The role of Rock Composition in the Deterioration of Wall Paintings, Saqqara Area, Egypt: Information from Petrography and Mineralogy

Abstract: Petrographical and mineralogical studies have been combined to decipher the causes of deterioration of the wall paintings inside the burial chambers of the rock tombs in the Saqqara area of Egypt, which are hewn within Upper Eocene (Maadi Formation) sediments. The samples were analyzed by polarizing microscope, X-rays diffraction (XRD) and thermal differential (DTA) techniques; along with acid insoluble residues (AIR) were examined using the binocular microscope. The studies reveal that the quality of the support of the wall paintings (the mother rock, or bedrock) in this area is particularly poor. It comprises two various types of lithotopes, marlstone and limestone (mostly argillaceous limestone), which are particularly susceptible to weathering processes in this region. The combination of rock texture, mineralogical composition and diagenetic processes contribute, to a great extent, to the deterioration recognized in the bedrock and in the preparatory plaster layer. The composition of the bedrock and plaster layer being too weak to resist the interaction between the exogenic (surrounding climate) and endogenic (related to the nature of the rock types) conditions over the thousands of years since their construction. The durability of the bedrock is mainly related to clay content and porosity. Furthermore, the obtained results will enhance the stones' conservation and development of suitable conservation/ restoration techniques