A STUDY OF THE DETERIORATION OF EGYPTIAN LIMESTONE SCULPTURE
S.M. Bradley, & A.P. Middleton
THE RESULTS OF THIS STUDY indicate that the chemical and physical properties of the various limestones used for the sculptures have exerted a strong influence upon the degree and mode of their subsequent decay in museum storage. A major factor leading to decay appears to be the presence of a relatively high proportion of acid insoluble materials, which consists in large part of clay (see Table 2). The deleterious effects of high levels of acid insoluble material have been recognized for some time, 1–2 and the effects of clay in particular on stone strength and durability have also been noted. 8–10,13
The results of the present investigation indicate that the mineralogical character of the acid insoluble fraction is of crucial importance in determining the durability of the stone. Thus, although the mean acid insoluble content of the relatively well preserved sculptures from Giza and Saqqara is high (x¯ = 5.8%), it is present as silica (opal-CT), which is likely to be a more stable component than those found in the sculptures from Thebes/Abydos. Another factor which appears to be significant is the presence of high levels of soluble salts, especially when these include a high proportion of nitrates. There is considerable evidence for the harmful effects of soluble salts on stone stability (e.g. 3–7) and published work 22,23 also suggests that nitrates in particular can be highly deleterious. This is because they are deliquescent at low relative humidities (e.g. sodium nitrate goes into solution at an RH in excess of 53%). Thus, following their initial crystallization, some salts may undergo repeated dissolution/recystallization cycles in response to changes in relative humidity, thus in turn inducing more damage in the stone. Sodium chloride is only deliquescent at higher relative humidity (75%) and has been found to be generally less damaging than the nitrate containing salts, although the degree of damage which it can cause varies from powdering to delamination, depending of the crystal habit, which in turn depends of the environmental conditions under which the salt forms.
The importance of high levels of nitrate in rendering the stone susceptible to decay is indicated in the present results for the “deteriorated” and “undeteriorated” sculptures from Thebes/Abydos. All of the objects studied contain high levels of soluble chlorides, above the 0.1% threshold of stability established by Oddy et al., 1 and the “deteriorated” and “undeteriorated” groups are indistinguishable statistically. However, as noted above (section 3.1), statistical analysis of the results for soluble nitrate shows that, on average, the “deteriorated” group have significantly higher levels than the “undeteriorated” group.
A third factor which appears to be significant is the petrographic character of the limestone. Those from Thebes/Abydos have textures in which the calcite is highly fragmented, occurring as dispersed grains and aggregates separated by fine grained clay and open pores, providing easy access for moisture, as well as greater surface area for chemical reaction. These rocks also contain subhedral-euhedral crystals of dolomite but the significance of these to the decay process is not known. In contrast, the better preserved sculptures from Cairo and from El Bersha exhibit less fragmented textures, with solid islands of calcite separated by regions of higher porosity (see section 3.5).
It is thus possible to identify for each of the three groups of sculptures particular features which have influenced their mode and extent of the decay.
4.1 The Thebes/Abydos group
This group of sculptures includes those which exhibit the most severe decay. This seems to be because of the combination of several adverse features, inherent in the stone, viz. high acid insoluble fractions which are predominantly of clay; high soluble salt contents which are high in nitrate; and a highly fragmented petrographic texture.
Reasons for the observed differences in condition between the “deteriorated” and “undeteriorated” sculptures from this area are less easy to pinpoint. One factor, concerning which we have no information, is the past conditions of storage of the objects; it may be, at least in some cases, that the present condition of the sculptures reflects their previous storage history. However, the analytical results do suggest that several other factors may also be significant, and it appears likely that a combination of high soluble nitrate content, high clay content, and high microporosity, perhaps exacerbated by adverse environmental conditions, has resulted in the advanced deterioration observed in some samples.
Observation of the chemical and physical properties of the stone may thus be useful in identifying those sculptures from Thebes/Abydos which are most at risk. The statistical technique of discriminant analysis 24 would be well suited to this purpose. This technique would have the particular advantage that it could be adapted to include information on storage, as well as scientific data, in an index of risk. Such an approach would however require a much larger database than is available from the present study, in order to obtain the best estimates of the various parameters of the index, and also to assess its reliability as a predictor.
4.2 The Giza/Saqqara group
The decay pattern observed in the sculptures from around Cairo (Giza and Saqqara) is one of pitting, powdering and some flaking, the body of the stone being firm and undeteriorated. The relative soundness of these sculptures is thought to be attributable to several factors, including the presence of only low levels of nitrate, despite relatively high levels of chloride (see Table 2); and a relatively solid, unfragmented petrographic texture. Surface powdering of these pieces has perhaps been caused by the movement of soluble salts close to the surface of the sculptures, the minor pitting observed may result from localized chemical attack in the regions of higher porosity between the more solid islands of calcite.
4.3 The El Bersha group
The sculptures from El Bersha, which show only minor pitting are very pure calcite limestones with low acid insoluble fractions and low soluble salts and they therefore fit well with previously established criteria for limestone durability. 1,2 As for the Giza/Saqqara sculptures, the minor pitting which is observed probably results from localized chemical attack in the regions of higher porosity between the more solid islands of calcite.