Changes in the relative humidity during the drying process affect both the ground layer and support of a painting, promoting crack propagation. Paintings involving hygroscopic materials like wood supports or gesso ground layers are especially susceptible to variations in relative humidity. Gesso is brittle at relative humidities (RH) below 75%; as RH increases, gesso becomes less stiff and transitions to a ductile state. Variations in RH cause highly non-uniform tensile strains across the gesso surface, and when the material contracts upon drying, it fractures. Craquelure formed during gesso drying are particularly noticeable.