One of the most frequent questions asked by rare book curators and librarians is: “At what RH, especially with respect to dry conditions, does a serious risk of irreversible mechanical stress occur?” Mechanical (physical) damage due to dryness or excessive dampness is the principal reason why special collection materials require controlled environmental conditions. For many years, recommendations have emphasized close control around a target of 45-55% RH. What is not well established from actual experimentation, however, are the practical limits where irreversible damage takes place. This area of research—safe limits for RH—has received considerable attention in the fine and decorative arts, but not for the complex and diverse mechanical structures of bound volumes. To overcome the difficulty of studying mechanical behavior of complex book structures IPI will employ a new technology, Digital Image Correlation (DIC) to dynamically assess expansion and contraction of composite objects.