THE ROLE OF SEPIOLITE-PALYGORSKITE IN THE DECAY OF ANCIENT EGYPTIAN LIMESTONE SCULPTURES

An ancient Egyptian limestone sculpture was found to be undergoing maj or structural decay when stored in a museum environment. Mineralogical and petrographic analysis of the limestone showed a high proportion o f clay (greater than or equal to 10% by weight) that was concentrated along bedding planes. The clay fraction consisted mostly of sepiolite (>90%) and palygorskite (<10%). Minor quantities (less than or equal t o 1%) of soluble salts (NaCl and NaNO3) were also found. Wetting/dryin g with distilled water and relative humidity cycling resulted in the s ame delamination cracking damage as that observed in the museum enviro nment. Thermomechanical analyses (TMA) confirmed that the damage was d ue to expansion (>4.5%) parallel to bedding planes when the limestone was immersed in water. The expansion due to swelling of the clays was directly observed at high magnification in an environmental scanning e lectron microscope (ESEM) when wetting/drying cycles were performed. X -ray diffraction (XRD) analysis showed that crystalline swelling of se piolite occurred. This was determined by a shift of (IIO) reflection ( from 12.07 to 12.20 Angstrom) and a decrease of (060) reflection (4.47 Angstrom, to 4.44 and 4.41 Angstrom), when in contact with ethylene g lycol (EG) and dimethyl sulfoxide (DMSO), respectively. Swelling also occurred due to hydration of the clay surfaces and to electrostatic fo rces between clay particles, which, it was assumed, was promoted by th e presence of Na counterions in water solution. Possible treatments fo r the conservation of these artistic objects are proposed and discusse d.