Films of rabbit-skin glue size were found to contract at elevated relative humidities as a result of the recovery of strains formed during drying after preparation. The phenomenon is investigated at different relative humidities and associated with changes in molecular structure in the glass transition region of size. Conformational changes during gelation and drying are discussed. The viscoelastic recovery of a layer of size in biaxial restraint leads to in-plane movements believed to be responsible for development of a common pattern of craquelure on paintings.