Slow, progressive moisture expansion is a general feature of fired-clay ceramics. In masonry structures, it may produce cracking and must be allowed for in design. Expansion continues indefinitely although at a diminishing rate. Using long-term data from four sources and extending up to 58 years, we show that a (time)(1/4) power law model describes accurately the rate of expansion. The model is remarkable in having only two adjustable parameters, and provides a firm basis for predicting expansion in structural masonry. The results support also the use of the (time)(1/4) model to represent the kinetics of long-term mass gain in archaeological ceramics.