A microstratigraphic investigation of iron ion migration in some ancient Egyptian wall paintings using PIXE Analysis

Abstract

The painted murals in the tombs at Beni Hasan (circa 2100 B.C.) are obscured by a rock-like grey surface layer. Its formation involves subsurface removal of slightly soluble calcite (CaCO3) under the action of surface-condensed water. To study the mechanism of surface layer formation and the effect of mural cleaning during recent restorations, the PIXE microprobe was used. Iron depth profiles in red-painted samples taken from the murals, both uncleaned and cleaned, have been obtained over depths of 80 μm (the cleaned sample) and 100 μm (the uncleaned sample). It has been found that the uncleaned sample has more Fe at all depths than does the cleaned sample and that portions of the uncleaned pigment appear to be pure Fe2O3. The shapes of depth profiles of Fe in the cleaned and uncleaned samples are similar to the shapes of diffusion curves; this finding is in agreement with the mechanism of surface layer formation proposed in our earlier papers. Apparent diffusion coefficients for the ion migration of Fe, DFe, in the limestone painting substrate have been determined, assuming constant temperature diffusion over 4 000 years: DFe(u) = (1.4 ± 0.5) × 10−16 cm2/s for the uncleaned sample and DFe(c) = (0.7 ± 0.4) × 10−16 cm2/s for the cleaned sample, where errors are given as the 95% C.I. based on standard deviations of the slope of the fitted curve. These coefficients are consistent with the mechanism of slow migration of Fe in the painting substrate in an arid environment. The difference between DFe values secured for the cleaned and uncleaned samples is attributed to a partial removal of the paint layer in the cleaned sample during the restoration efforts. Thus, if it is assumed that either 20 or 40 μm of paint layer was removed during HCl-cleaning of murals, DFe(c) = (1.0 ± 0.4) × 10−16 and DFe(c) = (1.4 ± 0.6) × 10−16 cm2/s, respectively, are obtained. Removal of the top 40 μm of the mural corresponds to elimination, during the restoration, of the order of 60% of the uncleaned paint layer. Key words: Egypt, PIXE, ancient murals deterioration.