Examination, Conservation and Analysis of a Gilded Egyptian Bronze Osiris

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Examination, conservation and analysis of a gilded Egyptian bronze Osiris

Article in Journal of Cultural Heritage · October 2002 Impact Factor: 1.57 · DOI: 10.1016/S1296-2074(02)01238-4

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Examination, conservation and analysis of a gilded Egyptian bronze Osiris David A. Scott a,*, Lynn Swartz Dodd b

aThe Getty Conservation Institute, Museum Research Laboratory, 1200 Getty Center Drive, Los Angeles, CA, 90049 - 1684, USA bUniversity of Southern California, Taper Hall of the Humanities, 328 MCO 355, Archaeological Research Collection, Los Angeles, CA, 90089-0355, USA

Abstract

A heavily corroded Egyptian bronze figurine of the god Osiris was examined and shown to have been originally gilt with gold leaf and inlaid with blue glass. Detailed formal comparison between this Osiris figure and the known corpus of bronze and stone sculpture leads to the inference that the statuette dates to the time between the Third Intermediate Period and the fourth century BC, with a greater probability of originating from the Third Intermediate Period through to the 26th Dynasty, or even possibly as late as the fourth century on the basis of stylistic similarities. An extensive corrosion crust of atacamite and chalconatronite completely obscures inlaid glass decoration, found during the investigation, together with remnants of a gilded surface. Analysis of the glass by electron microprobe showed a composition consistent with early Egyptian blue glass with high sodium oxide and low potassium oxide content. The solid cast bronze is a leaded tin bronze, and the gold is a gold foil applied to the bronze surface, originally alternating in decoration with the blue glass. The chalconatronite and atacamite patina appear to be closely associated in the development of the unusual but extensive chalconatronite crust that now covers part of the surface, as a natural corrosion process in this case, not derived from subsequent conservation treatment. The loss of the light blue corrosion crust was prevented by consolidation with Paraloid B72, as examination over several months showed no sign of continued chemical instability. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.

1. Research aims 2. Description

The purpose of the present paper is to undertake a The bronze Osiris now appears completely covered in a detailed scientific investigation of the corrosive deteriora- thick, green corrosion crust with lighter blue patches. The tion and morphological characteristics of an Egyptian principally verdant colour of the once-gilded God is quite bronze Osiris in the Archaeological Research Collection of appropriate for his symbolism as a god of regeneration in the School of Religion, University of Southern California the afterlife. The Papyrus of Ani from 1250 BC in the (USC 5047). The bronze appears to be actively corroding British Museum [1] illustrates a green Osiris, enthroned, with the continual loss of small fragments of light blue sitting in judgement over the dead. However appropriate corrosion from the heavily mineralized surface. As the this modern colour may be, this Osiris figure would not have bronze is of unknown provenience, it is also important to appeared green originally. This figurine was gilded and evaluate the technical art historical background of the details of the beard and royal insignia were highlighted with Osiris, which is shown in Figs. 1 and 2, with some detail of inset blue glass, as reconstructed in Fig. 5. The eyebrows the remaining gilding and a closer view of part of the may also have been inlaid. The Osiris figurine is, as usual, corrosion crust in Figs. 3 and 4. The requirements for depicted with his arms and legs bound to his body by conservation of the bronze are also assessed in the context mummy bandages. Osiris is customarily shown mummi- of this research. form, and when colour is indicated, he is seen wrapped in white cloth as would have been the case when a body was prepared for burial during mummification. The bronze figure shown here holds the crook and the flail, symbols of royalty, and also wears the white crown of Upper Egypt, the * Corresponding author. E-mail address: [email protected] (D. Scott). southern portion of the country. Visual examination does not

Fig. 1. Gilded bronze Osiris (USC 5047) from the collections of the University of Southern California. Frontal view. Copyright Bruce Zucker- man, West Semitic Research Collection. Scale in cm. Height 255.1 mm. reveal any difference in the corrosion on the sides of the crown, where fittings for feathers might have been attached, and so we assume that the crown was simply the White Crown and that no atef feathers were affixed in antiquity. Had the gilding of this ancient bronze Osiris figurine remained intact, he would have been entirely gilded (with blue inlay) as an example from the Bastis collection, shown in Fig. 6 illustrates. The ancient Egyptians referred to the flesh of their gods as gold, cf. The Destruction of Mankind/Myth of the Heavenly Cow: Haw = f m nbw “…his Fig. 2. Gilded bronze Osiris (USC 5047) from the collections of the body (was) as/like gold...” [2]. Gilding was used not only on University of Southern California. Side view. Copyright Bruce Zuckerman, metal but also on wood as examples from the Brooklyn West Semitic Research Collection. Scale in cm. Height 255.1 mm. Museum and the Pelizaeus-Museum, Hildesheim ([3], Fig. 68) illustrate. Gilding may have been used to create contrast detached flakes within the voluminous corrosion crust. This with the bronze material that might have had a duller glow corrosion crust is primarily a thick, dark green, mineralized than the gold in its original cast state, or may have been layer incorporating rounded quartz grains on the exterior polished to a high sheen as was the case with mirrors, or surface, together with chalky-blue patches of corrosion that special alloys may have been subjected to a surface treat- protrude beyond the dark green patina. ment, producing “black bronze”. Many objects are not During exploratory mechanical cleaning of a small re- entirely gilt, instead the gold is used as a partial decoration, gion of the surface near the folded arms, we found the leaving other parts of the surface free for glass inlay or remains of blue glass inlay that were buried beneath the chasing [4]. thick green corrosion crust. Following this discovery, care- Figure 4 shows that the gilding is especially well ful examination showed that the blue glass is part of the preserved between the crossed arms and is present as original decoration of the arms and also occurs in the beard. D. Scott, L. Dodd / Journal of Cultural Heritage 3 (2002) 333–345 335

Fig. 5. Gilded bronze Osiris (USC 5047) reconstruction sketch of left side profile and front. Drawing not to scale: left profile approximately same as Fig. 3. Gilded bronze Osiris (USC 5047) View of the back with extensive object. alteration to chalconatronite at the base and at the shoulders. Scale in cm. investigative cleaning in search of further remnants of glass There are instances of white (travertine limestone) and inlays would damage the object so that it is difficult to say black (copper-rich metal) inlays ([5], plate 25; §206f) used with surety whether the eyebrows were once inlaid. It is in the decoration of Osiris’s crook and flail and rarely red or very likely that the now-vacant eyes were once inlaid and green inlays ([3], fig 78) are found; the preserved glass on they may also have been lined with blue glass or black inlay. the USC Osiris is uniformly blue. Glass still remains in the The dark green corrosion crust, overlying cuprite, is beard and in the crook and flail. The eyebrows were incised cracked in many areas of the surface, revealing further green but the area is now so corroded and fragile that any layers, approximately 1.5 mm below, and with cuprite below that. The fissures that have opened up in this dark green layer suggest periodic hydration and dehydration of

Fig. 4. Detailed view of the region between the crossed arms which retains remnants of the gilded surface. Part of the corrosion crust can be seen which comprises dark green atacamite and light blue chalconatronite, with Fig. 6. Osiris. Bronze fragment of head with inlaid eyes. Height 4.2 cm, occasional cuprite pustules, which have formed over the atacamite and width 4.0 cm and depth 2.4 cm. Stylistically and technically dated to the chalconatronite surface. Magnification × 110. Third Intermediate Period. 336 D. Scott, L. Dodd / Journal of Cultural Heritage 3 (2002) 333–345 the corrosion crust with longitudinal cracking, a feature dates to the Third Intermediate Period or slightly later in the which is not uncommon in bronzes which have suffered Late Period. It is possible to attribute the latter date for this from very severe corrosion during burial, commonly asso- figurine, based on the technical examination of the evi- ciated with chlorides in the soil. Overlying this crust, and dence, including the propensity for Third Intermediate intercalated with it, is the chalky-light blue corrosion crust Period and Late Period (especially 25th and 26th Dynasty) that locally varies in colour from very pale to deep blue, also archaism, incorporating the use of models from the New incorporating quartz grains and other soil minerals. Kingdom. Thus the probable date of this figurine is between In some regions, dark red-brown to black pustules erupt the late 11th and 4th centuries BC. from this green surface and occur as isolated excrescences In seeking to specify the particular dynastic context in within that layer. There is a continual loss of small miner- which this Osiris was created, the problem of specificity alized particles from the surface, but no sign of light blue, looms larger, even allowing for the obfuscation of the powdery, “bronze disease” corrosion processes. features due to corrosion. The numerous bronze figurines The current weight of the Osiris is 599.3 g; 255.1 mm in found in the sacred lake at Karnak, those from a temple of length, 74.07 mm across the shoulders and 40 mm from the Nectanebo II in North Saqqara and the more recent finds at back to the outer edge of the folded arms. Gilded and inlaid the Persian temple of Douch in the el-Kharga Oasis have bronze figurines were fashioned in a variety of sizes and given researchers an indication of the range of facial and styles, from small to large. The size of the present figurine formal types present in a single deposit. Both the stylistic has numerous archaeological parallels, including examples and material analyses of comparable figurines are compli- discovered in what was possibly a mudbrick temple to cated, in the case of stylistic analysis, by the variations in Osiris at Douch in the Kharga oasis of the Persian Period, postures dependent on the place of manufacture within [6] and a kneeling statuette of a king holding a wedjat eye Egypt and, in the case of material analysis, by composi- found in Saqqara of the Late Period [4]. The size of a wood, tional variations at any given time according to the subject gilt and bronze figure of Amun, 48 cm high, has led some to portrayed or according to the technique of manufacture or speculate that it may have been the focus of a small source of materials used. sanctuary, as was the case in North Saqqara [7]. Indeed, it The formal characteristics of this Osiris include several has been suggested by Ogden [8] that these figurines may features that help to situate it stylistically. The body is come from temples rather than tombs. This hypothesis is modelled with some attention to the form beneath the supported by the horde of bronze figurines found in the mummy bindings. For instance in the rear, the bulk of the Karnak lake when it was drained; by the cache of Osiride buttocks is visible and to a certain extent, the shape of the figurines from North Saqqara and by the recent discovery of legs can be discerned as well. The attention to the shape of numerous figurines at the Persian period temple in Douch the body is restrained, and affords only a minimal outline of and probably dating from the mid 27th to early 29th the calves and thighs, shown as swellings on the outer sides Dynasty. of the lower body. A similar body shape on a Late Period Osiris (broken) was recently found in Karnak [12]. The feet and lower legs of the USC Osiris are missing, as is the case 3. Technical archaeological studies with many Osiris figurines, including a Late Period example from the Frankfurt Am Main Museum [13]. The extensive corrosion of this bronze Osiris presents a The neck of the USC Osiris is compact. Figs. 2 and 3 challenge to formal analysis. Even without corrosion, sty- show that the shoulders curve naturally forward to join the listic and iconographic analyses of small bronze figurines base of the neck beneath the crown. This forms a contrast to are not without complications. Roeder’s [5,9] seminal many Osiris figurines where a sharp hunchback appearance studies form a foundation for technical art studies. Consid- can be created by the extension of the cloth worn by the god erable progress has been made in defining a stylistic canon, out from the body. Examples of Osiride statues without this especially for certain narrowly defined categories of sculp- ridge are known, including the preceding example, and ture. Recent technical and formal approaches, such as others [14]. The short, thick neck of the USC figurine joins Josephson’s [10] monograph on royal fourth century por- the back in a smooth line reminiscent of the Osiris figure in traiture and Griffen’s work [11] on gilding, manufacture, Hannover ([5], abb. 201). This figure also wears the white and materials, offer analytic footholds. crown but with the addition of atef feathers. Fig. 7 shows an Due to the massive corrosive deterioration, details of example more distant in sculptural genre that is very close particular features, and in some cases, the shape or model- to the posture of the USC Osiris, a statue of Amasis, of the ling of the features, is in doubt. There are a few formal 26th Dynasty. parallels between the USC Osiris figure and those from the The USC Osiris’s eyes in their present damaged state are early to mid 18th Dynasty, a period to which few bronze somewhat difficult to reconstruct with certainty. It is rea- figurines are conclusively dated, but there are more signifi- sonable to conjecture that they were originally inlaid and cant parallels found from the Third Intermediate Period may have resembled the large bronze head of a fourth through to the fourth century BC. We suspect that this Osiris century king from the Pelizaeus-Museum ([3], Fig 87–8) or D. Scott, L. Dodd / Journal of Cultural Heritage 3 (2002) 333–345 337

Fig. 8. Shabako relief block reused in the Osireion of Taharqa, Karnak. Fig. 7. Statuette of Amasis. 26th Dynasty. Height: 22.5 cm. Provenience 25th Dynasty. Height 32.5 cm. Sandstone. (Plate XXXI in Mysliwiec, unknown, now in Copenhagen National Museum Inv no 3603. Limestone. Karol. Royal portraiture of the dynasties XXI–XXX. Mainz am Rhein: P. (Pl. LXIV in Mysliwiec, Karol. Royal portraiture of the dynasties von Zabern, c1988.). XXI–XXX. Mainz am Rhein: P. von Zabern, c1988.). In this case, the feature may be seen as an archaizing tendency of a later period. the charming face from the Bastis collection ([15], Fig. 9) or The nose on the USC Osiris begins at the level below the the kneeling figure of Thutmosis IV ([16], 128). Unlike the eyeball and descends in a pronounced curve until the nose fourth century examples, the eyes of the USC Osiris have a rejoins the face, a feature of the distinctive noses crafted by lower lid that is nearly horizontal and the shape of the eye 18th Dynasty artists sculpting for Amenhotep I and Thuth- is articulated by the vertical lift of the curve that defines the mosis III, as well as Hatshepsut. In these cases, the upper lid. These eyes are not almond-shaped as is common pronounced nose profile is similar to the nose of the USC during the reigns of Thuthmosis II and IV [17] and on Osiris but the 18th and 19th Dynasty noses all originate Ptolemaic and Ramesside representations as well. The eyes between the eyebrows, whereas the USC Osiris nose starts of the USC Osiris also do not slant downward at the inner much lower. The USC Osiris’s nose is fairly thin above its canthus, another common feature of Ptolemaic faces. The root between the eyes but it widens once it springs outward space between the eyes is fairly wide, equivalent to slightly from the face near the bottom of the eye. The widening less than the width of one of the eyes. The eyebrows and the continues through the nostril area, a feature common on eyeliner are both articulated as incised lines and these many Late Period figures, as well as in portraits of Thut- parallel lines tilt downward toward the ear, rather than mosis IV, to offer but one earlier example. extending straight out horizontally. The eyebrows do not Even amid the corrosion, it is possible to see the curve of meet in the middle. Despite the corrosion, or perhaps the side of the nostril as it meets the cheek. This well- because of it, it is possible to see a suggestion of a flat defined feature has parallels in some of the relief sculpture inverted triangular space between the eyebrows and this of Shabako and Taharqa from Karnak, such as Fig. 8. could be construed as a stylistic similarity to fourth century The mouth is approximately the width of the nostrils and royal portrait sculpture [10], or as reminiscent of the is curved slightly upward, almost in the manner of the portraiture of Thutmosis IV where the area between the Ptolemaic smile, but to a lesser degree. The two lips are of brows is flat and the nose root is low in relation to the eyes. equal thickness and the outer corners of the mouth are 338 D. Scott, L. Dodd / Journal of Cultural Heritage 3 (2002) 333–345

in royal portraits ceased to be inlaid before the Late Period and they only began to appear again in the late Ptolemaic period. The applicability of this assertion to non-royal, small-scale bronze sculpture requires investigation else- where. The broad collar is modelled but does not seem to have been further inlaid and so only was decorated with gilding. The royal insignia of the uraeus includes the still-extant body of the cobra that winds upward behind the open hood after winding horizontally to form a single opposing loop behind the hood of the cobra. The hood is partly still preserved but its head is gone. The bottom of the single loop appears to be symmetrical, although strictly speaking, the loop on the proper right is minutely higher than the loop on the left. One loop extends horizontally to the right and the other to the left side. The arrangement of such loops was once held to be temporally diagnostic, but subsequently this has been rendered problematic ([15], 76–7, 79; [18], 127; [10], 5, 18, 21 n. 164, 27–9). The ear is fleshy and, although its thickness may be exaggerated by corrosion, the ear is situated at the level just above the eyebrow (if a horizontal line was extended to the face) and the ear ends at the level of a line that bisects the area between the lips and nose. The ears are set back from the fleshy face giving a sense of depth to the head, a feature that has echoes in the sculpture of the 25th Dynasty. The ears are set in an axis parallel to the line of the crown and beard. The beard is attached to the chest. In summary, formal comparison between the USC Osiris and the known corpus of bronze and stone sculpture is a Fig. 9. Polarized light photomicrograph for a sample of atacamite removed major factor leading to our assumption that this Osiris dates from the front of the Osiris. Light green tablets with some more rounded to the period between the Third Intermediate Period and the crystals present. Partially crossed polars; magnification × 128. fourth century BCE. We do not feel that it is definitely possible to attribute a specific date to this bronze given its indented. Full lips undercut at the corners are known from state of conservation, but it seems more likely to date from the 30th Dynasty and also from the Tanite burials of the Third Intermediate Period through 26th Dynasty, or even Dynasties 21 and 22. Josephson [10] emphasizes them for possibly as late as the fourth century on the basis of stylistic fourth century royal sculpture in particular. similarities, than to the latest Ptolemaic periods or to the The shape of the face is notable. It is very slightly wider early New Kingdom, where certain parallels have been than it is long. The overall effect is a compact, squarish noted. visage. This proportioning of the face is independent of the plumpness of the cheeks, which may be found in a variety of periods on faces with varying proportions. For example, 4. X-ray fluorescence analysis many Ptolemaic faces share the plump cheeks but are generally longer than they are wide, and there is a 19th Non-destructive, in situ, X-ray fluorescence analysis was Dynasty example in which the pudgy cheeks appear on a carried out using the Omicron micro-X-ray fluorescence face which shares the same proportions as the USC system [19]. The alloy of the casting was shown to be a Osiris—face slightly wider than long. Other closely analo- bronze of the following composition: 0.4% iron, 0.03% gous examples to the USC Osiris include the face of a small nickel, 86.7% copper, 7.3% tin, 4.5% lead and 0.1% bronze Osiris in the collection of Frankfurt am Main, antimony, with arsenic, zinc, cobalt and silver not detected; possibly 25th Dynasty and the Copenhagen Amasis of the the total was determined to be 99.03%. Tin is present in a 26th Dynasty. majority of analysed copper alloy objects from the Rames- The beard, crook and flail were inlaid with blue glass. side period onwards, and arsenic is rarely found in copper- Bothmer [15] suggests that the inlaid blue glass was only –tin alloys beyond the New Kingdom [8], corroborated by common during the Third Intermediate Period after which an extensive series of analyses by Riederer [20–24] and its use was rare. Josephson ([10], 20) indicates that the eyes Cowell [25], with the exception of Late Period cat heads, D. Scott, L. Dodd / Journal of Cultural Heritage 3 (2002) 333–345 339

Table 1 Electron microprobe data for the blue glass inlay from the Osiris: sample taken from the upper forearm just above the bent elbow. Analysis spots 1–4 show the elemental oxide compositions in weight percentages Oxide Spot 1 Spot 2 Spot 3 Spot 4 dect. limit k-ratio correction

Na2O 16.512 16.308 16.54 16.177 0.0351 0.0657 1.8647 P2O5 0.412 0.525 0.457 0.388 0.0528 0.0012 1.479 NiO 0.121 0.081 0.176 0.176 0.066 0.0008 1.2128 PbO 0.004 0 0.069 0.002 0.1068 0 1.555 MgO 4.416 4.318 4.376 4.211 0.0235 0.016 1.6622

Al2O3 2.898 2.912 2.971 2.942 0.018 0.0106 1.452 SiO2 62.25 60.642 62.3 60808 0.0264 0.2287 1.2725 K2O 1.304 1.32 1.341 1.288 0.0174 0.0093 1.1592 CaO 9.044 9.141 8.834 9.197 0.0196 0.0574 1.1261

TiO2 0.103 0.091 0.074 0.097 0.0203 0.0005 1.2274 MnO 0.263 0.268 0.239 0.257 0.0371 0.0016 1.2424 FeO 0.455 0.468 0.455 0.4 0.0524 0.0029 1.2217 CoO 0.267 0.26 0.293 0.296 0.066 0.0017 1.2507 ZnO 0.47 0.474 0.494 0.391 0.106 0.0029 1.2902 CuO 0324 0.203 0.267 0.262 0.0966 0.002 1.2824 SnO 0.019 0.02 0 0.057 0.059 0.0001 1.407

Sb2O3 1.758 2.407 1.353 2954 0.068 0.0104 1.4134 SrO 0.514 0.51 0.546 0.601 0.2747 0.0025 1.7505 BaO 0.013 0.013 0.14 0.012 0.0099 0.0001 1.4884 Total 101.15 99.96 100.8 100.52 which, in several instances, have been found to have higher possible to quantify glass analysis by this method reliably, arsenic levels. especially since sodium and magnesium are not detectable, This composition, showing the use of a leaded tin alloy, even in vacuum. Consequently, a small sample, would be quite typical for later Iron Age Egyptian figurines, 2mm× 1 mm in section, was mounted, polished and coated e.g. a 25th Dynasty bronze statuette (BM 63595), showed for electron microprobe analysis. Examination of the glass 0.19% iron, 0.04% nickel, 86.4% copper, 7.10% tin, 4.20% section reveals that the condition of the glass is excellent lead, 0.07% antimony, 0.1% arsenic and 0.01% zinc [26]. and very little, if any, weathering has taken place. Possibly With a few exceptions, lead levels above 2% are rarely these fragments, which must have acted cathodically in the encountered prior to the late New Kingdom when during the context of corrosion, were rapidly covered over with bronze 19th Dynasty, high lead levels in copper alloys first become corrosion products during burial and became embedded in a common [8,27]. Exclusion of a later date using this criterion thick, hard copper corrosion crust which had protected them is not possible, however, because examples dating to the from decay. 20th Dynasty contain only 5% lead [26] and in the Third Intermediate Period, many items still have lead levels under 5% [8]. 5. Electron microprobe analysis XRF analysis of the gilding, probably applied in the form of a gold leaf 0.023 mm (23 µm) thick, showed a composition of 81.3% gold, 15% copper, with about 3.7% silver, The electron microprobe analysis of the glass sample was although it is uncertain if the copper content is actually part undertaken on a Cameca SX100 electron microprobe em- of the gilding metal, or just derived from the underlying ploying a 20 µm spot size at 9.00 nA, 15 kV for 100 s bronze. Investigation of a tiny flake of gilding, removed acquisition time. Nineteen elements were sought during the from the surface did not provide a clear answer to this analysis and the results, expressed as element oxides, are question, since analysis gave a composition of 84.4% gold, given in Table 1. 12.4% copper and 3.15% silver, which suggests that some The glass is essentially a soda-lime glass. Sayre and diffusion of copper into the gold leaf may have occurred, Smith [28] found that Egyptian glass of the second millen- since it is very difficult to prepare gold foil containing this nium BC and early first millennium BC shows a sodium much copper. Gold purities ranging between 70% and 85% oxide content from 15% to a little over 20%, a calcium characterize products of the Middle and New Kingdom, oxide content of 5–10%, a silica content of about 60–70% while Late Period examples more often have high gold together with small concentrations of minor component purities of 85% and above, which may indicate refining, oxides. Magnesia (2–5%) and potash (1–3%) were found to although rare high purity examples are known earlier, as are be consistently high, and that is exactly the range of these later low purity examples [20]. two constituents found here, with 4.3% MgO and 1.3%

XRF, in vacuum, of the associated glass fragments K2O. Interestingly, the high level of zinc oxide in this Osiris revealed that the detectable components are principally Si, blue glass was also noted as an anomaly in New Kingdom K, Ca, Co, Ni, Cu, Fe, Zn, and Sb. However, it is not cobalt blue glasses by Sayre and Smith, and must originate 340 D. Scott, L. Dodd / Journal of Cultural Heritage 3 (2002) 333–345 from unusual copper or cobalt ore sources, since otherwise, ite, one of the copper trihydroxychlorides Cu2(OH)3Cl. A such a high zinc content is inexplicable. chart of the X-ray diffractogram for the latter is shown in The cobalt, nickel, zinc, and copper concentrations in our Fig. 9. The in situ identification was followed by Debye- Osiris glass are also comparable, except that the New –Scherrer powder diffraction analysis, the data for chalcon- Kingdom glass samples contained only 0.1–0.01% copper, atronite being given in Table 2. whilst the Osiris glass contains 0.25% copper, showing that Examination of the dark brown pustules that occur over this blue glass is coloured by both cobalt and copper. An the surface was also carried out by Debye–Scherrer powder Egyptian cobalt source has been suggested by Kaczmarczyk diffraction, which showed the same atacamite diffraction and Hedges [29]. The alum deposits of the Kharga and data as the principal corrosion crust. This result indicates Dahkla oases also contain manganese, iron, nickel, and zinc. that the pustules are principally composed of atacamite, but On the basis of trace mineral analysis, Kaczmarczyk and the result did not account for the dark brown colour of the Hedges ([29], 373–4) suggest that the Egyptian cobalt surface. Consequently, the sample used for X-ray diffraction source was only in use from the 16th to the 11th centuries was crushed, mounted, and examined under the polarized BC, while an Iranian source was in use in later times [9]. light microscope. In some places, the atacamite crystals The opacifying agent in the Osiris glass is antimony, could be seen to be tipped with cuprite which had formed on present at relatively high amounts of about 2%, again the immediate surface of these pustules, but which repre- characteristic of Egyptian glass from the second-first mil- sents a very thin surface phenomenon. lennium BC. Antimony is known from faience manufacture during the reign of Thutmosis III, while in glass it is so far documented as early as the mid 14th century, during the 8. Polarized light microscopy Amarna period, and also later [20,30]. In the later part of the First Millennium BC, the compo- The atacamite particles appear as equant crystalline sition of Egyptian glass changed and a “Roman” composi- fragments, tending towards boulder-shaped, rounded par- tion started to assert itself, with low magnesia and low ticles which appear pale green in bright-field illumination potassia [30,31]. The earliest glasses of this type found by and have a refractive index greater than that of the mounting Sayre and Smith were from glass inlays from funeral objects medium of 1.662 (reference data: ␣ = 1.831; b = 1.861; associated with the late Pharaoh, Nectanebo II (360–341 v = 1.880). A photomicrograph is shown in Fig. 10. Mc- BC). The magnesium oxide content of the Osiris glass is Crone et al. [34] state that the orthorhombic mineral is typical for the high magnesia glass (HMG) group, which slightly pleochroic, but this was not evident in the samples uses plant alkali, differentiated from glass of the Roman examined here. Under crossed polars, the particles are period, where both magnesia and potash are present at levels predominantly yellow with tinges of red; some remaining a usually below 1%, consistent with the use of natron [32]. pale grey-white. Occasional particles are seen with finely The ratio of potassium oxide to magnesium oxide found radiating fibrous characteristics, from the mode of growth of here is very similar to examples of Egyptian glass from Tell the corrosion product. el Amarna published by Nicholson and Henderson [33]. The evidence from the glass analysis therefore suggests a date Examination of a microsample of one of the very dark for the Osiris of earlier than 500BC. pustules on the surface showed that this too was indeed composed of atacamite; some of these crystals could be seen to be covered with a very thin layer of cuprite, too small to have been able to be detected by the X-ray diffraction study. 6. X-ray radiography This explains the very dark, almost black, appearance of these excrescences: the red-brown of the cuprite over the A Phillips 450 kV X-ray tube was used in order to assess dark green of the atacamite producing a black appearance. whether the bronze is actually a solid cast or a hollow lost-wax bronze casting. The bronze could barely be pen- Under plane polarized light, the chalconatronite particles etrated using X-radiography at 420 kV, 10 mA for 180 s, appear almost colourless and have no obvious blue or green confirming the impression that the statuette is a solid bronze tone at all. Some of the particles show radiate, fibrous or casting. finely disseminated appearance in melt-mount of RI 1.662, while some crystalline fragments can be seen to have a refractive index less than that of the medium (reference ␣ b c 7. X-ray diffraction data: = 1.483; = 1.530; = 1.576). Under crossed polars, some of these crystalline tablets X-ray diffraction, in situ, using a Gobels mirror and a show a second-order blue colour parallel to the long Siemens D5005 X-ray diffractometer, showed that the light direction of the wave-plate, indicative of a negative sign of blue patches of corrosion are chalconatronite, sodium cop- elongation. Extinction in clear particles tends to be parallel per (II) carbonate trihydrate, Na2Cu(CO3)2·3H2O; the dark to the long axis of the crystal, whilst more fibrous-looking green corrosion crust that covers the entire figure is atacam- particles show undulose extinction, similar to some syn- D. Scott, L. Dodd / Journal of Cultural Heritage 3 (2002) 333–345 341

Fig. 10. X-ray diffraction data, measured in situ, for the dark green corrosion showing atacamite to be present. thetic preparations of the mineral made in the laboratory magnification of × 40 to assess the degree of saturation of [35,36]. the surface or alteration of surface gloss. Following conser- Photomicrographs can be seen in Figs. 11 and 12.A vation treatment, the visual appearance of the surface did reference sample of chalconatronite from the collections of not undergo significant change. the British Museum (mineral identification: BM1973:460), Observation of the bronze over a period of 12 months appears as angular, colourless, crystalline fragments in clear following treatment suggested that this consolidation has relief since all three refractive indices of chalconatronite are helped to retain the mineralized surface as the loss of parts well below that of the medium. Under crossed polars, these of this surface has now abated. crystals show a grey-white birefringence with some particles revealing a second-order straw yellow and a red- purple tinge: most particles possess clear extinction. Evidence for some calcite particles, mixed with chalcon- 10. Discussion atronite, was found in some preparations, as clear, crystalline fragments, showing typical birefringence under crossed Thomas [37] found that for appreciable quantities of polars, and with one refractive index very close to that of the atacamite to be found on an object, it is necessary for the mounting medium at 1.66. equilibrium concentrations of copper and chloride ions at the metal surface to be higher than those found in most groundwater. Typically, a chloride ion activity greater than 9. Conservation 10–2 g-ion per litre, 3550 ppm, is necessary for atacamite to form. The dry, natron-rich environs of Egypt provide ideal The principal conservation issue is the continued loss of opportunities for this kind of chloride-rich patina to de- small fragments of atacamite and chalconatronite from the velop. One possible sequence being: thick, heavily mineralized crust. It was decided to consoli- date this layer with an application of 3% Paraloid B72 in + − acetone (known as Acryloid B72 in the United States; a Cu + Cl = CuCl (1) co-polymer of ethylmethacrylate and methylacrylate), ap- + + = ͑ ͒ + + + − plied by brush, observing the surface under binocular 4CuCl O2 4H2 O 2Cu2 OH 3 Cl 2H 2Cl (2) 342 D. Scott, L. Dodd / Journal of Cultural Heritage 3 (2002) 333–345

Table 2 Debye-Scherrer X-ray powder diffraction data for the light blue corrosion, identiﬁed as chalconatronite

Osiris light blue corrosion Na2Cu(CO3)2H2O ICDD 22–1458 I/I* d d I/I* 20 9.198 – – 40 7.879 7.820 50 100 7.101 – – 100 6.951 – – 100 6.870 6.900 100 20 6.055 5.590 40 50 5.505 – – 50 5.167 5.180 70 20 4.584 4.570 40 90 4.181 4.180 80 80 3.681 3.680 90 40 3.450 3.450 40 Fig. 11. Polarized light photomicrograph for a sample of chalconatronite 5 3.220 3.290 10 removed from the back of the Osiris. The light blue corrosion is almost 5 3.139 3.120 40 20 3.032 3.040 10 colourless under the microscope so a partially crossed polar view is 20 3.014 3.000 50 illustrated here. The crystals have a grey-white birefringence with some 20 2.994 2.980 40 crystals showing a second-order straw yellow and a red-purple tinge. × 70 2.883 2.890 60 Partially crossed polars; magniﬁcation 128. 40 2.783 2.780 30 10 2.694 2.690 30 20 2.625 2.630 20 atacamite precipitates under more alkaline conditions. 30 2.531 2.530 60 40 2.438 2.430 60 50 2.274 2.280 30 5 2.217 2.210 40 50 2.153 2.150 20 40 2.084 2.082 60 40 2.076 – – 20 2.068 2.061 60 30 2.020 2.015 30 30 2.006 2.010 60 20 1.994 1.991 60 10 1.917 1.917 50 10 1.859 1.860 40 5 1.824 1.821 30 10 1.777 – – 30 1.723 1.710 10 5 1.679 1.981 2 10 1.619 – – 5 1.564 1.552 1 5 1.498 – – 10 1.432 1.425 3 5 1.412 1.418 3 5 1.390 1.387 2 5 1.373 1.370 2 3 1.268 – – 3 1.235 – – 3 1.208 – – 3 1.188 – – 3 1.160 – –

Nota bene: the last 7 entries for 22-1458 are from the author’s reference data and do not appear in the ICDD ﬁles.

Pourbaix [38] illustrates diagrams for the copper–chlo- rine–water system at this concentration of chloride ions which show that the ﬁeld of stability for the copper trihydroxychlorides occurs in more oxidizing and acidic environments from about pH 6.0. Woods and Garrels [39] found that under more acidic conditions, paratacamite, Fig. 12. The same specimen as Fig. 12 under crossed polars; magniﬁca- rather than atacamite, is the favoured product, and that tion × 128. D. Scott, L. Dodd / Journal of Cultural Heritage 3 (2002) 333–345 343

These alkaline conditions may also be produced by these Further reaction may then take place between available Egyptian natron deposits. For example, with a natron cupric ions and sodium chloride: deposit rich in sodium carbonate, the reaction between cupric chloride, sodium carbonate and water may produce ++ + = + + atacamite, along with an array of other products, as the Cu NaCl CuCl2 2Na (5) following rather complex equation suggests: The reaction provides the possibility of a cyclical chain of events, since the sodium chloride formed in reaction (4) 16Na CO + 18CuCl + 27H O = 9Cu ͑ OH ͒ Cl + 2 3 2 2 2 3 may then be consumed in reaction (5) to produce more − + − + + + 27Cl 5HCO3 11H2 CO3 32Na (3) CuCl2. Sharkey and Lewin [40] found that the concentrations of The atacamite appears to occur on the surface in two modifications, firstly as thick sheets of corrosion, and CuCl2 and NaCl were critical in determining which isomer of the copper trihydroxychloride system might be expected secondly, as eruptions of atacamite in the form of small to form. With copper and NaCl alone, paratacamite formed, pustules over this surface, which are covered with a thin – layer of cuprite, which accounts for their brown/black the addition of NaCl to CuCl2 between 0.1 and 1 M Cl favoured atacamite, with higher concentrations again pro- appearance, as the thin skin of red cuprite is underlain by the ducing paratacamite. dark green of the atacamite. The association of atacamite, Another interesting aspect of this study is the extensive chalconatronite and cuprite may be significant in the envi- formation of chalconatronite that has taken place over the ronmental parameters required for chalconatronite forma- atacamite corrosion crust, where large patches of the surface tion. are of chalconatronite, as can be seen from Figs. 2 and 3. The bronze has undergone no prior conservation work, and the chalconatronite retains quartz and calcite mineral grains from the burial environment, as does the atacamite layer. 11. Conclusions Chalconatronite was first identified by Frondel and Gettens [41] in 1955 as a bluish-green chalky crust within The study of this bronze Osiris, from the different the hollow interior of an Egyptian bronze figurine of the viewpoints discussed in this paper, has provided new deity Sekmet from the Saite-Ptolomaic period (663–630 insights concerning patina and corrosion of Egyptian BC) in the Fogg Museum of Art. The mineral was also bronzes from very chloride-rich environments. Firstly, it is identified on an Egyptian bronze group of a cat and kittens not obvious, from previously published work, that it is in the Gulbenkian Collection in Lisbon; on a Coptic censer, possible for an overall patina of atacamite to exit as a dating from about the seventh century AD, in the Freer coherent crust. The assumption has been that such occur- Gallery of Art, Washington, DC; on a copper pin from the rences of the copper trihydroxychlorides are limited to basilica of St. Mark’s in Venice [42], and as isolated crystals excrescences over the patina but do not form a continuous on Roman copper and iron armour from an excavated site at covering. The publication of this heavily corroded Osiris Chester, England, where conservation treatment had been shows that these assumptions are incorrect. Secondly, the carried out many years earlier [43]. Interestingly, Frondel aetiology of the chalconatronite was found to be unrelated and Gettens [41] also found that atacamite and cuprite were to prior conservation treatment, and represented an associa- associated with the chalconatronite, very similar to the tion between atacamite, chalconatronite and cuprite, which associations seen here. may be one of the requirements for its natural formation in One of the synthetic methods for the preparation of highly saline environments. Despite the heavy corrosion of sodium copper (II) carbonate trihydrate is the precipitation the bronze, an extensive amount of technical art historical of the crystals from a concentrated solution of sodium interpretation was possible. Formal comparison between the carbonate containing bicarbonate and copper ions, and this USC Osiris and the known corpus of bronze and stone was the route utilized by the first laboratory synthesis of the sculpture enabled us to infer that this Osiris dates to the compound by Deville [44] in 1852. The nature of the period between the Third Intermediate Period and the fourth possible atacamite to chalconatronite transition, was also century BC. highlighted by an experiment carried out by Thomas [37] in The determination that the chalconatronite was spalling which a 4.7 mmol solution of cupric chloride dihydrate was away from the surface of the bronze due to thermomechani- added to 100 ml of a stirred solution of 50 mmol of sodium cal rather than chemical instability, led to the decision to carbonate at 25 °C. When this solution was allowed to apply a surface consolidant to the corroded surface, in an evaporate to dryness, the resulting mixture is of soluble attempt to provide greater cohesion of this layer. The sodium salts and chalconatronite; viz. treatment appears to have been successful, as observation of the bronze over a period of 12 months reveals no further + = ͑ ͒ + CuCl2 2Na2 CO3 Na2 Cu CO3 2 2NaCl (4) loss of the mineralized surface. 344 D. Scott, L. Dodd / Journal of Cultural Heritage 3 (2002) 333–345

Author’s biography bronze, 22.1 cm high, Late Period, 712–332 BC; Saqqara; Inlaid statue of Nefertum, 363, possibly JE39483, 46.5 cm high, bronze David A. Scott, BSc., BA, Ph.D., FIIC, FRSC, is the with remnants of possible blue inlay in lotus blossom on head; Statue of a kneeling king holding a wedjat eye, JE91436, 26 cm Senior Scientist in charge of the GCI Museum Research high, Saqqara; Late Period, 712–332 BC; the kilt and wedjat eye and Laboratory. He was a lecturer in conservation at University white crown are gilt. And also JE94436, 26 cm complete: 359. College, Institute of Archaeology, Department of Conserva- [5] G. Roeder, Staatliche Museen zu Berlin: Mitteilungen aus der tion and Materials Science, from 1981 to 1987. In 1987, he Ägyptischen Sammlung (vol. 6) (1956) Ägyptische Bronzefiguren. joined the GCI as Head of the Museum Services Laboratory. Berlin. §206f, plate 25 a-d Berlin 8671. Osiris with blue, and red and white alternating inlays. Possibly traces of green. He was appointed as an editor for Studies in Conservation in [6] M. Wuttmann, B. Bousquet, B. M. Chauveau, P. Dils, S. Marchand, 1984. His principal interests are the analysis of Museum A. Schweitzer, L. Volay, First preliminary report on the work at the objects, the characterization of pigments, ancient metals and Site of ‘Ayn Manâwîr (Kharga Oasis), Bulletin de l’Institut français their microstructure, and the archaeometallurgy of preHis- d’archéologie orientale 96 (2000) 385–451. panic Colombia and Ecuador. [7] W. B. Emery, Preliminary report on the excavations at North Saqqara, 1968–9, J. Egyptian Archaeology 56 (1970) 5–10. Lynn Swartz Dodd,BA in Art History from Smith Col- [8] J. Ogden, Metals, in: P.T. Nicholson, I. Shaw (Eds.), Ancient lege, Northampton, MA, 1984, MA in Near Eastern Lan- Egyptian Materials and Technology, Cambridge University Press, guages and Cultures, UCLA, 1997, Ph.D 2002, research on Cambridge, 2000, pp. 153. cultural identity and the recreation of statehood in the early [9] G. Roeder, Ägyptische Bronzewerke, J.J. Augustin, Gluckstadt (1937). Iron Age-Late Bronze Age transition of North Syria. Since [10] J. Josephson, Egyptian Royal Sculpture of the Late Period. 400–246 1998 she has been at USC where she is visiting assistant B.C., Verlag P. von Zabern, Mainz on Rhine, 1997. professor and curator of the use Archaeological Research [11] P. S. Griffin, The selective use of gilding on Egyptian polychromed Collection. bronzes, in: T. Drayman-Weisser (Ed.), Gilded Metals: History, Technology and Conservation, Archetype Publications, London, 2000, pp. 104–120. 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