A STUDY OF THE DETERIORATION OF EGYPTIAN LIMESTONE SCULPTURE
S.M. Bradley, & A.P. Middleton
ABSTRACT—The decay of some Egyptian limestone sculptures during storage in a museum environment has been investigated using a variety of analytical techniques. These have included the chemical determination of soluble salts, gravimetric estimation and analysis by X-ray diffraction of the acid insoluble fraction, petrographic examination, observation and analysis in the scanning electron microscope and determination of porosity using mercury porosimetry. The results suggest that the mineralogy of the limestone, in particular the clay content, and the level and nature of soluble salts, particularly of soluble nitrates, are important factors in determining the nature and degree of any decay. The results also indicate that different types of limestone were used at different sites which accounts for the high proportion of badly deteriorated sculpture attributed to sites around Thebes and Abydos. This also suggests a possible technique for determining the provenance of Egyptian limestone objects.
THE BRITISH MUSEUM has an important collection of Egyptian limestone sculptures. Since 1966 this collection has been the subject of a programme of conservation which has included cleaning, consolidation and repair 1–2. A comparison of the present condition of the sculptures with photographs taken when they were added to the collection in the late nineteenth and early twentieth century showed that a small number of sculptures had undergone considerable deterioration in the intervening period (Figures 1 and 2). These sculptures have been stored and displayed in a variety of locations in the museum but, because of a lack of detailed environmental records, it is not possible to relate the degree of deterioration to conditions of storage or display.
Block Statue of “Hati” with two squatting figures of Anuris and Mehyt (EA 1726), attributed to Abydos or Thinis; photographed about 1920.
Same sculpture as Figure 1, photographed in about 1940.
Same sculpture as Figure 1, photographed in about 1983.
The adverse effect of changes in relative humidity (RH) and subsequent salt movement in contaminated stone have been well documented 3–7, and it is possible that the now badly deteriorated sculptures could have suffered most of this damage from exposure to unusually adverse conditions during or after their removal from the dry conditions of Egypt to the damp maritime climate of England. However it seemed unlikely that this was the only factor involved, since the majority of the sculptures in this collection are still in good condition. It was thought that the chemical and physical properties of the stone itself might render it more or less liable to decay. Factors other than salt content which have been identified as indicators of the durability of the stone include the nature and proportion of clay content 8–10, and pore size distribution, in particular the proportion of the micropores, less then 5 μm diameter 11–13.
To investigate the problem, a group of sculptures was selected for detailed examination and analysis; the sculptures included some which had suffered extensive deterioration in the Museum as well as some showing little or no evidence of decay. On examination of the Museum records it was found that the sculptures were attributed to three geographical areas: Thebes and Abydos; El Bersha; and sites around Giza and Saqqara, close to Cairo (Figure 3). The consistency of the analytical results within each geographical group (see below) suggests that the stated provenances of the objects are reliable. The pieces from Thebes/Abydos included those showing the most serious deterioration involving “structural decay,” in which the integrity of the stone is lost by splitting, fragmentation and flaking such as can be seen in Figure 1. This group of sculptures was identified as the “deteriorated” group. Many other sculptures from Thebes/Abydos show little evidence of decay and a number of these were selected to form a comparative group of “undeteriorated” sculptures from that area. All of the sculptures for El Bersha and from the Cairo area (Giza and Saqqara) show only minor deterioration, which is limited to “textural decay,” i.e. the surface of the sculpture shows localized pitting and powdering with some evidence of flaking but the underlying stone is sound, as illustrated in Figures 4 and 5. Details of all the sculptures examined are given in Table 1.
Map to show the attributed provenances of sculptures included in this study.
Sculpted head (EA 114), attributed to Gizeh.
Painted relief showing a procession of armed retainers (EA 1147), attributed to E1 Bersha.
TABLE 1 Details of Sculptures
The investigation involved in the analysis of soluble salts, the determination of the amounts of acid insoluble matter, X-ray diffraction analysis (XRD) of the acid insoluble matter, petrographic examination of polished thin sections, and scanning electron microscopy on polished thin sections and fracture surfaces. In addition, porosity determinations were made on a number of samples using mercury porosimetry.
TWO SAMPLES WERE TAKEN from each sculpture, one by drilling 50mm into the body of the stone using a 3mm diameter drill bit in a hand drill. The first 1mm of the drilling was discarded to avoid contamination. This sample was used for analysis of soluble salts and for the determination of acid insoluble matter. For the second sample, a small fragment of stone was removed from the underside or back of the sculpture. From this fragment a polished thin section for optical and scanning electron microscopy and also a fracture surface for examination in the SEM were prepared. When required, samples for mercury porosimetry were also obtained from these fragments.
2.1 Soluble salt analysis
A portion of the powder obtained by drilling the sculpture was weighed in to a 25ml volumetric flask and distilled water added to the mark. The sample was left to soak for at least 24 hours before the mixture was filtered. Soluble salt analysis was carried out on the aqueous extract using specific ion electrodes with a specific ion meter to measure soluble chloride and nitrate levels. The acidified barium chloride spot test was used to test for the presence of sulphate ions 14. The results of the chloride and nitrate analyses were corrected for variations in solution temperature and were calculated as weight/weight percent soluble salt to stone sample.
2.2 Acid insoluble matter determination
The proportion of acid insoluble matter was determined by weighing a portion of the drilling into a beaker and adding 1M hydrochloric acid. The mixture was left to react for one hour at ambient temperature and then filtered under vacuum through a pre-weighed Gelman DM450 membrane filter. The filter was allowed to dry to constant weight under ambient conditions. The weight/weight percent acid insoluble matter to stone sample was then calculated.
2.3 X-ray diffraction analysis
Acid insoluble residues and stone powder were analyzed by X-ray diffractometry (XRD) using nickel-filtered copper radiation with a Philips PW1050 goniometer and PW1710 controller. Samples of air dried, acid insoluble matters were analyzed directly on the membrane filters prepared as above (2.2.). A selection of these samples was also analyzed after exposure to ethylene glycol vapor (48 hours, 55°C) in order to determine whether the clays had expanded (see reference 19). Portions of some of the undigested stone powder samples were ground in an agate pestle and mortar, dispersed in distilled water in an ultrasonic tank and similarly deposited on membrane filters for analysis.
2.4 Petrographic examination
Polished thin sections were prepared from the fragments of stone taken from the sculptures and were examined using a Nikon Optiphot petrological microscope at 20–400x magnification. Uncovered polished thin sections were used rather than the normal covered sections to allow subsequent examination and analysis of the sections in the SEM.
2.5 Scanning electron microscopy and X-ray micro-analysis
Polished thin sections and fracture surfaces from the fragments sampled were examined in a JEOL JSM-840 scanning electron microscope (SEM) fitted with a Link Systems model 500, series II energy dispersive X-ray analyzer (EDXA). The samples were coated with carbon to make them conducting and to facilitate elemental analysis of features identified morphologically.
2.6 Mercury Porosimetry
Mercury porosimetry measurements were carried out by the Hydrology Research Institute. Small samples of stone, approximately 10mm × 5mm × 5 mm were taken from a total of seven sculptures - five from Thebes/Abydos (three “deteriorated,” two “undeteriorated”) and one each from Saqqara and Giza. The samples were dried at 60° C to a constant weight before being placed in the sample holder. The pressure in the sample holder was reduced to 1 PSI and mercury gradually introduced into the specimen with increasing pressure up to 35000 PSI. The intrusion of mercury into the specimen was followed and the data obtained used to produce curves of (i) mercury intrusion versus absolute pressure, and (ii) volume of mercury per gram of stone versus absolute pressure. Percentage mercury intrusion was calculated by taking the amount of mercury forced into the stone at 35000 PSI as 100% and calculating the amount of mercury present at all other pressures as a percentage of that weight. The work on pore size distribution has assumed that the pore system consists of open pores of circular section, the pore size distribution being the volumetric distribution of the pores expressed as a percentage of the total pore volume 15–16. The relationship between pore size and intrusion pressure is given by the formula,
where d is the diameter of the pore throat, p the intrusion pressure, θ the liquid solid/contact angle and σ the liquid-air surface tension. This formula was used to calculate the pore sizes from the intrusion data.
3.1 Soluble salt analysis
TESTS OF THE AQUEOUS extracts for the presence of sulphate ion were negative for all samples. Both chloride and nitrate were present in the “deteriorated” samples from Thebes/Abydos, but in several of the “undeteriorated” sculptures only chloride ions were detected, the level of nitrate ions being below the limit of detection (ca. 1ppm). Statistical analysis using a non-parametric test 17 showed that the nitrate levels tend to be significantly higher in the “deteriorated” group of sculptures but there was no statistical difference in chloride levels between the two groups.
The soluble salt levels determined for sculptures from the Cairo area are similar to those for the “undeteriorated” group from Thebes/Abydos. The sculptures from El Bersha were found to have very low levels of both chloride and nitrate. The results are given in Table 2.
TABLE 2 Summary of Analytical Results
3.2 Acid insoluble matter determination
The results of the acid insoluble matter determination are also given in Table 2. The samples from El Bersha have very low acid insoluble fractions whilst those from Cairo and Thebes/Abydos generally have high acid insoluble fractions, indicating differences in the mineralogy of the stones used at the various locations. Although the mean acid insoluble fraction level for the “undeteriorated” group from Thebes/Abydos is lower than for the “deteriorated” group of sculptures there is considerable overlap in the results and the two groups could not be distinguished by statistical tests.
3.3 Identification of acid insoluble fraction (XRD, EDXA)
The X-ray diffraction traces obtained on the acid insoluble fractions showed differences between the objects; three broad categories of residue were recognized. These appeared to correlate with the geographical provenance of the sculptures. The results are summarized in Table 2.
There was considerable similarity between the acid insoluble fractions obtained from the “undeteriorated” and “deteriorated” sculptures from Thebes/Abydos, and the XRD results indicated that they generally consist of a mixture of non-expanding clay and quartz. Analysis of the clay in the SEM, using EDXA, showed that in most samples it is a magnesium silicate, very low in aluminum, consistent with its identification as either a member of the sepiolite-palygorskite group or as a low aluminum smectite such as hectoraite or stevensite.19,20 The XRD data, particularly the apparently non-expanding nature of the clay, suggest that it is probably an aluminum-poor member of the sepiolote/palygorskite group, since smectite minerals would have been expected to expand on exposure to ethylene glycol vapor. In a few samples, those from EA 33275 and EA 636, the XRD traces indicated that the clay was not of this type, and analysis using EDXA showed that these clays contained aluminum, potassium and iron in addition to magnesium and silicon.
Examination of the XRD traces indicated that, in general, the quartz content of the acid insoluble fraction in the “undeteriorated” sculptures from Thebes/Anydos was higher than in residues from the “deteriorates” sculptures. This was tested for the samples from Thebes/Abydos (exception EA 805 for which insufficient sample was available) by determining the quarts peak areas (101 peak) and plotting values normalized for sample weight against the percentage of acid insoluble fraction in the limestone (Figure 6). Statistical analysis of these data, using a non-parametric test, confirmed the significance (at the 95% level) of this difference between the two groups of sculptures.
Graph of acid insoluble fraction (weight %) vs log (quartz peak area/weight of sample analyzed) for sculptures from Thebes/Abydos.
X-ray diffraction analysis was also carried out on stone powder from some of the sculptures from Thebes/Abydos. The traces obtained showed the presence of dolomite in the samples, and normalized peak area measurements (104) peak were obtained for dolomite, as described above for quartz. Although initial inspection of the values suggested that more dolomite was present in the samples from the “deteriorated” group of sculptures, subsequent statistical analysis did not confirm this. X-ray diffraction patterns from the acid insoluble fraction of the sculptures from Saqqara and Giza indicate that they consist mainly of opal-CT, a metastable form of silica,18 sometimes with minor amounts of quartz and/or clay. The very low acid insoluble fractions of the limestone from El Bersha were identified as being mainly quartz.
3.4 Petrographic examination
Petrographic Examination of the polished thin sections indicated that most of the samples are fine grained micritic limestones containing a few fossil fragments (fossilifereous micrites, 21), although some are highly fossiliferous biomicrites (Figure 7). In general it was not possible to observe the non-carbonate fraction in any of the sections in detail, although some EA 33275 from Thebes exhibits form birefringence due to the strongly developed preferred orientation of the clay platelets. The limestones from Thebes and Abydos generally show evidence for partial dolmitisation in the form of small rhombohedra of dolomite (Figure 8). No significant differences were observed between the “deteriorated” and “undeteriorated” groups of sculptures from Thebes/Abydos.
Optical photomicrograph (plane polarized light) of thin section of EA 636 (stela, attributed to Thebes), showing abundant microfossils in a fine grained matrix (biomicrite). The scale bar represents 0.1 mm.
Optical photomicrograph (plane polarized light) of thin section from EA 339 (stela, attributed to Abydos), showing euhedral rhombs of dolomite (some arrowed) in fine grained matrix (fossiliferous micrite). The scale bar represents 0.1 mm.
3.5 Scanning electron microscopy
The polished thin sections and fracture surfaces from the sculptures were examined in the SEM. Although in the fracture surfaces detail such as the presence of sodium chloride crystals, grain morphology and distribution and morphology of the non-carbonate fraction were visible, most useful information was derived from the polished thin sections.
The observations revealed significant differences between the sculptures, dividing them into three groups which correlate with the geographical provenance. The sculptures from Thebes/Abydos are characterized by the presence of subhedral to euhedral rhombohedra of dolomite in a fine grained micritic matrix of low magnesium calcite (Figure 9). In these samples porosity and the non-carbonate, clay fraction are apparently fairly uniformly distributed throughout the stone. In fracture surfaces the clay could be seen as coatings of flakey or fibrous material on the surface of the calcite grains. The sculptures from Giza and Saqqara have a radically different texture to those from Thebes/Abydos, with islands of relatively solid calcite separated by areas of higher porosity containing lepispheres of silica (opal-CT) (Figure 10). A similar texture was shown by the sculptures from El Bersha (Figure 11), but in these the acid insoluble fraction, present at very low levels, was not readily apparent in the SEM.
SEM photomicrograph (BEI) of polished section of block statue of “Hati” (EA 1726, attributed to Abydos or Thinis) showing fine grained fragmented texture with euhedral, rhombohedra of dolomite. The scale bar represents 10μm.
SEM photomicrograph (BEI) of polished section of standing figure (EA 35, attributed to Giza) showing solid “islands” of calcite and lephispheres of opal-CT (arrowed) in porous areas. The scale bar represents 10μm.
SEM photomicrograph (BEI) of polished section of painted relief (EA1150, attributed to E1 Bersha) showing solid “islands” of calcite with more porous, fragmented areas between. The scale bar represents 10μm
3.6 Mercury Porosimetry
Histograms showing pore size distributions of EA 1726 (Thebes, “deteriorated” group), EA 636 (Thebes, “undeteriorated” group) and EA 1011 (Giza) are presented in Figure 12. The data obtained from the determinations are summarized in Table 3. These results suggest that the “undeteriorated” sculptures from Thebes/Abydos have higher total porosity and also higher median pore throat sizes than the “deteriorated” group. Sculptures from the Cairo area have somewhat higher values still of porosity and median throat size. All of the sculptures tested were found to have a high proportion of micropores (i.e. those less than 5 μm in diameter).
Histograms showing pore size distributions for “deteriorated” and “undeteriorated” samples from Thebes/Abydos and a sample from Giza.
TABLE 3 Results of Mecury Porosimetry Determination
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.
THIS INVESTIGATION OF EGYPTIAN sculpture has demonstrated that a number of factors, especially in combination, may be important in determining the durability of limestone objects.
- A high proportion of acid insoluble material, which is predominantly clay would appear to render the stone liable to structural decay.
- Elevated levels of soluble salts, particularly nitrates, seem to be significant, especially in respect of the susceptibility of the sculptures to localized powdering and pitting.
- Porosity, in particular the proportion of micropores, may also be significant but in this study all of the objects for which porosity data are available have a high proportion of micropores (less than 5μm) and so, on the basis of established criteria 11–13 would be expected to be of low durability.
As noted above, the sculptures from Thebes/Abydos, because of the particular properties of the limestones used are most at risk amongst those included in this study. The deterioration of sculptures of this type, which are of low durability, can be minimized by controlling environmental conditions in storage and display areas. This technique has been used successfully on another collection of sculptures in the Museum which are now stored under conditions of constant relative humidity and temperature to avoid wet-dry cycles affecting clays and to avoid hydration-dehydration cycles of soluble salts,. Conditions recommended are RH 45% ± 2% and temperature 19 ± 1°C.26
An area of study which this investigation has incidentally touched upon is the determination of provenance for Egyptian limestone sculpture. An analytical approach, using neutron activation analysis, was described by Meyers and van Zelst25 and their results permitted a distinction between limestone sculptures from around Theses and those from further north. The present study has shown that several petrographically distinct types of limestone were used for Egyptian sculpture and that these can be characterized by several analytical techniques, especially X-ray diffraction and scanning electron microscopy. Possible implications for the determination of the provenance of Egyptian limestone, using these techniques, are being actively explored.
WE ARE GRATEFUL to several of our colleagues for their constructive comments and discussion throughout this project, especially Dr. I.C. Freestone, Dr. M.N. Leese, Mr. W.A. Oddy and Dr. M.S. Tite. We are also grateful to Dr. M. Price of the Hydrogeology Research Institute, British Geological Survey, Wallingford, England for organizing the pore size distribution measurements on our behalf, and Dr. H.F. Shaw of Imperial College, University of London for assistance with the interpretation of the XRD data. We would like to thank members of the Department of Conservation, Stone Section for their help in sampling the objects and Mr. T.G.H. James, Keepter of the Department of Egyptian Antiquities and his staff for their support and cooperation throughout this project.
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