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 COz-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 C 0 2 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. Chem. 66, 2348Chem. 66, (1988. Les tombes tgyptiennes ii Beni Hasan (environ 2100 B.C.) ont developptes une couche superficielle grise qui dtfigure progressivement les surfaces peintes. Rtcemment, on a publiC une strie de spectres photo-tlectroniques des rayons-X effectuts sur des tchantillons provenant de ces tombes et on a proposC un mtcanisme pour expliquer la formation de ces couches superficielles. I1 implique de I'eau saturCe par du C 0 2 qui provoque une trosion graduelle des murales. Toutefois, on ne posstdait aucune donnte directe concernant cette trosion. Afin de verifier le mtcanisme propost, on a utilisC la microscopic lumineuse pour ttudier des sections droites des murales. Sous la surface, on a dkcouvert des surfaces ayant subi des solutions et prtcipitations rtgtCes, sous la forme de structures dendritiques et particulibrement dans les Cchantillons pigmentts avec du jaune. Dans les rtgions pigmenties en rouge, on n'a pas observe un tel degrt de dkgradation. On a aussi effectut une strie d'analyses par micro-sondes tlectroniques ii la surface et dans les regions sous la surface de petits fragments peints. Dans les Cchantillons pigmentks en jaune, on a observt sous la surface des regions poreuses et hkttrogbnes qui foment des poches de phosphates. Ces rtsultats sont en accord avec le mtcanisme pr...

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

“…However, the subsurface degradation associated with the surface layer formation was seen in only one red-pigmented sample (5), although not at the same depth as in the yellow samples. Furthermore, the red paint layer-substrate separation line can be seen both under the microscope and by the electron microprobe (5).…”

“…It is this ion migration that also led to eventual subsurface removal of slightly soluble material, mainly CaCO,, from the substrate upon which the murals were painted and the subsequent deposition of this material on the surface. The migration of this material appeared to be possible under the action of surface-condensed water (5).…”

“…Investigations of the grey degradation layer and its formation have been conducted (5,8) on a small collection of fragments, coloured with the yellow and the red iron-based pigments assembled bv the authors on site from the Beni Hasan tomb paintings. The first few nanometres of the surface deposit have been analyzed earlier by the X-ray photoelectron spectroscopy (8).…”

“…Although all these predictions could not have been verified from the XPS observations (8), some of them were subsequently confirmed' using light microscopy and electron microprobe analyses (5): painted fragments, examined in cross-section, showed some subsurface depletion and associated formation of dendritic structures within the substrate, below the painted layers (5). Such confirmation provides qualitative support for the proposed (8) mechanism; however, this work (8) provided no direct evidence for ion migration within the murals.…”

“…The degradation process which results in the subsurface depletion below the paint layer has been suspected also to act directly on the paint layers (5): in the depleted yellow-painted samples, degradation was so extensive that a clear demarcation of the paint layer could not be seen. However, the subsurface degradation associated with the surface layer formation was seen in only one red-pigmented sample (5), although not at the same depth as in the yellow samples.…”