A continuous threat: detection of unusual salt phases on the painted wall reliefs of Khonsu temple at Karnak complex, Egypt: a case study

Abstract

Salt damage is a serious threat to the ancient Egyptian structures in Upper Egypt. This study documents unusual salt phases on the painted wall reliefs of Khonsu temple at the Karnak complex. Optical and petrographic examinations allowed observing characteristic details of stone, plaster layers and salt encrustations. High resolution field emission scanning electron microscope (FE-SEM)-together with its microanalyzer- and X-ray diffraction analysis (XPD) successfully identified the microstructure and the crystalline mineral phases contained in the samples. The molecular observation on some samples was evaluated by Fourier transform infrared spectroscopy (FTIR). The results showed common phases of calcium sulphates (gypsum CaSO₄·2H₂O, bassanite CaSO₄·0.5H₂O and anhydrite CaSO₄), calcium carbonate (calcite polymorph, CaCO₃), and halite (NaCl). While, unusual salt phases of urea [CO(NH₂)₂] and archerite [H₂K(PO₄)] were reported for the first time in our case. For the two latter salt phases, the in-situ observations to the temple and interpretations of results allowed a possible hypothesis for their formation. The bats’ guano that accumulated on the temple walls was considered as a major reason. Thus, the uric acid produced by the bats’ urine causes a severe damage to the porous sandstone reliefs. Further, the other salt phases are highly attributed to the combination of weathering impacts.