Chemical Aspects of the Conservation of Archaeological Materials

2 Chemical Aspects of the Conservation of

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Archaeological Materials N. S. BAER Conservation Center of the Institute of Fine Arts, New York University, 1 East 78th Street, New York, NY 10021 Long term burial of artifacts recovered in archaeological excavations often leads to friability, salt encrustation, physical damage, and severe corrosion. Field conservation is limited to such measures as are required to preserve the artifact until it may receive the attention of specialists in the museum laboratory. Typical conservation treatments for textiles, waterlogged wood, bone and ivory, cuneiform tablets, and cast and wrought marine iron are reviewed with particular emphasis on the effects such treatments may have on the subsequent technical examination of the artifact.

Archaeological artifacts seldom are recovered in a perfectly preserved state. Often throughout many years of burial the artifact has been subjected to chemical and biological attack, leaving it in a state of extreme friability. In order to recover and in some cases to save the object, conservation measures may be required in the field (1,2). For example, waterlogged wood unable to bear its own weight must be supported, dried, and consolidated; pottery whose decorations may be significant to the progress of an excavation but which are obscured by mud and deposits of lime and chalk must be cleaned; some cuneiform clay tablets, sun-dried but not baked, must be fired before they may be studied. Once the artifact has been brought to the museum laboratory, a wide range of conservation procedures are undertaken not only to assure the long term survival of the artifact but also to return it to an approximation of its original state. All of these field and laboratory treatments will in some way change the artifact, and some treatments may have profound effects on the suitability of the artifact for technological examination. It is therefore essential for the scientist engaged in analyzing and examining archaeo0-8412-0397-0/78/33-171-025$05.00/l © 1978 American Chemical Society Carter; Archaeological Chemistry—II Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

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logical materials to be familiar with conservation practice. Similarly, the conservator has a professional obligation to limit his initial treatments to those essential for the survival of the artifact; to keep detailed treatment records; and, where possible, to retain some representative material in an untreated state. Under the heading, "Safeguarding Evidence," Plenderleith and Werner (3) consider the problem of record keeping. It must be clear that laboratory treatment of antiquities and works of art carries with it a great responsibility. The excitement of discovery and the urge to reveal hidden interests must be held in check and kept subservient to the duty of maintaining full laboratory records of the work as it proceeds, illustrated, where necessary, with sketches and photographs for incorporation in a permanent archive. Similarly, Jedrejewska (4), considering the ethics of conservation, writes about the treatment process: During this time something is removed, something is added, and something changed in the object. In consequence some of the historical evidence inherent in the object can be damaged, changed, or lost. It may also happen that new materials introduced to the object, or used for treatment, may prove harmful immediately, or after a lapse of time. In the following discussion a number of conservation methods commonly used to treat archaeological artifacts are examined for their effects on subsequent technical examination. The preservation of archaeological textiles, waterlogged wood, archaeological bone and ivory, cuneiform tablets, and marine iron are among the problems considered. Archaeological Textiles Archaeological textile artifacts often are recovered in a severely deteriorated state, embrittled and extremely friable (5,6). However, the art historical and anthropological importance of materials such as Coptic and Precolumbian textiles requires their conservation treatment to make them available for study and display. Among the operations variously used by textile conservators are bleaching; brushing; washing with detergents; dry cleaning; use of enzymes; exposure to fungicides, insecticides, and moth-proofing agents; stain removal; steam cleaning; washing; and use of wetting agents (1). On washing and drying, ancient textiles often become still more brittle and so are treated with lubricating materials, most often glycerine added to the final aqueous rinse (7,8). In the most severe cases, methods of consolidation and reinforcement are used to facilitate the handling of these artifacts (6,9,10,11). That these consolidative procedures are irreversible is readily acknowledged, although it is suggested that consolidation may offer the only hope for the survival of the artifact (6).

Carter; Archaeological Chemistry—II Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

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Once restored, the textile is then often subjected to the vicissitudes of the museum environment. Airborne dirt, sulfur dioxide, ozone, light, and microorganisms all affect additional change on the object (12). Such natural dyestuffs as madder, cochineal, and indigo can all be expected to undergo substantial fading under long term exposure to average museum lighting conditions (13,14). Museum illumination may cause even further deterioration in the fibers themselves (15,16). Thus it is not surprising that archaeological textiles subject to chemical investigation present serious problems of contamination and alteration requiring extensive pretreatment for even the most routine analyses (17).

Waterlogged Wood Waterlogged wood artifacts, especially prehistoric dug-out canoes, boats, and ships, have been recovered for museum display for more than a century (18). In recent years, the growth of underwater archaeology has accelerated the rate of discovery of such materials. The conservation process usually involves removal of excess water to reduce shrinkage and distortion followed by consolidation to strengthen the artifact (18-24). The major concerns of the conservator are the elimination of shrinkage and control of the surface appearance after consolidation (24). The most commonly used procedure involves immersing the waterlogged wood in a 10% polyethylene glycol solution in distilled water at 60°C with controlled evaporation over several months (1,2,19,23). In a case of wooden writing tablets where the main objective was to render the ink inscriptions as legible as possible, the excess water was removed by an alcohol/ether dehydration process (20). Although the treatment was considered to be successful, the wood was very fragile and still might require backing or consolidation. Consolidative methods include alcohol/ ether drying followed by dammar resin consolidation (1,24); heating the wood at 96°G with molten potassium alum (1, 24); washing in water, impregnation with aqueous melamine/formaldehyde which is then polymerized in situ (1,24); and impregnation with epoxy resins (21). That none of the above methods is completely satisfactory as a conservation procedure is generally agreed. One interesting recent development involves the application of freeze-drying technology (24,25) combined with polyethylene glycol impregnation. In an interesting comparative study using scanning electron microscopy Oddy (24) demonstrated the substantial changes in cell structure which accompany the several treatments. Clearly most, if not all, of the consolidation methods described are irreversible, introducing materials which may obscure subsequent examination.


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Bone and Ivory Fossil tooth and bone material and artifacts of bone and ivory are among the most important materials recovered by anthropologists and archaeologists. A substantial literature (26,27) exists describing the many forms of examination applied to these materials. The degree of preservation of these materials is a function of many environmental variables (28,29). On long term burial systematic changes may occur involving replacement of hydroxyl groups in the hydroxyapatite with fluoride to form fluorapatite. In the proteinaceous component, significant change will occur in the amino acid composition (27,28,29,30). Since the middle of the 19th century researchers have sought to take advantage of these changes in order to develop methods for dating tooth and fossil material (26,27,28,32,33). Most recently, considerable attention has been focused on racemization of the constituent amino acids as an indicator of age (34,35). In some cases radiocarbon methods have been applied with success (26,27,30). With the exception of radiocarbon dating, all of the methods rely on chemical changes occurring in the original bone or tooth material. However, these same changes can lead to such severe deterioration that the bone itself may disappear almost completely, leaving only a silhouette or stain in the ground (36). When an artifact rather than a simple skeletal remain is found, substantial intervention may occur. Layard, writing in 1849, provides an extreme example of the treatment some ivory specimens have received (31). The chamber V is remarkable for the discovery of a number of ivory ornaments, of considerable beauty and interest. These ivories, when uncovered, adhered so firmly to the soil, and were in so forward a state of decomposition, that I had the greatest difficulty in extracting them, even in fragments. I spent hours lying on the ground, separating them, with a penknife, from the rubbish by which they were surrounded. The ivory separated itself in flakes. With all the care that I could devote to the collection of fragments, many were lost, or remained unperceived, in the immense heap of rubbish under which they were buried. Since they have been in England, they have been admirably restored and cleaned, and the ornaments have regained the appearance and consistency of recent ivory, and may be handled without risk of injury. More common are consolidation procedures (1,2,37) where various waxes and resins are introduced into the interstices produced by the deterioration process. The cleaning and consolidation processes may interfere seriously with any or all of the various dating methods. New organic matter will bias radiocarbon dates; animal glue consolidants will introduce extraneous proteinaceous material, thus confusing dating methods based on amino acid analysis; and nitrogen-bearing consolidants such


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as cellulose nitrate, soluble nylon, and animal glues will introduce errors in nitrogen measurements. Though one may use separation techniques in an attempt to isolate the original matrix materials, it is likely that these techniques will be only moderately successful.


Cuneiform Tablets Artifacts of unbaked clay recovered in archaeological excavations present a singular series of problems. If washed, they may turn to mud; if left unfired, they may be damaged in transit. One particular class of material, cuneiform tablets, has received particularly severe treatment. Dowman describes accurately the general attitude (38). Tablets come into a somewhat different category. All that is ever wanted of them is their inscriptions and it is common practice to fire tablets so that they can be handled indefinitely. Should only clay be wanted for analysis an unscribed section can be set aside and left unfired. Any treatment to be given to tablets in the field depends on whether there is a kiln on the site or not and, if not, on how desperate the epigraphist is to read them on the spot. Several authors have reported the general methods for firing these tablets in the field as well as in the museum laboratory (1,38,39,40). The specific problems associated with removing soluble salts are considered in detail by Organ (39). Those whose research interests involve the examination of trace element distribution in ceramic materials as an indication of provenance and those making thermoluminescence measurements will find few useful clay samples among the materials preserved by archaeologists. Thus, a potentially valuable source of * documented clay materials is lost when unbaked clay tablets are routinely fired and cleaned without systematic sorting to preserve in their original state those specimens which do not require firing for their survival. Cast and Wrought Marine Iron In wrought and cast irons the examination of the metallurgical structure may provide significant insight into the method of fabrication of the artifact. Cold working in wrought iron, chill casting, and annealing in cast iron may all be observed on metallographic examination (43,44, 45). Iron samples recovered from shipwreck are typical of materials usefully examined by such methods. Unfortunately, iron artifacts recovered from the sea deteriorate extremely rapidly because of the presence of large amounts of chlorides in the corrosion products (45). Thermal stabilization processes involving heating in air at 860°C (41) or heating in a hydrogen reduction furnace at 800°C and 1060°C (42)



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have been used to drive off the metallic chlorides. It has been demonstrated (43) that such heating destroys the original metallurgical structure in the iron vitiating a significant part of the evidence provided by the artifact. It was shown that hydrogen reduction at temperatures above 400°C could not be used without serious loss of microstructure. Unfortunately, at this temperature the treatment is only partially successful, allowing sufficient chlorides to remain to leave the artifact unstable. Secondary treatments will have to be developed to remove the remaining chlorides. It is interesting to note a recent announcement of the installation in the Portsmouth City Museum, United Kingdom of a large reduction furnace to stabilize ion antiquities, cannon in particular (41). It is indicated that "experiments will be conducted to determine the feasibility of treating archaeological iron using the furnace, but not destroying the metal's microstructural characteristics." Conclusion Conservators have long recognized that their professional responsibilities include the information inherent in the structure as well as in the composition of the artifact under treatment. Unfortunately, the need to preserve an artifact or to prepare it for display has often required treatments which significantly changed the artifact. Cooperation between scientists engaged in the study of archaeological materials and conservators is required to develop conservation treatments which minimize structural and compositional alteration. Guidelines must be developed for the preservation of representative untreated materials where satisfactory treatments are not yet available. It is also essential that in the examination of archaeological artifacts the previous treatment history be considered. Acknowledgment This project was sponsored by the National Museum Act (administered by the Smithsonian Institution).

Literature Cited 1. Plenderleith, H.J.,Werner, A. E. A., "The Conservation of Antiquities and Works of Art," 2nd ed., Oxford, London, 1971. 2. Dowman, E. A., "Conservation in Field Archaeology," Methuen, London, 1970. 3. Plenderleith, H.J.,Werner, A. E. A., "The Conservation of Antiquities and Works of Art," 2nd ed., pp. 16-17, Oxford, London, 1971. 4. Jedrzejewska, H., "Ethics in Conservation," p. 7, Institutet för Materialkunskap, Stockholm, 1976.



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5. Bery, G. M., Hersh, S. P., Tucker, P. A., Walsh, W. K., "Reinforcing Degraded Textiles. Part I: Properties of Naturally and Artificially Aged CottonTextiles,"ADV. CHEM. SER. (1977) 164, 228-248. 6. Jedrzejewska, H., "Some New Techniques for Archaeological Textiles," in "Textile Conservation," J. E. Leene, Ed., Chap. 22, Butterworths, Lon don, 1972. 7. Delacorte, M., Sayre, E. V., Indictor, N., "Lubrication of Deteriorated Wool," Stud. Conserv. (1971) 16(1), 9-17. 8. Textile Museum Workshop Notes, The Textile Museum, Washington, D.C., papers no. 4, 8, and 24. 9. "The Conservation of Cultural Property," UNESCO, Paris, 1968. 10. Berger, G. A., "Testing Adhesives for the Consolidation of Paintings," Stud. Conserv. (1972) 17, 173-194. 11. Berry, G. M., Hersh, S. P., Tucker, P. A., Walsh, W. K., "Reinforcing Degraded Textiles. Part II: Properties of Resin-Treated, Artificially Aged Cotton Textiles," ADV. CHEM. SER. (1977) 164, 249-260. 12. Thomson, G., "Textiles in the Museum Environment," in "Textile Conser vation," J. E. Leene, Ed., Chap. 7, Butterworths, London, 1972. 13. Padfield, T., Landi, S., "The Lightfastness of Natural Dyes," Stud. Conserv. (1966) 11, 181-196. 14. Thomson, G., "A New Look at Colour Rendering, Level of Illumination, and Protection from Ultraviolet," Stud. Conserv. (1961) 6, 49-70. 15. Little, A. H., "Deterioration of Textile Materials," 1964 Delft Confernce on the Conservation of Textiles, 2nd edition, pp. 67-78, International Institute for Conservation, London, 1965. 16. Padfield, T., "The Deterioration of Cellulose," in "Problems of Conserva tion in Museums," p. 119-164, Allen and Unwin, 1969. 17. Baer, N. S., Delacorte, M., Indictor, N., "Chemical Investigations on Pre -Columbian Archaeological Textile Specimens," ADV. CHEM. SER. (1977) 164, 261-271. 18. Barker, H., "Early Work on the Conservation of Waterlogged Wood in the UK," in "Problems in the Conservation of Waterlogged Wood," W. A. Oddy, Ed., pp. 61-63, National Maritime Museum, Greenwich, Lon don, 1975. 19. Stark, B. L., "Waterlogged Wood Preservation with Polyethylene Glycol," Stud. Conserv. (1976) 21 (3), 154-158. 20. Blackshaw, S. M., "The Conservation of the Wooden Writing-Tablets from Vindolanda Roman Fort, Northumberland," Stud. Conserv. (1974) 19(4), 244-246. 21. Munnikendam, R. A., "Low Molecular Weight Epoxy Resins for the Con solidation of Decayed Wooden Objects," Stud. Conserv. (1972) 17(4), 202-204. 22. Oddy, W. A., Van Geersdaele, P. C., "The Recovery of the Graveney Boat," Stud. Conserv. (1972) 17, 30-38. 23. Gregson, C. N., "Progress on the Conservation of the Graveney Boat," in "Problems in the Conservation of Waterlogged Wood," W. A. Oddy, Ed., pp. 113-114, National Maritime Museum, Greenwich, London, 1975. 24. Oddy, W. A., "Comparison of Different Methods of Treating Waterlogged Wood as Revealed by Stereoscan Examination and Thoughts on the Conservation of Waterlogged Boats," in "Problems in the Conservation of Waterlogged Wood," W. A. Oddy, Ed., pp. 45-49, National Mari time Museum, Greenwich, London, 1975. 25. Rosenqvist, A. M., "Experiments on the Conservation of Waterlogged Wood and Leather by Freeze-Drying," in "Problems in the Conservation of Waterlogged Wood," W. A. Oddy, Ed., pp. 9-23, National Maritime Museum, Greenwich, London, 1975.



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26. Baer, N. S., Majewski, L.J.,"Ivory and Related Materials in Art and Archaeology: An Annotated Bibliography," Art andArchaeol.Tech. Abstr. (1970) 8(2), 229-275. 27. Ibid., (1971) 8(3), 189-228. 28. Hare, P. E., "Geochemistry of Proteins, Peptides, and Amino Acids," in "Organic Geochemistry," Eglinton and Murphy, Eds., Chap. 18, Springer Verlag, New York, 1969. 29. Hare, P. E., "Organic Geochemistry of Bone and Its Relation to the Sur vival of Bone in the Natural Environment," in press. 30. Hassan, A. A., Ortner, D. J., "Inclusion in Bone Material as a Source of Error in Radiocarbon Dating," Archaeometry (1977) 19(2), 131-135. 31. Layard, A. H., "Nineva and Its Remains," H. W. F. Saggs, Ed., p. 245, Praeger, New York, 1970. 32. Baer, N. S., Indictor, N., "Chemical Investigations of Ancient Near Eastern Archaeological Ivory Artifacts," ADV. CHEM. SER. (1975) 138, 236-245. 33. Baer, N. S., Jochsberger, T., Indictor, N., "Chemical Investigations on Ancient Near Eastern Ivory Artifacts: III. Fluorine and Nitrogen Com position," ADV. CHEM. SER. (1978) 171, 139. 34. Bada, J. L., "Racemization of Isoleucine in Calcareous Marine Sediments: Kinetics and Mechanism," Earth Planet. Sci. Lett. (1972) 15, 1-11. 35. Bada, J. L., "The Dating of Fossil Bones Using the Racemization of Iso leucine," Earth Planet. Sci. Lett. (1972) 15, 223-231. 36. Keeley, H. C. M., Hudson, G. E., Evans,J.,"Trace Element Contents of Human Bones in Various States of Preservation. I. The Soil Silhouette," J. Archaeol. Sci. (1977) 4, 19-24. 37. Crawford, V. E., "Ivories from the Earth," Metropolitan Museum of Art Bulletin (1962) 21 (4), 141-148. 38. Dowman, E. A., "Conservation in Field Archaeology," p. 122-123, Methuen, London, 1970. 39. Organ, R. M., "The Conservation of Cuneiform Tablets," British Museum Quarterly (1961) 23(2), 52-57. 40. Crawford, V. E., "Processing Clay Tablets in the Field," in "The Preserva tion and Reproduction of Clay Tablets and Conservation of Wall Paint ings," Colt Archaeol. Monogr. Ser. (1966) 3. 41. Eriksen, E., Thegel, S., "Conservation of Iron Recovered from the Sea," Tojhusmuseets Skrifter (1966) 8. 42. Arrhenius, O., Barkman, L., Sjostrand, E., "Conservation of Old Rusty Iron Objects," Swed. Corros. Inst. Bull. (1973) 61E. 43. North, N., Owens, M., Pearson, C., "Thermal Stability of Cast and Wrought Marine Iron," Stud. Conserv. (1976) 21, 192-197. 44. North, N., Pearson, C., "Alkaline Sulphite Reduction of Marine Iron," Proceedings, ICOM Committee for Conservation, 4th Triennial Meetin Venice, 1975, 75/13/3, 1-14. 45. North, N. A., Pearson, C., "Thermal Decomposition of FeOCl and Marine Cast Iron Corrosion Products," Stud. Conserv. (1977) 22, 146-157. 46. IIC Conservation News (March 1977) 2. RECEIVED December 19, 1977.


Chemical Aspects of the Conservation of Archaeological Materials

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