THE ROLE OF CLAYS IN THE DECAY OF ANCIENT EGYPTIAN LIMESTONE SCULPTURESCARLOS RODRIGUEZ-NAVARRO, ERIC HANSEN, EDUARDO SEBASTIAN, & WILLIAM S. GINELL
ABSTRACT—One type of Egyptian limestone from Naga el-Deir (Abydos/Thebes region) exhibits an ongoing problem of deterioration typified in the form of continued delamination of the surface in a stela from Naga el-Deir acquired from archaeological investigations carried out in the early 20th century. Previous testing of this limestone type indicated the presence of sodium chloride and sodium nitrate. The sculptures have been treated and desalinated either by immersion in water or by aqueous poulticing, but the decay process was not halted, and major loss of surface stone was still noticeable after storage for a period of years.Mineralogical and petrographic data (x-ray diffraction [XRD], scanning-electron microscopy [SEM], and optical microscopy) indicate that this stone has a high proportion of clays (up to 10% by weight). Laboratory tests suggest that the clays, concentrated along bedding planes, are largely responsible for the type of deterioration noted. The role of clay minerals in the decay of this type of limestone was demonstrated by performing a series of experiments, including wetting/drying cycles and relative humidity changes, thermomechanical analysis, and accelerated decay tests using water and ethylene glycol. One conclusion of this study is that in some instances, desalination procedures can induce more deterioration than can rigid environmental control. Another conclusion is that attribution of deterioration to the presence of salts may be insufficient, and further petrographic analysis should be initiated prior to desalination of clay-rich limestones. Unconventional methods for possible stabilization of the clay structure (by means of ion exchange and/or surfactant treatment) are also discussed. [Spanish Abstract] [French Abstract]
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