Understanding the hygral reactivity of ceramic materials is essential to understanding the long-term behavior of building materials and of archeological pottery, especially in relation to dating. We explore the literature on the expansion of fired-clay ceramics, reviewing strain and mass measurements at the bulk scale, and rehydroxylation (RHX) and dehydroxylation (DHX) processes in clay minerals at the molecular level. We present open questions on the nature of ceramic rehydroxylation and its apparent adherence to a sub-diffusive kinetic (time)1/4 power law. We discuss measurement of the RHX process through mass gain in relation to a proposed new dating method for archaeological ceramics.