Humidity-induced movements were studied on biaxially stretched samples of sized and gessoed fabric and on films of size and gesso without fabric. In-plane strains were detected through observation of crack patterns formed in a brittle coating applied on the samples. Different patterns recorded during drying and humidification were found to depend on tension, humidity range, and time. The explanation of results is based on viscoelastic and anisotropic properties of the size layer resulting from changes in molecular conformation brought about by drying in restraint.