A STUDY OF THE DETERIORATION OF EGYPTIAN LIMESTONE SCULPTURE
S.M. Bradley, & A.P. Middleton
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.