In an article just published online in Environmental Geology [“Clay Swelling Mechanism in Clay-Bearing Sandstones”, T. Wangler, G. W. Scherer, Env. Geo. DOI 10.1007/s00254-008-1380-3], Tim demonstrates that the expansion of brownstone results from intracrystalline swelling of chlorite clay, which is present in the cementing phase of Portland Brownstone. It was unclear previously whether intercrystalline or osmotic effects might contribute to the measured swelling. It is the expansion of the clay that causes deterioration of Portland brownstone in buildings. In another paper (submitted to J. Materials Research), he shows that four layers of water enter the interlayer space. If the stone is treated with diaminoalkanes, only two layers of water can be absorbed, so the swelling is reduced by about a factor of two.