| Abstrakt: | SUMMARIES. On ancient Egyptian artifacts, paint containing Egyptian blue pigment is often found to have become brownish green or even black. Darkened paints from several coffins have been investigated using gas-chromatography/mass-spectrometry (GC/MS) and other methods, to identify the cause of the discoloration. Different phenomena have been seen: one type of discoloration involved the browning of gum arabic throughout the paint layer; other examples showed surface discoloration in the varnish, the paint underneath being still bright blue. Surface dirt also contributes to darkening. Artificial aging of paints demonstrated that gum arabic binder can discolour sufficiently for paint to appear black. The poor hiding power and transparency of Egyptian blue contribute to the overall dark appearance.
CONCLUSIONS. The observations reported here suggest that the darkening of Egyptian blue-pigmented paint, which can range from relatively minor darkening of the paint to transformation to black, may be attributable to several distinct causes. There is certainly evidence that the straightforward deposition of dirt onto the surface of Egyptian blue paint may contribute to darkening. Sometimes it appears that this atmospheric dirt is held by a thin layer of calcium sulphate, analogous to the deposition of dark gypsum crusts on stone surfaces. The gypsum is probably formed by the reaction of atmospheric pollutant vapours with the calcite substrate that is used on many of the painted surfaces. However, it appears that the main cause of darkening on the objects studied is the darkening of organic materials used as binders and varnishes. In several instances the authors found direct evidence that binders and varnishes that were probably originally clear and colourless are now more or less brown. It proved possible to replicate the browning of gum arabic using accelerated aging tests, and the effect on the perceived colour of several Egyptian blue paints prepared in the laboratory was observed. These observations suggest that Egyptian blue paint is susceptible to colour modification, probably because of the transparency and relatively poor covering power of the pigment. When mixed with brown binder, the darkest types of Egyptian blue produce a black paint, but mixtures made with lighter blue pigment are not so dark and appear brownish green. Gum arabic has a reputation for stability [18] and has been used for thousands of years in a wide variety of paints; however, the browning reaction has not hitherto been commented on in the conservation literature. The reason for this is probably that the paints are usually made with pigments that have better hiding power and are more or less opaque, so that the browning of the binder affects the colour of the paint very little. In some accelerated aging tests there was apparently enhanced darkening of the gum caused by Egyptian blue, but these results were not reproducible. When enhanced darkening was observed it was not produced uniformly over the paint surface, suggesting that some local condition was responsible. It is possible that several factors may cause gum to darken faster than would normally be expected. One of these mechanisms is the catalytic effect of copper ions released from the Egyptian blue; however, no evidence has been found for gross degradation ot the cuprorivaite or other phases in the blue pigments. In all cases these have appeared 'fresh', with no indication of corrosion or the deposition of new phases, such as black copper oxide, tenorite (CuO). Other factors which may influence the rate of darkening are pH, impurities and changes in composition due to botanical source, climate, etc. Clearly, though, these conclusions are based on the observation of only a small number of coffins and papyri. Confirmation of the mechanisms of darkening suggested here would require the examination of a wider selection of painted artifacts on a range of substrates — perhaps the most notable omission from the present study is painted stone objects — and representing a wider range of dates. It seems quite possible that other mechanisms may be found for transformations in the colours perceived. Such studies have implications for our understanding ot the materials and techniques used to decorate these objects and for our perception and interpretation of the colour schemes employed by the artists of ancient Egypt, as well as for the proper cleaning and conservation of artifacts. Table 3 is included to summarize most likely reasons for the discoloration of the paints studied. |
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