenty-four hours in saturated air they were 

 kept some four or five hours in a freezing mixture. No cracks 

 developed, and sections showed all coats firmly adhering. 



While, then, the result of the experiments has failed to reveal the 

 cause of cracking in the pictures examined, considerable negative 

 evidence has been accumulated which should give us confidence in 

 modern methods of painting. Cracking seems to be due to an expan- 

 sion of the under layers (possibly due to the action of moisture and 

 frost), which shows in cracks when the top coat is hard either from 

 excess of pigment or from the presence of a medium like mastic. 



On the other hand, a sound canvas, properly sized and primed and 

 painted with pure oil and good oil varnishes, with the exclusion of 

 fugitive pigments, seems to withstand very severe treatment without 

 appreciable injury. 



There is also very little evidence to confirm the usual statements 

 about expanding surfaces when the paint is drying under normal 

 conditions. 



