Shrinkage stresses in art and conservation coatings based on synthetic polymers

The shrinkage stresses that are created in the application of modern commer cial and homemade art and conservation paints, media, and gesso based on sy nthetic polymers have been measured. Commercial art products tended to dry to essentially stress-free films. Coatings formulated with hard (high T-g o r Young's modulus) polymers, however, tended to dry with substantial shrink age stresses, and those stress levels increased with pigment loading up to the critical pigment volume concentration. In these tests, some films were produced having drying stresses that exceeded the cohesive or adhesive stre ngth of the coating. Even for coatings that remained intact after drying, r esidual stresses remained in the films, which would leave them at greater r isk of subsequent damage from additional applied stresses due to environmen tal changes or handling. Material analyses to determine the nature of the c oating resin or paint binder may provide some means of assessing the likeli hood of residual drying stresses being present, although stress measurement s on the particular materials found would be needed to estimate the magnitu de of those stresses. Treatments to relieve the stresses in coatings may be possible, but extensive testing on the artifact is necessary to determine the safety and efficacy of the method. Climate control and careful handling , to minimize additional stresses on objects that may be at risk, are the m ost prudent steps currently available.