Electron microprobe studies of chemical reactions in ancient painted murals: The Beni Hasan Tombs, Egypt

Abstract

The Egyptian tombs at Beni Hasan (ca. 2100 B.C.) have developed an ubiquitous, progressive grey surface layer which disfigures their painted surfaces. Recently, we published a series of X-ray photoelectron spectra of samples from these tombs and proposed a mechanism for surface layer formation. It involves CO2-saturated condensed water which gradually erodes the mural substrate. However, no direct evidence for this erosion was available. To test the proposed mechanism, light microscopy was used to study cross-sections of the murals. Areas of repeated solution and precipitation, in the form of dendritic structures were discovered in the subsurface regions, especially in yellow pigmented samples. In red pigmented samples, this degradation is not seen to the same extent. A series of electron microprobe analyses of the surface and subsurface regions of small painted fragments also were performed. Porous, heterogenous subsurface regions with pockets of phosphates have been found, predominantly in the yellow samples. These findings support the mechanism proposed earlier. Pore size measurements, estimated by photographic measurements, indicate that subsurface alteration took place before the bulk of the surface deposit layer had formed and that the existing surface deposit protects, as well as obscures, the painted walls. The surface layer development described here has some aspects common to other natural systems, i.e. the interaction of CO2 in water with carbonate-bearing rocks and deposits. However, at Beni Hasan there is effectively no source of groundwater and the painted surfaces are sheltered from direct exposure to the environment. Therefore the Beni Hasan deterioration has unique mechanistic features.