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Chemistry in the conservation of archaeological materials
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ELSEVIER SCIENCE IRELAND
the Science of the Total Environment The Science of the Total Environment 143 (1994) 121-126
Chemistry in the conservation of archaeological materials W.A. Oddy Department of Conservation, The British Museum, London WC1B 3DG, UK
Abstract This paper briefly reviews the deterioration of antiquities from a chemical point of view and then discusses the application of synthetic polymers in conservation. Materials which are mentioned include the use of polyethylene glycol for preserving waterlogged wood, cellulose ether gels as thickening agents for retaining solvents on surfaces, and epoxy resin and UV-curing methacrylates for the restoration of ceramics and glass• Finally, mention is made of ways of preventing objects deteriorating once they are on display in a museum.
Key words: Conservation; Archaeology; Museum; Polymers; Chemistry
1. Introduction
The application of chemistry to studying the decay and conservation of antiquities is not new. In the mid 19th century Michael Faraday studied the deterioration of easel paintings [1], and others were already applying the methods of chemical analysis to determining the composition of objects, particularly those made of metal [2]. The first scientist to have been employed in a museum, however, appears to have been Friedrich Rathgen (1862-1942) [3], who headed the laboratory in the Royal Museums in Berlin from 1888 to 1927. Rathgen published his methods as a book [4], which was subsequently translated into English [5]. Also in the mid 19th century, the National Museum in Copenhagen, faced, in particular, with the problem of preserving an increasing number of waterlogged wooden objects found in peat bogs, employed its first conservator, V.F. Steffensen, in 1867 [6]. In 1896 he was joined by Gustav Rosenberg (approx. 1870-1940) who also left a testament to his career in book form [7]. Sadly the scientific initiative in Germany did not survive the Second World War, although a
new laboratory, named after Rathgen, was founded in Berlin in 1975 [8]. The First World War, however, can be credited with initiating the scientific conservation of antiquities in the UK, because when British Museum objects were unpacked in 1919, after war-time storage in the underground railway tunnels, some of them were found to have noticeably deteriorated. As a result, the Department of Scientific and Industrial Research appointed Dr. Alexander Scott, FRS, as a consultant to The British Museum, and in 1926 Dr. Harold Plenderleith, MC, was recruited as a full-time conservation scientist. The career of Plenderleith is well known, culminating in the establishment of the International Centre for Conservation in Rome in 1959 with Plenderleith as its first director. In recent years, many other museums have recruited conservation scientists and some countries have established research institutes to service the conservation profession. Notable among these are Hungary, The Netherlands, Belgium, Canada and Japan. Although the number of conservation scientists in the world has increased significantly in the last
0048-9697/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved. SSDI 0048-9697(94)003683-S
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30 years, it is still small in relation to the problems faced in trying to conserve the cultural heritage of mankind. Many problems have been solved but many remain, and it is the purpose of this paper to look at some of the problems and to show how chemistry has contributed to their solution.
2. The decay of archaeological materials Almost all objects deteriorate in use because of wear and tear, and deteriorate once they are lost and become buried because of chemical or biological processes. When an object is excavated it will usually have reached an equilibrium with its surroundings, which means that the processes of decay are at a standstill, or virtually so. The equilibrium may have been reached for two reasons; first, that the particular process may have proceeded to completion giving rise to alteration products which are stable in the burial environment; and second that the decay process has stopped because of a shortage of one of the agents of decay. Typical examples of objects which stabilise because decay processes have proceeded to completion are the corrosion of metals. With the exception of gold (and platinum), the metals known in antiquity (copper, silver, iron, lead, tin, mercury, and, rather later in time, zinc) are unstable under average burial conditions. Thus copper is frequently converted into basic copper carbcnate, which is stable, and, as it is insoluble in water, it remains in situ until excavated. Pottery, on the other hand, is usually stable in the ground under most conditions of burial, although it may break into fragments because of the weight of the soil pressing down on it. However, if the pottery was only fired to a low temperature, as was much prehistoric pottery, burial in a damp environment may lead to break down of the fabric. This poses problems in even lifting the remains from the excavation trench - - they usually resemble a damp biscuit! The survival of organic remains is less usual, but, again, does depend on the burial conditions. For instance, a body buried in a very acid soil will often decay completely, so that not even the teeth
W.A. Oddy / Sci. Total Environ. 143 (1994) 121-126
or fingernails survive, but yet the shape of the body may be preserved as a stain, particularly in a sandy soil [9]. Under wet and anaerobic conditions, however, not only bones, but also the soft tissue may be preserved. Good examples are the numerous bodies found in or excavated from peat bogs in northern Europe [10]. At the opposite end of the spectrum, hot dry conditions will also lead to the preservation of bones and soft tissues, as the survival of naturally mummified bodies in Egypt testifies [11]. Wood, together with most other organic materials, also survives in either very dry or very wet conditions. In recent years, the possibility of excavating shipwrecks under water has led to a huge increase in the recovery of waterlogged wood and to the development of new methods for conserving it [12]. Again, the process of decay may proceed almost to completion, with the loss of all the cellulose but the retention of the lignin. The wood usually retains its shape but is then like a sponge full of water, and if the water is allowed to dry the wood will shrink to a fraction of its former size. However, it is more usual for wood to partially decay, so that there is an outer layer of very decayed wood, and a core, in which much of the cellulose survives. The process of decay has usually virtually stopped at this point because of the slowness of diffusion of oxygenated water into the wood. Once an object is excavated, the equilibrium which has been established with the surroundings is disturbed and the same or a different process of deterioration may start up. Copper alloys, for instance, may have formed patches of cuprous chloride below the layers of cuprite (cuprous oxide) and malachite (basic cupric carbonate). Once the object is exposed to the air, a rapid cyclical corrosion process starts, resulting in the appearance of light green eruptions of loose powdery cupric chloride on the surface, which is known as 'bronze disease' [13]. This process must be stopped, otherwise the object will be totally destroyed. By understanding the bronze disease mechanism it is possible to conclude that the process will stop if either oxygen or moisture are excluded from the object - - or if the cuprous chloride can
W.A. Oddy / Sci. Total Era,iron. 143 (1994) 121-126
be removed from the corrosion layers. The total removal of cuprous chloride, especially when the corrosion layers are very thick, has proved to be remarkably difficult, but stabilisation by excluding moisture has been more effective. The simplest way is to store or exhibit the object in an atmosphere where the relative humidity is controlled to less than 40%. This, however, is expensive. A cheaper way is to block the pores in the corrosion layers so that moisture cannot enter. This can be done by inmersing the bronze object in a 3% solution of benzotriazole in either water or industrial alcohol, which causes the precipitation of a copper-benzotriazole complex in the pores of the corrosion layers [14]. Finally a lacquer is applied onto the surface of the object to assist in keeping out the moisture contained in the surrounding atmosphere. 3. The conservation of archaeological materials One field of chemistry which has had a major impact on the conservation of antiquities is that of synthetic polymers [15]. These have a number of different applications: as a consolidant for strengthening an object; as an adhesive for sticking fragments together; as a gap-fill in restoration; and as a surface coating. Polymers are also used as moulding and casting materials in the making of replicas and for making mounts for use in display and storage. Rathgen seems to have been the first to recognise that cellulose nitrate made a useful adhesive and an invisible lacquer for antiquities [16], and so it has remained in spite of misguided attempts to relegate it to history [17]. The best quality cellulose nitrate adhesives do not yellow or become brittle with age [18] and the danger of spontaneous combustion, which is such a problem with the storage of nitrate film, does not occur with the adhesive provided that the antiquity is not packed in an airtight container. Cellulose nitrate will maintain its adhesion for decades and does not become insoluble with age [19]. Other polymers have fared less well. In the late 1950s, soluble nylon was being recommended for the consolidation of powdery surfaces on pottery and stone, particularly when they supported painted
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decoration [20]. Soluble nylon, which dissolved in industrial alcohol, was said to form a water-permeable barrier, and was hence widely used for consolidating the surfaces of pottery and stone before washing to remove soluble salts. Within 10 years the euphoria had disappeared when it was realised that soluble nylon cross-linked with age and became virtually impossible to remove [21]. In many cases the problems were exacerbated by the use of excessive amounts of the nylon, particularly as a consolidant for fragile ethnographic objects. One of the polymers which has stood the test of time is polyethylene glycol (PEG), a watersoluble wax. It is available in various 'grades', each with a different average molecular weight. PEG is a white, waxy solid which can be melted like a normal wax, but which will dissolve freely in water. If the concentrated solution is kept at approximately 60°C, the water will slowly evaporate to leave a bath of molten wax which solidifies on cooling. In the late 1950s it was recognised that this could be the answer to the conservation of waterlogged wood [22], and so it proved to be. Pieces of waterlogged wood could be immersed in a bath of approximately 10% PEG in water, the concentration and temperature of which were gradually increased over a period of several months. Eventually, all the water diffused out of the wood, to be replaced with molten PEG, so that when the wood was removed from the bath and allowed to cool, the PEG solidified and prevented shrinkage. This treatment of waterlogged wood is still widely practised, but PEG can also be used to partially impregnate the waterlogged wood, which is then freeze-dried [23]. This process leaves the wood feeling unnaturally light in weight, but a much more natural colour. This technique has also been used successfully to conserve the soft tissues of Lindow Man, a body found in a peat bog in Cheshire, England, in 1984 [24]. Polyethylene glycol has not been as successful in other applications. An attempt to use it to replace soluble nylon for the consolidation of powdery limestone [25] failed because soluble salts continued to make their way to the surface, where their removal by poulticing also removed some
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PEG, leaving 'tide marks' and the stone looking patchy, and also because the PEG on the surface makes it impossible for other adhesives to reattach the fragments of a sculpture together again. One other field of polymer chemistry which is making an impact is the use of inert gels to hold solvents in position on only a part of an object. One example from outside the field of archaeology is the use of a thick cellulose ether gel to remove patches of old water soluble adhesive from paper and papyrus without having to wet the whole sheet [26]. In the field of archaeological conservation, a synthetic sodium lithium magnesium silicate (Laponite) has been used as a poultice with both chemical solvents and water. On metals it has been mixed with complexing agents like alkaline glycerol for the selective removal of green corrosion layers on bronzes, and in ceramics conservation it is used as a poultice to draw stains out of glazed pottery through broken edges [27]. Other poulticing materials have also been used [28]. But it is in the field of adhesive technology that synthetic polymers have made their greatest contribution to the conservation of archaeological materials. Although 'impact' and 'contact' adhesives have little use, because of the need to have a few minutes working time to ensure that the join is correctly aligned, the new generation of methacrylate resin adhesives which harden when irradiated with ultra-violet light will revolutionise glass conservation by their ability to be hardened at will. One of these adhesives was used with great success in the recent re-restoration of the Portland Vase, the most important and famous surviving piece of Roman glass [29]. It was used in conjunction with a new epoxy resin (Hxtal NYL-1), the first adhesive ever to be produced specifically for the conservation profession. Hxtal is a waterwhite epoxy which is particularly resistant to yellowing with age. Its only disadvantages are that it takes a long time to cure completely (1-2 weeks) and it is rather expensive. Hxtal was used on the Portland Vase in conjunction with Loctite Engineering Adhesive 350, a UV sensitive methacrylate adhesive. The Hxtal was applied to most of the broken edge of one sherd, but a drop of Loctite was put at one end.
W.A. Oddy/Sci. Total Environ. 143 (1994)121-126
The two sherds were then fitted together and adjusted until they were correctly aligned. Then radiation from a UV lamp was directed onto the drop of Loctite, which immediately hardened, thus holding the two sherds in the correct register while the Hxtal was allowed to cure over several days. Being very resistant to discoloration, Hxtal can be used as a gap fill for the restoration of porcelain and glass, and can be coloured by the addition of suitable dyes and pigments. However, there is a tendency these days, when a translucent coloured fill is required, to add the colour as a paint on the surface of the infill rather than to add it as a dye to the infill itself. The reason for this is that if the infill does discolour with time, so that it no longer matches or tones in with the surrounding original glass or enamel, the painted surface can be removed and re-applied without having to re-do the infill. This is particularly important in the case of thin-walled vessel glass for which the actual restoration process can often be a hazard in itself [30]! The examples discussed so far demonstrate the use of either pre-polymerised materials in solution (PEG, cellulose nitrate and soluble nylon) or monomers which are polymerised in situ by mixing with a catalyst (epoxy resin) or by using UV radiation as an initiator (methacrylate). These systems work well as adhesives, gap-fillers and surface coatings, but present serious problems for consolidation. When solutions of polymers are used it is possible to achieve considerable depths of penetration in porous materials by prolonged immersion in an appropriate solution, but when the object is removed from the solution to allow evaporation of the solvent, much of the polymer will migrate towards the surface together with the solvent. This may not be a problem if the alteration layer is not very thick, but in marble and limestone for instance, it has been shown that the effects of weathering can penetrate deep into the stone. If the consolidant does not only consolidate the weathered layer but also firmly bond the weathered layer to the unaltered core it is not doing its job. Similar problems of achieving a sufficient depth of penetration occur with the use of catalysed
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W.A. Oddy /Sci. Total Environ. 143 (1994) 121-126
monomers. The monomer and catalyst must he mixed before application to the porous material, and are often rather viscous. Solvents may sometimes be used to reduce viscosity and increase penetration, but there is then a 'race' to effect sufficient penetration before the polymerisation is complete. The use of the UV curing methacrylate would be one way of overcoming this problem, but most archaeological materials are not transparent to UV light. However, the application of gamma-rays or neutrons to polymerise methyl methacrylate monomer after it has been absorbed by a porous material has been used on antiquities [31]. The problem is that suitable radiation facilities are not available to the average conservator and the process is not reversible. Once antiquities have been conserved, the museum curator is faced with maintaining the objects in good condition and it is a fallacy to think that objects are safe once they are on display or in storage in a museum. Metals are prone to corrosion as a result of industrial pollutants in the atmosphere, and there are a number of ways of protecting objects against these. One of the oldest is to use silver 'wool' or active charcoal to react with sulphides or to absorb impurities, respectively. Recently, however, pellets of zinc oxide have shown their worth for 'cleaning up' the atmosphere inside showcases containing polished silver [32]. This has proved to be more effective than using vapour phase inhibitors which sometimes leave the silver with a yellowish tinge. A different approach is to seal the object into an inert atmosphere, but this is not easy to engineer when curators want frequent access to their objects. Inert atmospheres of nitrogen or argon have been used, but are really only practical for objects in store. If objects are unlikely to be required for study they can be sealed in an inert atmosphere in a flexible container [33]. The flexibility means that the inert atmosphere can expand and contract with fluctuations in ambient temperature without creating additional pressure at the seams of the airtight container. This system can be adapted for use in a rigid showcase by fitting a 'lung' made of a flexible airtight material
in the base of the showcase to allow for the expansion and contraction of the atmosphere inside. Such storage systems are likely to assume greater importance in the future as conservators are more and more faced with the preservation of inherently unstable materials, such as natural rubber and some plastics. These can be sealed in flexible oxygen-impermeable containers, which also include an oxygen scavenger, and then purged with nitrogen. One such product currently on the market contains finely divided iron which reacts with virtually all traces of any oxygen which is present [33]. Increasingly conservators are looking towards less interventive ways of preserving objects for the future and so storage and exhibition conditions are now high on the agenda for attention. Many museums now routinely test all adhesives, paints, wood and textiles which are to be used for constructing storage cupboards and showcases to ensure that they will not emit gases or vapours which are harmful to antiquities. The harmful effect on metallic lead of acetic acid vapour emitted from oak is well known, but research in recent years has shown that many other timbers, especially modern 'synthetic' particle boards, will affect lead and other metals [34]. Silver, in particular, is tarnished by sulphur-containing gases which are often released by proteinaceous fibres, such as wool, and some modern adhesives will corrode lead and copper [35]. This is a very brief survey of some of the ways in which chemistry has contributed, and is contributing, to the conservation of antiquities. There are, however, whole fields of interest, such as the selective dissolution of corrosion products and the removal of surface dirt from objects as different as stone sculpture and fragile textiles which could not be covered in the space available. 4. References 1 N. Brommelle, Material for a history of Conservation: the 1850 and 1853 reports on the National Gallery. Stud. Conserv., 2(4) (1956) 176-188. 2 E.R. Caley, Early history and literature of archaeological chemistry. J. Chem. Educ., 28 (1951) 64-66.
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M. Gilberg, Friedrich Rathgen: the father of modern archaeological conservation. J. Am. Inst. Conserv., 26(2) (1987) 105-120. F. Rathgen, Die Konservierung von Alterthumsfunden, Berlin, 1898. F. Rathgen, The Preservation of Antiquities: A Handbook for Curators, Cambridge University Press, Cambridge, 1905. Information kindly supplied by Helge Brinch-Madsen in 1990. G.A. Rosenberg, Antiquiti6s en Fer et en Bronze, Copenhagen, 1917. J. Riederer, Das Rathgen-Forschungslabor der Staatlichen Museen Preussischer Kulturbesitz. Berl. Beitr. Arch~iometrie, 1 (1976) 13-31. A.C. Evans, The Sutton Hoo Ship Burial, British Museum Publications, London, 1986, pp. 119-121 and Fig. 99. P.V. Glob, The Bog People: Iron-Age Man Preserved, Faber and Faber, London, 1969. I have been unable to find a general account of air-dried bodies. However, see C. Johnson and B. Wills, The conservation of two pre-dynastic Egyptian bodies, in S.C. Watkins and C.E. Brown (Eds), Conservation of Ancient Egyptian Materials, UKIC/IAP, Butterworth, London, 1988, pp. 79-84. C. Pearson, Conservation of Marine Archaeological Objects, London, 1987; L.H. de Vries-Zuiderbaan, Conservation of Waterlogged Wood: International Symposium on the Conservation of Large Objects of Waterlogged Wood, The Hague, 1979; B.B. Christensen, The Conservation of Waterlogged Wood in the National Museum of Denmark, Copenhagen, 1970. T. Stambolov, The Corrosion and Conservation of Metallic Antiquities and Works of Art, Central Research Laboratories for Objects of Art and Science, Amsterdam, 1985, pp. 99-101. T. Stambolov, The Corrosion and Conservation of Metallic Antiquities and Works of Art, Central Research Laboratories for Objects of Art and Science, Amsterdam, 1985, pp. 109-110, and references therein. C.V. Horie, Materials for Conservation: Organic Consolidants, Adhesives and Coatings, Butterworth, London, 1987. F. Rathgen, The Preservation of Antiquities, Cambridge University Press, Cambridge, 1905, Appendix B, pp. 168-170. S. Koob, The instability of cellulose nitrate adhesives. The Conservator, 6 (1982) 31-34. e.g. HMG Heat and Waterproof Adhesive available from H. Marcel Guest Ltd., Riverside Works, Collyhurst Road, Manchester M10 7RU. Y. Shashoua, S.M. Bradley and V.D. Daniels, Degradation of Cellulose Nitrate Adhesive. Stud. Conserv., 37(2) (1992) 113-119. A.E.A. Werner, Technical notes on a new material in conservation. Chron. d'Egypte, 33 (1958) 273-278. C. Sease, The case against using soluble nylon in conservation work. Stud. Conserv., 26 (1981) 102-110.
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23
24
25
26
27
28
29
30
31
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33
34
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R.M. Organ, Carbowax and other materials in the treatment of waterlogged Paleolithic wood. Stud. Conserv., 4(3) (1959) 96-105. W.R. Ambrose, Freeze-drying of swamp-degraded wood, in Conservation of Wooden Objects, 2nd edn., International Institute for Conservation, London, 1971, pp. 53-57. I.M. Stead, J.B. Bourke and D. Brothwell (Eds), Lindow Man: The Body in the Bog, British Museum Publications, London, 1986. W.A. Oddy, S.M. Blackshaw and S. Baker, The consolidation of limestone sculptures with polyethylene glycol for indoor display, in R. Rossi-Manaresi (Ed.), The Conservation of Stone I, Centro per la Conservazione delle Sculture au'Aperto, Bologna, 1976, pp. 485-497. F. Elliot and E. Harding, A modern approach to papyrus conservation: materials and techniques as applied at the British Museum. The Paper Conservator, 11 (1987) 63-68. S. Dove, Laponite RD as a gelling agent, Conserv. News (24) (1984) 16; D. Ling, Laponite poulticing. Conserv. News, (46) (1991) 10-11 M. Sawada, A new technique for removal of corrosion products on gilded copper alloy artifacts, in: Current Problems in the Conservation of Metal Antiquities, Tokyo National Research Institute of Cultural Properties, Tokyo, 1989, pp. 187-190. N. Williams, The Breaking and Remaking of the Portland Vase, London, 1989; S. Smith, The Portland Vase, in A. Oddy (Ed.), The Art of the Conservator, British Museum Press, London, 1992, pp. 42-58. P. Fisher, The restoration of historical enamels, in M Stefanaggi (Ed.), Les Arts du Verre Histoire, Technique et Conservation, Champs-sue-Marne, 1991, pp. 181-191; P. Fisher, Hxtal NYL-1, an epoxy resin for the conservation of glass, in C. Daintith (Ed.), Glass and Enamel Conservation, London, 1991, pp. 6-9. R.A. Munnikendam, Conservation of waterlogged wood using radiation polymerization. Stud. Conserv., 12 (2) (1967) 70-75; C. de Tassigny, The suitability of gamma radiation polymerization for conservation treatment of large size waterlogged wood, in L.H. de Vries-Zuiderbaan op. cit. pp. 77-83. S.M. Bradley, Hydrogen sulphide scavengers for the prevention of silver tarnishing, in Environmental Monitoring and Control. Scott. Soc. Conserv. Restor., (1989) 65-68. Y. Shashoua and S. Thomsen, A field trial for the use of Ageless in the preservation of rubber in museum collections, in: Post-prints of Saving the 20th Century: The Conservation of Modern Materials, 1991. D. Grattan (Ed.). Canadian Conservation Institute, Ottawa, 1993, pp. 363-372. G. Werner, Corrosion of metal caused by wood in closed spaces, in J. Black (Ed.), Recent Advances in the Conservation and Analysis of Artifacts, London, 1988, pp. 185-187. W.A. Oddy, An unsuspected danger in display. Museums J., 73(1) (1973) 27-28; S.M. Blackshaw and V.D. Daniels, Selecting safe materials for use in the display and storage of antiquities, Preprints of the International Council of Museums Committee for Conservation, 5th Triennial Meeting, Zagreb, 1978.
)
,
!
/R
ELSEVIER SCIENCE IRELAND
the Science of the Total Environment The Science of the Total Environment 143 (1994) 121-126
Chemistry in the conservation of archaeological materials W.A. Oddy Department of Conservation, The British Museum, London WC1B 3DG, UK
Abstract This paper briefly reviews the deterioration of antiquities from a chemical point of view and then discusses the application of synthetic polymers in conservation. Materials which are mentioned include the use of polyethylene glycol for preserving waterlogged wood, cellulose ether gels as thickening agents for retaining solvents on surfaces, and epoxy resin and UV-curing methacrylates for the restoration of ceramics and glass• Finally, mention is made of ways of preventing objects deteriorating once they are on display in a museum.
Key words: Conservation; Archaeology; Museum; Polymers; Chemistry
1. Introduction
The application of chemistry to studying the decay and conservation of antiquities is not new. In the mid 19th century Michael Faraday studied the deterioration of easel paintings [1], and others were already applying the methods of chemical analysis to determining the composition of objects, particularly those made of metal [2]. The first scientist to have been employed in a museum, however, appears to have been Friedrich Rathgen (1862-1942) [3], who headed the laboratory in the Royal Museums in Berlin from 1888 to 1927. Rathgen published his methods as a book [4], which was subsequently translated into English [5]. Also in the mid 19th century, the National Museum in Copenhagen, faced, in particular, with the problem of preserving an increasing number of waterlogged wooden objects found in peat bogs, employed its first conservator, V.F. Steffensen, in 1867 [6]. In 1896 he was joined by Gustav Rosenberg (approx. 1870-1940) who also left a testament to his career in book form [7]. Sadly the scientific initiative in Germany did not survive the Second World War, although a
new laboratory, named after Rathgen, was founded in Berlin in 1975 [8]. The First World War, however, can be credited with initiating the scientific conservation of antiquities in the UK, because when British Museum objects were unpacked in 1919, after war-time storage in the underground railway tunnels, some of them were found to have noticeably deteriorated. As a result, the Department of Scientific and Industrial Research appointed Dr. Alexander Scott, FRS, as a consultant to The British Museum, and in 1926 Dr. Harold Plenderleith, MC, was recruited as a full-time conservation scientist. The career of Plenderleith is well known, culminating in the establishment of the International Centre for Conservation in Rome in 1959 with Plenderleith as its first director. In recent years, many other museums have recruited conservation scientists and some countries have established research institutes to service the conservation profession. Notable among these are Hungary, The Netherlands, Belgium, Canada and Japan. Although the number of conservation scientists in the world has increased significantly in the last
0048-9697/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved. SSDI 0048-9697(94)003683-S
122
30 years, it is still small in relation to the problems faced in trying to conserve the cultural heritage of mankind. Many problems have been solved but many remain, and it is the purpose of this paper to look at some of the problems and to show how chemistry has contributed to their solution.
2. The decay of archaeological materials Almost all objects deteriorate in use because of wear and tear, and deteriorate once they are lost and become buried because of chemical or biological processes. When an object is excavated it will usually have reached an equilibrium with its surroundings, which means that the processes of decay are at a standstill, or virtually so. The equilibrium may have been reached for two reasons; first, that the particular process may have proceeded to completion giving rise to alteration products which are stable in the burial environment; and second that the decay process has stopped because of a shortage of one of the agents of decay. Typical examples of objects which stabilise because decay processes have proceeded to completion are the corrosion of metals. With the exception of gold (and platinum), the metals known in antiquity (copper, silver, iron, lead, tin, mercury, and, rather later in time, zinc) are unstable under average burial conditions. Thus copper is frequently converted into basic copper carbcnate, which is stable, and, as it is insoluble in water, it remains in situ until excavated. Pottery, on the other hand, is usually stable in the ground under most conditions of burial, although it may break into fragments because of the weight of the soil pressing down on it. However, if the pottery was only fired to a low temperature, as was much prehistoric pottery, burial in a damp environment may lead to break down of the fabric. This poses problems in even lifting the remains from the excavation trench - - they usually resemble a damp biscuit! The survival of organic remains is less usual, but, again, does depend on the burial conditions. For instance, a body buried in a very acid soil will often decay completely, so that not even the teeth
W.A. Oddy / Sci. Total Environ. 143 (1994) 121-126
or fingernails survive, but yet the shape of the body may be preserved as a stain, particularly in a sandy soil [9]. Under wet and anaerobic conditions, however, not only bones, but also the soft tissue may be preserved. Good examples are the numerous bodies found in or excavated from peat bogs in northern Europe [10]. At the opposite end of the spectrum, hot dry conditions will also lead to the preservation of bones and soft tissues, as the survival of naturally mummified bodies in Egypt testifies [11]. Wood, together with most other organic materials, also survives in either very dry or very wet conditions. In recent years, the possibility of excavating shipwrecks under water has led to a huge increase in the recovery of waterlogged wood and to the development of new methods for conserving it [12]. Again, the process of decay may proceed almost to completion, with the loss of all the cellulose but the retention of the lignin. The wood usually retains its shape but is then like a sponge full of water, and if the water is allowed to dry the wood will shrink to a fraction of its former size. However, it is more usual for wood to partially decay, so that there is an outer layer of very decayed wood, and a core, in which much of the cellulose survives. The process of decay has usually virtually stopped at this point because of the slowness of diffusion of oxygenated water into the wood. Once an object is excavated, the equilibrium which has been established with the surroundings is disturbed and the same or a different process of deterioration may start up. Copper alloys, for instance, may have formed patches of cuprous chloride below the layers of cuprite (cuprous oxide) and malachite (basic cupric carbonate). Once the object is exposed to the air, a rapid cyclical corrosion process starts, resulting in the appearance of light green eruptions of loose powdery cupric chloride on the surface, which is known as 'bronze disease' [13]. This process must be stopped, otherwise the object will be totally destroyed. By understanding the bronze disease mechanism it is possible to conclude that the process will stop if either oxygen or moisture are excluded from the object - - or if the cuprous chloride can
W.A. Oddy / Sci. Total Era,iron. 143 (1994) 121-126
be removed from the corrosion layers. The total removal of cuprous chloride, especially when the corrosion layers are very thick, has proved to be remarkably difficult, but stabilisation by excluding moisture has been more effective. The simplest way is to store or exhibit the object in an atmosphere where the relative humidity is controlled to less than 40%. This, however, is expensive. A cheaper way is to block the pores in the corrosion layers so that moisture cannot enter. This can be done by inmersing the bronze object in a 3% solution of benzotriazole in either water or industrial alcohol, which causes the precipitation of a copper-benzotriazole complex in the pores of the corrosion layers [14]. Finally a lacquer is applied onto the surface of the object to assist in keeping out the moisture contained in the surrounding atmosphere. 3. The conservation of archaeological materials One field of chemistry which has had a major impact on the conservation of antiquities is that of synthetic polymers [15]. These have a number of different applications: as a consolidant for strengthening an object; as an adhesive for sticking fragments together; as a gap-fill in restoration; and as a surface coating. Polymers are also used as moulding and casting materials in the making of replicas and for making mounts for use in display and storage. Rathgen seems to have been the first to recognise that cellulose nitrate made a useful adhesive and an invisible lacquer for antiquities [16], and so it has remained in spite of misguided attempts to relegate it to history [17]. The best quality cellulose nitrate adhesives do not yellow or become brittle with age [18] and the danger of spontaneous combustion, which is such a problem with the storage of nitrate film, does not occur with the adhesive provided that the antiquity is not packed in an airtight container. Cellulose nitrate will maintain its adhesion for decades and does not become insoluble with age [19]. Other polymers have fared less well. In the late 1950s, soluble nylon was being recommended for the consolidation of powdery surfaces on pottery and stone, particularly when they supported painted
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decoration [20]. Soluble nylon, which dissolved in industrial alcohol, was said to form a water-permeable barrier, and was hence widely used for consolidating the surfaces of pottery and stone before washing to remove soluble salts. Within 10 years the euphoria had disappeared when it was realised that soluble nylon cross-linked with age and became virtually impossible to remove [21]. In many cases the problems were exacerbated by the use of excessive amounts of the nylon, particularly as a consolidant for fragile ethnographic objects. One of the polymers which has stood the test of time is polyethylene glycol (PEG), a watersoluble wax. It is available in various 'grades', each with a different average molecular weight. PEG is a white, waxy solid which can be melted like a normal wax, but which will dissolve freely in water. If the concentrated solution is kept at approximately 60°C, the water will slowly evaporate to leave a bath of molten wax which solidifies on cooling. In the late 1950s it was recognised that this could be the answer to the conservation of waterlogged wood [22], and so it proved to be. Pieces of waterlogged wood could be immersed in a bath of approximately 10% PEG in water, the concentration and temperature of which were gradually increased over a period of several months. Eventually, all the water diffused out of the wood, to be replaced with molten PEG, so that when the wood was removed from the bath and allowed to cool, the PEG solidified and prevented shrinkage. This treatment of waterlogged wood is still widely practised, but PEG can also be used to partially impregnate the waterlogged wood, which is then freeze-dried [23]. This process leaves the wood feeling unnaturally light in weight, but a much more natural colour. This technique has also been used successfully to conserve the soft tissues of Lindow Man, a body found in a peat bog in Cheshire, England, in 1984 [24]. Polyethylene glycol has not been as successful in other applications. An attempt to use it to replace soluble nylon for the consolidation of powdery limestone [25] failed because soluble salts continued to make their way to the surface, where their removal by poulticing also removed some
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PEG, leaving 'tide marks' and the stone looking patchy, and also because the PEG on the surface makes it impossible for other adhesives to reattach the fragments of a sculpture together again. One other field of polymer chemistry which is making an impact is the use of inert gels to hold solvents in position on only a part of an object. One example from outside the field of archaeology is the use of a thick cellulose ether gel to remove patches of old water soluble adhesive from paper and papyrus without having to wet the whole sheet [26]. In the field of archaeological conservation, a synthetic sodium lithium magnesium silicate (Laponite) has been used as a poultice with both chemical solvents and water. On metals it has been mixed with complexing agents like alkaline glycerol for the selective removal of green corrosion layers on bronzes, and in ceramics conservation it is used as a poultice to draw stains out of glazed pottery through broken edges [27]. Other poulticing materials have also been used [28]. But it is in the field of adhesive technology that synthetic polymers have made their greatest contribution to the conservation of archaeological materials. Although 'impact' and 'contact' adhesives have little use, because of the need to have a few minutes working time to ensure that the join is correctly aligned, the new generation of methacrylate resin adhesives which harden when irradiated with ultra-violet light will revolutionise glass conservation by their ability to be hardened at will. One of these adhesives was used with great success in the recent re-restoration of the Portland Vase, the most important and famous surviving piece of Roman glass [29]. It was used in conjunction with a new epoxy resin (Hxtal NYL-1), the first adhesive ever to be produced specifically for the conservation profession. Hxtal is a waterwhite epoxy which is particularly resistant to yellowing with age. Its only disadvantages are that it takes a long time to cure completely (1-2 weeks) and it is rather expensive. Hxtal was used on the Portland Vase in conjunction with Loctite Engineering Adhesive 350, a UV sensitive methacrylate adhesive. The Hxtal was applied to most of the broken edge of one sherd, but a drop of Loctite was put at one end.
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The two sherds were then fitted together and adjusted until they were correctly aligned. Then radiation from a UV lamp was directed onto the drop of Loctite, which immediately hardened, thus holding the two sherds in the correct register while the Hxtal was allowed to cure over several days. Being very resistant to discoloration, Hxtal can be used as a gap fill for the restoration of porcelain and glass, and can be coloured by the addition of suitable dyes and pigments. However, there is a tendency these days, when a translucent coloured fill is required, to add the colour as a paint on the surface of the infill rather than to add it as a dye to the infill itself. The reason for this is that if the infill does discolour with time, so that it no longer matches or tones in with the surrounding original glass or enamel, the painted surface can be removed and re-applied without having to re-do the infill. This is particularly important in the case of thin-walled vessel glass for which the actual restoration process can often be a hazard in itself [30]! The examples discussed so far demonstrate the use of either pre-polymerised materials in solution (PEG, cellulose nitrate and soluble nylon) or monomers which are polymerised in situ by mixing with a catalyst (epoxy resin) or by using UV radiation as an initiator (methacrylate). These systems work well as adhesives, gap-fillers and surface coatings, but present serious problems for consolidation. When solutions of polymers are used it is possible to achieve considerable depths of penetration in porous materials by prolonged immersion in an appropriate solution, but when the object is removed from the solution to allow evaporation of the solvent, much of the polymer will migrate towards the surface together with the solvent. This may not be a problem if the alteration layer is not very thick, but in marble and limestone for instance, it has been shown that the effects of weathering can penetrate deep into the stone. If the consolidant does not only consolidate the weathered layer but also firmly bond the weathered layer to the unaltered core it is not doing its job. Similar problems of achieving a sufficient depth of penetration occur with the use of catalysed
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monomers. The monomer and catalyst must he mixed before application to the porous material, and are often rather viscous. Solvents may sometimes be used to reduce viscosity and increase penetration, but there is then a 'race' to effect sufficient penetration before the polymerisation is complete. The use of the UV curing methacrylate would be one way of overcoming this problem, but most archaeological materials are not transparent to UV light. However, the application of gamma-rays or neutrons to polymerise methyl methacrylate monomer after it has been absorbed by a porous material has been used on antiquities [31]. The problem is that suitable radiation facilities are not available to the average conservator and the process is not reversible. Once antiquities have been conserved, the museum curator is faced with maintaining the objects in good condition and it is a fallacy to think that objects are safe once they are on display or in storage in a museum. Metals are prone to corrosion as a result of industrial pollutants in the atmosphere, and there are a number of ways of protecting objects against these. One of the oldest is to use silver 'wool' or active charcoal to react with sulphides or to absorb impurities, respectively. Recently, however, pellets of zinc oxide have shown their worth for 'cleaning up' the atmosphere inside showcases containing polished silver [32]. This has proved to be more effective than using vapour phase inhibitors which sometimes leave the silver with a yellowish tinge. A different approach is to seal the object into an inert atmosphere, but this is not easy to engineer when curators want frequent access to their objects. Inert atmospheres of nitrogen or argon have been used, but are really only practical for objects in store. If objects are unlikely to be required for study they can be sealed in an inert atmosphere in a flexible container [33]. The flexibility means that the inert atmosphere can expand and contract with fluctuations in ambient temperature without creating additional pressure at the seams of the airtight container. This system can be adapted for use in a rigid showcase by fitting a 'lung' made of a flexible airtight material
in the base of the showcase to allow for the expansion and contraction of the atmosphere inside. Such storage systems are likely to assume greater importance in the future as conservators are more and more faced with the preservation of inherently unstable materials, such as natural rubber and some plastics. These can be sealed in flexible oxygen-impermeable containers, which also include an oxygen scavenger, and then purged with nitrogen. One such product currently on the market contains finely divided iron which reacts with virtually all traces of any oxygen which is present [33]. Increasingly conservators are looking towards less interventive ways of preserving objects for the future and so storage and exhibition conditions are now high on the agenda for attention. Many museums now routinely test all adhesives, paints, wood and textiles which are to be used for constructing storage cupboards and showcases to ensure that they will not emit gases or vapours which are harmful to antiquities. The harmful effect on metallic lead of acetic acid vapour emitted from oak is well known, but research in recent years has shown that many other timbers, especially modern 'synthetic' particle boards, will affect lead and other metals [34]. Silver, in particular, is tarnished by sulphur-containing gases which are often released by proteinaceous fibres, such as wool, and some modern adhesives will corrode lead and copper [35]. This is a very brief survey of some of the ways in which chemistry has contributed, and is contributing, to the conservation of antiquities. There are, however, whole fields of interest, such as the selective dissolution of corrosion products and the removal of surface dirt from objects as different as stone sculpture and fragile textiles which could not be covered in the space available. 4. References 1 N. Brommelle, Material for a history of Conservation: the 1850 and 1853 reports on the National Gallery. Stud. Conserv., 2(4) (1956) 176-188. 2 E.R. Caley, Early history and literature of archaeological chemistry. J. Chem. Educ., 28 (1951) 64-66.
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16
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M. Gilberg, Friedrich Rathgen: the father of modern archaeological conservation. J. Am. Inst. Conserv., 26(2) (1987) 105-120. F. Rathgen, Die Konservierung von Alterthumsfunden, Berlin, 1898. F. Rathgen, The Preservation of Antiquities: A Handbook for Curators, Cambridge University Press, Cambridge, 1905. Information kindly supplied by Helge Brinch-Madsen in 1990. G.A. Rosenberg, Antiquiti6s en Fer et en Bronze, Copenhagen, 1917. J. Riederer, Das Rathgen-Forschungslabor der Staatlichen Museen Preussischer Kulturbesitz. Berl. Beitr. Arch~iometrie, 1 (1976) 13-31. A.C. Evans, The Sutton Hoo Ship Burial, British Museum Publications, London, 1986, pp. 119-121 and Fig. 99. P.V. Glob, The Bog People: Iron-Age Man Preserved, Faber and Faber, London, 1969. I have been unable to find a general account of air-dried bodies. However, see C. Johnson and B. Wills, The conservation of two pre-dynastic Egyptian bodies, in S.C. Watkins and C.E. Brown (Eds), Conservation of Ancient Egyptian Materials, UKIC/IAP, Butterworth, London, 1988, pp. 79-84. C. Pearson, Conservation of Marine Archaeological Objects, London, 1987; L.H. de Vries-Zuiderbaan, Conservation of Waterlogged Wood: International Symposium on the Conservation of Large Objects of Waterlogged Wood, The Hague, 1979; B.B. Christensen, The Conservation of Waterlogged Wood in the National Museum of Denmark, Copenhagen, 1970. T. Stambolov, The Corrosion and Conservation of Metallic Antiquities and Works of Art, Central Research Laboratories for Objects of Art and Science, Amsterdam, 1985, pp. 99-101. T. Stambolov, The Corrosion and Conservation of Metallic Antiquities and Works of Art, Central Research Laboratories for Objects of Art and Science, Amsterdam, 1985, pp. 109-110, and references therein. C.V. Horie, Materials for Conservation: Organic Consolidants, Adhesives and Coatings, Butterworth, London, 1987. F. Rathgen, The Preservation of Antiquities, Cambridge University Press, Cambridge, 1905, Appendix B, pp. 168-170. S. Koob, The instability of cellulose nitrate adhesives. The Conservator, 6 (1982) 31-34. e.g. HMG Heat and Waterproof Adhesive available from H. Marcel Guest Ltd., Riverside Works, Collyhurst Road, Manchester M10 7RU. Y. Shashoua, S.M. Bradley and V.D. Daniels, Degradation of Cellulose Nitrate Adhesive. Stud. Conserv., 37(2) (1992) 113-119. A.E.A. Werner, Technical notes on a new material in conservation. Chron. d'Egypte, 33 (1958) 273-278. C. Sease, The case against using soluble nylon in conservation work. Stud. Conserv., 26 (1981) 102-110.
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28
29
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31
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R.M. Organ, Carbowax and other materials in the treatment of waterlogged Paleolithic wood. Stud. Conserv., 4(3) (1959) 96-105. W.R. Ambrose, Freeze-drying of swamp-degraded wood, in Conservation of Wooden Objects, 2nd edn., International Institute for Conservation, London, 1971, pp. 53-57. I.M. Stead, J.B. Bourke and D. Brothwell (Eds), Lindow Man: The Body in the Bog, British Museum Publications, London, 1986. W.A. Oddy, S.M. Blackshaw and S. Baker, The consolidation of limestone sculptures with polyethylene glycol for indoor display, in R. Rossi-Manaresi (Ed.), The Conservation of Stone I, Centro per la Conservazione delle Sculture au'Aperto, Bologna, 1976, pp. 485-497. F. Elliot and E. Harding, A modern approach to papyrus conservation: materials and techniques as applied at the British Museum. The Paper Conservator, 11 (1987) 63-68. S. Dove, Laponite RD as a gelling agent, Conserv. News (24) (1984) 16; D. Ling, Laponite poulticing. Conserv. News, (46) (1991) 10-11 M. Sawada, A new technique for removal of corrosion products on gilded copper alloy artifacts, in: Current Problems in the Conservation of Metal Antiquities, Tokyo National Research Institute of Cultural Properties, Tokyo, 1989, pp. 187-190. N. Williams, The Breaking and Remaking of the Portland Vase, London, 1989; S. Smith, The Portland Vase, in A. Oddy (Ed.), The Art of the Conservator, British Museum Press, London, 1992, pp. 42-58. P. Fisher, The restoration of historical enamels, in M Stefanaggi (Ed.), Les Arts du Verre Histoire, Technique et Conservation, Champs-sue-Marne, 1991, pp. 181-191; P. Fisher, Hxtal NYL-1, an epoxy resin for the conservation of glass, in C. Daintith (Ed.), Glass and Enamel Conservation, London, 1991, pp. 6-9. R.A. Munnikendam, Conservation of waterlogged wood using radiation polymerization. Stud. Conserv., 12 (2) (1967) 70-75; C. de Tassigny, The suitability of gamma radiation polymerization for conservation treatment of large size waterlogged wood, in L.H. de Vries-Zuiderbaan op. cit. pp. 77-83. S.M. Bradley, Hydrogen sulphide scavengers for the prevention of silver tarnishing, in Environmental Monitoring and Control. Scott. Soc. Conserv. Restor., (1989) 65-68. Y. Shashoua and S. Thomsen, A field trial for the use of Ageless in the preservation of rubber in museum collections, in: Post-prints of Saving the 20th Century: The Conservation of Modern Materials, 1991. D. Grattan (Ed.). Canadian Conservation Institute, Ottawa, 1993, pp. 363-372. G. Werner, Corrosion of metal caused by wood in closed spaces, in J. Black (Ed.), Recent Advances in the Conservation and Analysis of Artifacts, London, 1988, pp. 185-187. W.A. Oddy, An unsuspected danger in display. Museums J., 73(1) (1973) 27-28; S.M. Blackshaw and V.D. Daniels, Selecting safe materials for use in the display and storage of antiquities, Preprints of the International Council of Museums Committee for Conservation, 5th Triennial Meeting, Zagreb, 1978.