Flat c~eavage Buckli,ng The most common defect of paint is crackle. This is most certainly induced by mechanical stresses, but because we find crackle in paint on any support wemay conclude that the stresses to blame are inherent in the mechanical behaviour of paint. Similarly, cupped or curled paint islands, in which crackle edges are raised, are not restricted to paint supported by wood. These defects, therefore, may be dismissed from this discussion. Buckled paint-that is, paint islands which are elevated into a tiny tent-a defect commonly called a blister, is another matter. Buckling is only rarely found in paint on other supports; it is a characteristic defect of panel painting. So also is flat or blind cleavage. This is a separation, usually between paint layers and support, and sometimes of considerable area, in a plane parallel to the paint. Often blind cleavage is not visibly evident either by a crack or an elevation, but is detected by sounding. It is serious because under aggravation the condition may lead to a large loss of paint. Neither buckling nor flat cleavage can be explained in terms of the inherent behaviour of paint. An external agency must be responsible. ]f we take a typical cross-section through a panel painting, we have a layered structure of wood surmounted by several layers of paint. This is almost a textbook model for a study of shear stresses. The wood is an active material moving in a horizontal plane as it swells or shrinks. The paint, in another