Investigation of calcium migration across clay-cement interfaces in building bricks and other similar artifacts ranging in age from about 68 to 3,800 yr shows that calcium diffusion coefficients in the samples' clay layers are equivalent in all of the artifacts studied to date. Consequently, the age of the samples may be estimated from the distance that calcium migrated from the cement into the clay. To investigate the stability of clay barriers proposed for nuclear-waste containment systems, the rate of calcium diffusion from calcium-based materials (e.g., cement, plaster, and mortar) into adjacent clay-based materials (e.g., riverbed clay, mud brick, and fired clay brick) was determined from electron-microprobe analyses of Ca distribution in samples of known age. In seven samples with well-preserved clay-cement interfaces, an apparent Ca diffusion coefficient on the order of 10−20 m2/s was calculated from Ca distribution in the clay layer. The resulting calcium-concentration front migrates about 9 µm in 100 yr, and can be located to ±2 µm, corresponding to about ±25 yr error. Thus, although only preliminary, these results indicate that Ca diffusion may provide the basis for a rapid and relatively inexpensive method of dating building bricks and other similar materials that have hitherto defied conventional dating techniques.