- Email: [email protected]
Microbial induced corrosion of metallic antiquities and works of art: a critical review
International Biodeterioration & Biodegradation 29 (1992) 367-375
Microbial Induced Corrosion of Metallic Antiquities and Works of Art: a Critical Review
A. S ~ n c h e z d e l J u n c o ", D. A. M o r e n o b, C. R a n n i n g e r h, J. J. O r t e g a - C a l v o a & C. S f i i z - J i m r n e z a "Instituto de Recursos Naturales y Agrobiologia, CS1C, Apartado 1052, 41080 Sevilla, Spain ~'Departamento de Ingenieria y Ciencia de Materiales, Escuela Trcnica Superior de Ingenieros Industriales, Universidad Politrcnica de Madrid, Jos6 Gutirrrez Abascal, 2, 28006 Madrid, Spain
ABSTRACT Metallic antiquities and works of art are no exception to the phenomenon of microbial corrosion. Thus. all those which are buried, sunk or poorly conserved are susceptible to microbial corrosion. Although the last ,few decades have seen a marked development in the study of microbial corrosion of metals in industrial use, the same cannot be said for metallic antiquities and works of art. This work reviews the practical cases published, historical materials affected, and the micro-organisms involved and their mechanisms of action. Despite the great social importance of recovering these metallic cultural properties, and conserving them in a good state, there is a lack of documentation and of research groups dedicated to their study. The bringing together of researchers in works of art. microbiology, and corrosion in metallic materials is shown to be necessa~.
INTRODUCTION In antiquity m a n c o u l d h a r d l y i m a g i n e that the fate o f s o m e o f his b r o n z e sculptures, helmets, swords a n d o t h e r metallic objects c o n c e i v e d from a n d for their culture were to r e m a i n b u r i e d or s u n k during centuries until archaeologists" h a n d s b r o u g h t t h e m to o u r m u s e u m s . D u r i n g their close contact with terrestrial or a q u a t i c e n v i r o n m e n t s , those objects, n o w 367 International Biodeterioration & Biodegradation 0964-8305/92/$05.00© 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain.
368 A. Sgmchez del Junco, D. A. Moreno, C. Ranninger, J. J. Ortega-Calvo, C. Sdiz-JimOnez
considered of cultural a n d artistical value, have suffered corrosion of the constituting metal or alloy. The degree a n d type of corrosion depends on the nature of the metal a n d the characteristics of the e n v i r o n m e n t where the object has remained. Furthermore, as Stambolov (1985) pointed out, the behaviour of a metal artifact after excavation will be influenced by the conditions of burial. Consequently, the design of appropriate a n d scientifically based conservation strategies needs to be aware of these conditions. F r o m the point of view of chemists, engineers a n d metallurgists involved in industrial corrosion, such corroded antiquities provide realistic examples of long-term corrosion in nature, which is sometimes difficult to mimic. This may generate useful information for the knowledge of the durability of materials used today. Most studies on corrosion of metallic works of art do not consider the microbial life in the excavated sites, although the occurrence of microbial i n d u c e d corrosion (MIC) processes in nature is well known. There is no reason to exclude these processes in objects of historical value, especially when in some cases MIC of the same metals in similar e n v i r o n m e n t s has been considered to be industrially a n d technologically important. Furthermore, as in the industrial problems created by MIC, where engineers have traditionally lacked microbiological training (Fellers, 1989), MIC is not usually considered by archaeologists a n d conservators when studying the causes of corrosion of metallic antiquities. Thus, the aim of this review is to connect the existing knowledge on MIC in nature with cultural heritage a n d specifically with metallic antiquities a n d works of art. MIC m e c h a n i s m s in nature will be briefly discussed from this point of view, followed by a review of specific studies on MIC in metallic antiquities.
MIC IN N A T U R E MIC has been known to occur in a wide variety of e n v i r o n m e n t s a n d structures, such as u n d e r g r o u n d pipelines, aircraft fuel tanks and offshore oil a n d gas p r o d u c t i o n structures (Iverson, 1987). MIC is most c o m m o n l y associated with sulphate-reducing bacteria (SRB) according to H a m i l t o n (1985), a n d the e c o n o m i c relevance of this process in industry has been recently e m p h a s i z e d (Videla, 1991). F r o m the point of view of M I C in natural e n v i r o n m e n t s where archaeological specimens have been recovered, two different environments, terrestrial a n d aquatic, can be considered. Soils are complex entities whose formation is notably influenced by
MIC of metallic antiquities and works of art: review
369
different processes, such as weathering of rocks, microbial degradation of organic matter and redistribution of the organic and inorganic materials through water movement. The result is the formation of soils organized in layers or horizons, differing in colour, texture, in organic matter content, etc. Different physicochemical conditions are provided for the inhabiting microbial communities, which thus show a high degree of heterogeneity (Atlas & Bartha, 1987; Lynch, 1988). Local physicochemical characteristics of the soils can even be modified in the archaeological sites due to the presence of buried m a n - m a d e structures and artifacts. Tylecote (1979) and, more recently, Stambolov (1985) have discussed the corrosivity of the soil medium in the context of archaeology. Sandy soils are considered as highly aggressive to metals. Water and air move easily through their large pores, allowing rapid corrosion. Soils with a predominant clay texture, or those that are calcareous in composition, are generally less corrosive, because of their poor water percolation, their low air content and the low rate at which corrosive reactions occur in an alkaline environment. MIC in soils is often associated with the presence of SRB in anaerobic conditions. Unique features of corrosion caused by SRB are that it occurs at neutral pH in anaerobic environments, oxygen is not involved, and the corrosion products include iron sulphides (Hamilton, 1985). Perhaps one of the most studied cases of MIC is the anaerobic corrosion of iron by SRB. The role of these bacteria is a rather complex one, acting in different ways in anodic and cathodic reactions during metal passivity breakdown (Videla, 1991). On the one hand, sulphide and disulphide ions generated by SRB enhance the corrosive activity of chloride ions (anodic effect). On the other hand, a cathodic effect is exerted because: (1) biogenic sulphides may be used as alternative substrates in the cathodic reaction at low oxygen concentration and at not very low pH, and (2) iron sulphide formation involves the use of ferrous ions generated from metal solution. Underground corrosion can be markedly accelerated by alternate anaerobic and aerobic conditions, for example, during changes in water table level (Royuela, 1991). Sulphides formed by SRB in anaerobic conditions are transformed to polysulphides, sulphur and ferric sulphate, decreasing soil pH. The change to anaerobic conditions induces the reduction of corrosion products by cathodic hydrogen, raising the pH and favouring the development of SRB. Unlike archaeological material recovered from soils, with a very wide distribution, the main source of historically valuable artifacts recovered from the sea are shipwrecks. According to Tylecote (1977), this material
370 A. S(mchez del Junco. D. A. Moreno. C. Ranninger. J. J. Ortega-Calvo, C. S6iz-JimOnez
falls into two major categories: (1) Greek to R o m a n ships a n d their cargoes sunk in the Mediterranean, a n d (2) A r m a d a a n d later ships sunk o f f t h e coasts of the British Isles a n d western Europe. Two m i n o r groups are constituted by a few Bronze Age shipwrecks, a n d 17th century a n d later ships sunk off the Australian coast. M e a n seawater temperature a n d the nature of the turbulence are very i m p o r t a n t factors in the characterization of shipwreck sites (MacLeod, 1989). They determine the rate at which corrosion occurs. Once a metal artifact is placed in a m a r i n e e n v i r o n m e n t , it is progressively covered by biofouling. This process plays a major role in the corrosion of the artifact, because the biofilm act as a semipermeable m e m b r a n e . For example, it causes the chloride concentration to be increased by a factor of three in the i m m e d i a t e s u r r o u n d i n g s of the metal in c o m p a r i s o n to the m e a n content of the water. The pH can fall from the n o r m a l value of 8.2 to c. 4.8 (MacLeod, 1989). Anaerobic conditions created by biological colonization (Tylecote, 1977) or within sediments ( H a m i l t o n et al.. 1988) may p r o m o t e M I C due to the activity of SRB.
MIC CASES IN M E T A L L I C A N T I Q U I T I E S Walker (1980) reported that, in polluted atmospheres containing hydrogen sulphide, copper may form chalcocite (Cu2S) or covellite (CuS). The latter is also especially a b u n d a n t on copper a n d bronze recovered from wrecks of w o o d e n ships, because SRB are often found in the decaying wood, a n d they can convert sulphates in water to sulphides. These bacteria also survive u n d e r g r o u n d so that buried structures may show similar corrosion products. The corrosion patina of m a n y metallic objects is often decorative as well as protective. The surface layer formed may well have given protection to the u n d e r l y i n g metal over h u n d r e d s or t h o u s a n d s of years. This fact is interesting in the case of metallic objects of art. The first e n c o u n t e r e d report is that of Farrer et al. (1953)~ who investigated archaeological iron specimens f o u n d in an excellent state of preservation on a site at Hungate, York. These were nails a n d knives, some being R o m a n (about 2000 years old), some Saxon (about 1000 years old) a n d others mediaeval. According to Farrer et al.. such small iron specimens would not n o r m a l l y be expected to r e m a i n recognizable after burial for such periods of time, even in relatively n o n agressive soils, because in the waterlogged, anaerobic conditions of the Hungate site it would be expected that iron would be rapidly corroded. The most interesting level of the excavated site was a peat, with a black silty subsoil,
MIC of metallic antiquities and works of art: review
371
in which a great quantity of plants and some animal refuse were found. M a n y leather cuttings were found in the peaty layers, together with several mediaeval shoes and a n u m b e r of knives, at least one of which had probably been used in the working of leather. It was believed that the mediaeval Cordwainers Hall stood nearby, indicating the centre of a leather industry. Analysis of the soil revealed it to be anaerobic, to contain sulphate and to have a pH suitable for bacterial growth. M a n y of the soil samples contained highly coloured mottles and patches of a blue c o m p o u n d closely resembling in appearance the mineral vivianite, Fe3(PO4) 2. 8H20. In fact, X-ray examination of scrapings from a R o m a n knife revealed that the corrosion products were a mixture of ferric phosphate hydrate and ferrous phosphate hydrate, in approximately equal proportions. Chemical examination of the product also revealed the presence of traces of tannate. Farrer et al. (1953) considered that the presence of large quantities of wood had probably increased the concentration of tannate in the surrounding soil which, being peaty by nature, would also have a natural content of tannins. The burial of scrap leather would, however, greatly increase the concentration oftannate locally. Animal and especially dog refuse (blackened dog bones were found in the peat) is a rich source of phosphate, and this would locally increase the concentration of phosphate. The authors stated that these two factors would appear to have been of primary importance in the preservation of iron specimens on the site. The phosphate had functioned by providing a protective deposit upon the iron objects, and the tannate has e n h a n c e d this protection by suppresing bacterial activity. It is well known that the acid, anaerobic character of most peat deposits ensures that microbial degradation of buried organic materials is extremely slow. Thus, organisms or their parts, which are trapped in the peat, do not decompose and may be excavated at a later date, beautifully preserved. This is dramatically emphasized by the 700 or so Iron and Bronze Age h u m a n corpses which have been recovered from Scandinavian peat bogs (Richardson, 1981). Furthermore, the complex mixture of organic substances present in peat usually includes phenols and polyphenols. A correlation between phenolic structure and microbial inhibitory activity is evident (Fuchsman, 1980). Therefore, inactivation of bacteria involved in corrosion processes could be considered as the main cause of preservation of ancient buried iron objects in York. Farrer et al. (1953) demonstrated the inactivation o f Desulfovibrio desulfuricans by addition of 0-5% of soil from the excavations to culture media. Similarly, archaeological materials recovered from wrecks have been found in different preservation conditions, in which tannins were also
372 A. S6nchez deI Junco, D. A. Moreno. C Ranninger. J. J. Ortega-Ca&o, C. Sdiz-Jim¢;nez p r e s u m e d to be protective for metallic structures. A group of Greek a n d Punic wrecks from the 3rd century BC containing wood, lead sheet, bronze, copper a n d iron nails were recovered off the Sicilian coast near Marsala. Some of the nails attached to the timbers of the vessel were f o u n d to be of leaded bronze, a n d claimed to be possibly protected by tannins from corrosion. The nails released from the wood structure had suffered considerable corrosion and, by some m e c h a n i s m , the surface h a d been depleted in tin and lead, which is the reverse of what might have been expected from the relative insolubility of tin c o m p o u n d s . The corrosion conditions were anaerobic, caused by the growth a n d decay of P o s i d o n i a grass, which, at the time of excavation of the wreck, was giving offlarge a m o u n t s of hydrogen sulphide. There was also m a r k e d evidence of SRB, which were responsible for the conversion of the metal corrosion products to lead and copper sulphides (Tylecote, 1977). M a c L e o d (1982) distinguished three types of concretions on copper objects recovered from wrecks sites. The first appears as a cemented mixture o f c o r a l l i n e material a n d shell fragments, which is fairly porous a n d is stained with green copper corrosion products. The second type of aerobically formed copper concretions are often dense, thin layers of c e m e n t e d calcareous material, which give fairly adventitious iron corrosion products. The third type of concretion is associated with material which has been in an anaerobic environment. The source of sulphide ions is the activity of naturally occurring bacteria which are f o u n d in anaerobic m a r i n e sediments. T h e colour is that of the d o m i n a n t copper sulphide that form the bulk of the concretion. In cases of disturbance of the wreck site, a mixture of types of concretion can be found, shown by the appearance of green (aerobic) a n d deep blue-grey black (anaerobic) copper corrosion products. More recently, Daldorff (1987) also considered the MIC of iron objects. Different ions a n d total iron were analysed in the extracts obtained during conservation procedures at the Tromso M u s e u m (Norway), during the period 1982-1985, of 2500 to 3000 iron objects excavated from the soil. Organic matter was also determined for about 50% of the objects. Based on these analyses, the a u t h o r c o n c l u d e d that all elements necessary for the growth of micro-organisms such as bacteria a n d fungi were present. The presence of sulphide in all of the 50 objects tested was indicative of a contact with SRB in at least one stage of the deterioration process. Nevertheless, no microbiological studies were performed a n d the discussion of M I C in those objects was merely speculative. D u n c a n & Ganiaris (1987) studied the corrosion products of excavated metal objects from L o n d o n waterfront sites. Some of these
MIC of metallic antiquities and works of art: review
373
objects, made of copper alloys and lead alloys, showed a 'dull-gold' coloured layer which occasionally obscured the underlying alloy. Black corrosion products were also present on copper alloy objects. The analysis of such corrosion products revealed that they consisted of sulphides such as chalcopyrite, pyrrhotite, chalcocite and covellite. The anaerobic soils where those objects were excavated were typically black, wet and cohesive, with a foul sulphurous smell. The authors attributed the formation of the sulphide corrosion products to the SRB prevalent in those soils, through the generation of hydrogen sulphide. Oxidation of the sulphides may form sulphuric acid, which causes further corrosion. The stability of sulphide layers of the copper alloy objects was also studied by D u n c a n & Ganiaris (1987) in recently excavated material and compared with objects of similar origin that had been acquired by the Museum of London since the late 19th century. In both cases, a green corrosion product sometimes occurred associated with black corrosion layers. Experiments with recently excavated waste fragments showed that high relative humidity accelerated the colour change of sulphide surfaces. Mechanical and/or chemical cleaning was proposed for the removal of sulphide corrosion products. The deterioration of metallic works of art is not restricted only to buried or watery environments--incorrect preservation treatment or adverse environmental conditions leads to corrosion of m u s e u m objects. Brinch Madsen & Hjelm-Hansen (1979) observed that bronzes from Copenhagen, Stockholm, Oslo and London museums showed black spots on their surfaces. Chemical and X-ray diffraction analyses revealed that the black spots were copper sulphide. Because the spots resembled small mould spot, they were examined for micro-organisms. Three isolates from culture media containing increasing amounts of copper were obtained: one of them was probably a Cladosporium sp. Evidence for a chemical reduction of sulphur dioxide to hydrogen sulphide in the object's surroundings was lacking; sulphur-reducing micro-organisms are anaerobic, and museums do not provide ideal conditions for such growth. Therefore, a precise explanation of the origin of the black spots developed on the m u s e u m exhibits was not advanced. Makes (1985) reported that a 16th century chased helmet placed in a closed glass show-case was attacked by micro-organisms when the relative air humidity and temperature changed accidentally. The helmet was preserved with a mixture of lanolin, vegetable fats and water, which formed an emulsion favourable for microbial growth. The microorganisms produced lipolytic enzymes which hydrolysed the fats into fatty acids. The acids lowered the pH, which in turn increased the
374 A. S(mchezdel Junco, D. A. Moreno. C. Ranninger. J. J. Ortega-Cairo. C. S(fiz-Jimdnez corrosion of the helmet. The changes caused damage to the tin surface coating and the underlying steel, both acting as a galvanic cell, with the steel functioning as an anode and the tin coating as a cathode.
CONCLUSIONS The corrosion of archaeological metallic artifacts has usually been regarded as a chemical process, in which microbiological agents were rarely thought to be involved. However, bacteria have an important role in m a n y corrosion processes in nature, particularly u n d e r anaerobic conditions. In most cases it was only inferred that such a process could occur after burial or sinking of the object. This has resulted in a scarcity of papers on this topic, and those available are generally devoid of experimental and conclusive data on MIC. The intervention of MIC in the deterioration of metallic antiquities and works of art is today taken into consideration. This will undoubtedly allow the analysis, from both biological and metallurgical points of view, of the state of conservation of metallic works of art exposed to environments which will possibly contain microorganisms. Information and knowledge has been passed from the technical and industrial field, where economic and efficiency motives have pushed the study of MIC, to the area of cultural heritage. This could speed the adoption of measures to avoid the deterioration and facilitate the conservation of works of art for future generations.
REFERENCES Atlas, R. M. & Bartha, R. (1987). Microbial Ecology. Fundamentals and Applications, Benjamin/Cummings, Menlo Park, CA. Brinch Madsen, H. & Hjelm-Hansen, N. (1979). Black spots on bronzes -- a microbiological or chemical attack. Proceedings of the Symposium on the Conservation and Restoration of Metals, Scottish Society for Conservation & Restoration, Edinburgh, pp. 33-9. Daldorff, S. A. (1987). Microbial corrosion and museum iron objects. 8th Triennial Meeting, ICOM Committee for Conservation, Sydney, Australia. pp. 1063-6. Duncan, S. J. & Ganiaris, H. (1987). Some sulphide corrosion products on copper alloys and lead alloys from London waterfront sites. In Recent Advances in the Conservation and Analysis of ArtOeacts, ed. J. Black. Summer Schools Press, London. pp. 109-18. Farrer, T.W., Biek, L. & Wormweli, F. (1953). The role of tannates and phosphates in the preservation of ancient buried iron objects. J. Appl. Chem.. 3, 80-4.
MIC of metallic antiquities and works of art: review
375
Fellers, B. D. (1989). An interdisciplinary perspective on microbiological influenced corrosion. Corrosion 89. NACE, Houston, TX, Paper 510. Fuchsman, C. H. (1980). Peat. Industrial Chemist~ and Technology. Academic Press, New York. Hamilton, W. A. (1985). Sulphate-reducing bacteria and anaerobic corrosion. Ann. Rev. Microbiol.. 39, 195-217. Hamilton, W. A., Moosavi, A. N. & Pirrie, R. N. (1988). Mechanism of anaerobic microbial corrosion in the marine environment. In Microbial Corrosion L eds C. A. C. Sequeira & A. K. Tiller. Elsevier, Amsterdam. pp. 13-19. Iverson, W. P. (1987). Microbial corrosion of metals. Advances in Applied Microbiology. 32, 1-36. Lynch, J. M. (1988). The terrestrial environment. In Micro-organisms in Action." Concepts and Applications in Microbial Ecology. eds J. M. Lynch & J. E. Hobbie. Blackwell, Oxford, UK, pp. 103-31. MacLeod, I. D. (1982). Formation of marine concretions on copper and its alloys. Int. J. Nautical Archaeol. and Underwater Explor.. 11,267-75. MacLeod, I. D. (1989). Marine corrosion on historic shipwrecks and its application to modern materials. Corrosion Australasia, 14, 8-14. Makes, F. (1985). Preservation ofa 16th century Italian helmet. Livrustkammaren. 17, 79-92. Richardson, D. H. S. (1981). The Biology of Mosses. Blackwell, Oxford, UK, pp. 194-5. Royuela, J. J. (1991). Corrosirn en suelos. In Corrosirn y Protecci6n Metdtlicas. Vol II. Coleccirn Nuevas Tendencias. eds S. Feliu & M. C. Andrade. Consejo Superior de Investigaciones Cientificas, Madrid. pp. 3-21 Stambolov, T. (1985). The Corrosion and Conservation of Metallic Antiquities and Works of Art. CL Publication, Central Research Laboratory for Objects of Art and Science, Amsterdam. Tylecote, R. F. (1977). Durable materials for sea water: the archaeological evidence. Int. J. Nautical Archaeol. and Underwater Explor., 6, 269-83. Tylecote, R. F. (1979). The effect of soil conditions on the long-term corrosion of buried tin-bronzes and copper. J. Archaeol. Sci.. 6, 345-68. Videla, H. A. (1991). Corrosirn microbiol6gica. In Corrosirn y Proteccirn Metdllicas. Vol I. Coleccirn Nuevas Tendencias, eds S. Feliu & M. C. Andrade. Consejo Superior de Investigaciones Cientificas, Madrid. pp. 117-37. Walker, R. (1980). Corrosion and preservation of bronze artifacts.J. Chem. Educ.. 57, 277-80.
Microbial Induced Corrosion of Metallic Antiquities and Works of Art: a Critical Review
A. S ~ n c h e z d e l J u n c o ", D. A. M o r e n o b, C. R a n n i n g e r h, J. J. O r t e g a - C a l v o a & C. S f i i z - J i m r n e z a "Instituto de Recursos Naturales y Agrobiologia, CS1C, Apartado 1052, 41080 Sevilla, Spain ~'Departamento de Ingenieria y Ciencia de Materiales, Escuela Trcnica Superior de Ingenieros Industriales, Universidad Politrcnica de Madrid, Jos6 Gutirrrez Abascal, 2, 28006 Madrid, Spain
ABSTRACT Metallic antiquities and works of art are no exception to the phenomenon of microbial corrosion. Thus. all those which are buried, sunk or poorly conserved are susceptible to microbial corrosion. Although the last ,few decades have seen a marked development in the study of microbial corrosion of metals in industrial use, the same cannot be said for metallic antiquities and works of art. This work reviews the practical cases published, historical materials affected, and the micro-organisms involved and their mechanisms of action. Despite the great social importance of recovering these metallic cultural properties, and conserving them in a good state, there is a lack of documentation and of research groups dedicated to their study. The bringing together of researchers in works of art. microbiology, and corrosion in metallic materials is shown to be necessa~.
INTRODUCTION In antiquity m a n c o u l d h a r d l y i m a g i n e that the fate o f s o m e o f his b r o n z e sculptures, helmets, swords a n d o t h e r metallic objects c o n c e i v e d from a n d for their culture were to r e m a i n b u r i e d or s u n k during centuries until archaeologists" h a n d s b r o u g h t t h e m to o u r m u s e u m s . D u r i n g their close contact with terrestrial or a q u a t i c e n v i r o n m e n t s , those objects, n o w 367 International Biodeterioration & Biodegradation 0964-8305/92/$05.00© 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain.
368 A. Sgmchez del Junco, D. A. Moreno, C. Ranninger, J. J. Ortega-Calvo, C. Sdiz-JimOnez
considered of cultural a n d artistical value, have suffered corrosion of the constituting metal or alloy. The degree a n d type of corrosion depends on the nature of the metal a n d the characteristics of the e n v i r o n m e n t where the object has remained. Furthermore, as Stambolov (1985) pointed out, the behaviour of a metal artifact after excavation will be influenced by the conditions of burial. Consequently, the design of appropriate a n d scientifically based conservation strategies needs to be aware of these conditions. F r o m the point of view of chemists, engineers a n d metallurgists involved in industrial corrosion, such corroded antiquities provide realistic examples of long-term corrosion in nature, which is sometimes difficult to mimic. This may generate useful information for the knowledge of the durability of materials used today. Most studies on corrosion of metallic works of art do not consider the microbial life in the excavated sites, although the occurrence of microbial i n d u c e d corrosion (MIC) processes in nature is well known. There is no reason to exclude these processes in objects of historical value, especially when in some cases MIC of the same metals in similar e n v i r o n m e n t s has been considered to be industrially a n d technologically important. Furthermore, as in the industrial problems created by MIC, where engineers have traditionally lacked microbiological training (Fellers, 1989), MIC is not usually considered by archaeologists a n d conservators when studying the causes of corrosion of metallic antiquities. Thus, the aim of this review is to connect the existing knowledge on MIC in nature with cultural heritage a n d specifically with metallic antiquities a n d works of art. MIC m e c h a n i s m s in nature will be briefly discussed from this point of view, followed by a review of specific studies on MIC in metallic antiquities.
MIC IN N A T U R E MIC has been known to occur in a wide variety of e n v i r o n m e n t s a n d structures, such as u n d e r g r o u n d pipelines, aircraft fuel tanks and offshore oil a n d gas p r o d u c t i o n structures (Iverson, 1987). MIC is most c o m m o n l y associated with sulphate-reducing bacteria (SRB) according to H a m i l t o n (1985), a n d the e c o n o m i c relevance of this process in industry has been recently e m p h a s i z e d (Videla, 1991). F r o m the point of view of M I C in natural e n v i r o n m e n t s where archaeological specimens have been recovered, two different environments, terrestrial a n d aquatic, can be considered. Soils are complex entities whose formation is notably influenced by
MIC of metallic antiquities and works of art: review
369
different processes, such as weathering of rocks, microbial degradation of organic matter and redistribution of the organic and inorganic materials through water movement. The result is the formation of soils organized in layers or horizons, differing in colour, texture, in organic matter content, etc. Different physicochemical conditions are provided for the inhabiting microbial communities, which thus show a high degree of heterogeneity (Atlas & Bartha, 1987; Lynch, 1988). Local physicochemical characteristics of the soils can even be modified in the archaeological sites due to the presence of buried m a n - m a d e structures and artifacts. Tylecote (1979) and, more recently, Stambolov (1985) have discussed the corrosivity of the soil medium in the context of archaeology. Sandy soils are considered as highly aggressive to metals. Water and air move easily through their large pores, allowing rapid corrosion. Soils with a predominant clay texture, or those that are calcareous in composition, are generally less corrosive, because of their poor water percolation, their low air content and the low rate at which corrosive reactions occur in an alkaline environment. MIC in soils is often associated with the presence of SRB in anaerobic conditions. Unique features of corrosion caused by SRB are that it occurs at neutral pH in anaerobic environments, oxygen is not involved, and the corrosion products include iron sulphides (Hamilton, 1985). Perhaps one of the most studied cases of MIC is the anaerobic corrosion of iron by SRB. The role of these bacteria is a rather complex one, acting in different ways in anodic and cathodic reactions during metal passivity breakdown (Videla, 1991). On the one hand, sulphide and disulphide ions generated by SRB enhance the corrosive activity of chloride ions (anodic effect). On the other hand, a cathodic effect is exerted because: (1) biogenic sulphides may be used as alternative substrates in the cathodic reaction at low oxygen concentration and at not very low pH, and (2) iron sulphide formation involves the use of ferrous ions generated from metal solution. Underground corrosion can be markedly accelerated by alternate anaerobic and aerobic conditions, for example, during changes in water table level (Royuela, 1991). Sulphides formed by SRB in anaerobic conditions are transformed to polysulphides, sulphur and ferric sulphate, decreasing soil pH. The change to anaerobic conditions induces the reduction of corrosion products by cathodic hydrogen, raising the pH and favouring the development of SRB. Unlike archaeological material recovered from soils, with a very wide distribution, the main source of historically valuable artifacts recovered from the sea are shipwrecks. According to Tylecote (1977), this material
370 A. S(mchez del Junco. D. A. Moreno. C. Ranninger. J. J. Ortega-Calvo, C. S6iz-JimOnez
falls into two major categories: (1) Greek to R o m a n ships a n d their cargoes sunk in the Mediterranean, a n d (2) A r m a d a a n d later ships sunk o f f t h e coasts of the British Isles a n d western Europe. Two m i n o r groups are constituted by a few Bronze Age shipwrecks, a n d 17th century a n d later ships sunk off the Australian coast. M e a n seawater temperature a n d the nature of the turbulence are very i m p o r t a n t factors in the characterization of shipwreck sites (MacLeod, 1989). They determine the rate at which corrosion occurs. Once a metal artifact is placed in a m a r i n e e n v i r o n m e n t , it is progressively covered by biofouling. This process plays a major role in the corrosion of the artifact, because the biofilm act as a semipermeable m e m b r a n e . For example, it causes the chloride concentration to be increased by a factor of three in the i m m e d i a t e s u r r o u n d i n g s of the metal in c o m p a r i s o n to the m e a n content of the water. The pH can fall from the n o r m a l value of 8.2 to c. 4.8 (MacLeod, 1989). Anaerobic conditions created by biological colonization (Tylecote, 1977) or within sediments ( H a m i l t o n et al.. 1988) may p r o m o t e M I C due to the activity of SRB.
MIC CASES IN M E T A L L I C A N T I Q U I T I E S Walker (1980) reported that, in polluted atmospheres containing hydrogen sulphide, copper may form chalcocite (Cu2S) or covellite (CuS). The latter is also especially a b u n d a n t on copper a n d bronze recovered from wrecks of w o o d e n ships, because SRB are often found in the decaying wood, a n d they can convert sulphates in water to sulphides. These bacteria also survive u n d e r g r o u n d so that buried structures may show similar corrosion products. The corrosion patina of m a n y metallic objects is often decorative as well as protective. The surface layer formed may well have given protection to the u n d e r l y i n g metal over h u n d r e d s or t h o u s a n d s of years. This fact is interesting in the case of metallic objects of art. The first e n c o u n t e r e d report is that of Farrer et al. (1953)~ who investigated archaeological iron specimens f o u n d in an excellent state of preservation on a site at Hungate, York. These were nails a n d knives, some being R o m a n (about 2000 years old), some Saxon (about 1000 years old) a n d others mediaeval. According to Farrer et al.. such small iron specimens would not n o r m a l l y be expected to r e m a i n recognizable after burial for such periods of time, even in relatively n o n agressive soils, because in the waterlogged, anaerobic conditions of the Hungate site it would be expected that iron would be rapidly corroded. The most interesting level of the excavated site was a peat, with a black silty subsoil,
MIC of metallic antiquities and works of art: review
371
in which a great quantity of plants and some animal refuse were found. M a n y leather cuttings were found in the peaty layers, together with several mediaeval shoes and a n u m b e r of knives, at least one of which had probably been used in the working of leather. It was believed that the mediaeval Cordwainers Hall stood nearby, indicating the centre of a leather industry. Analysis of the soil revealed it to be anaerobic, to contain sulphate and to have a pH suitable for bacterial growth. M a n y of the soil samples contained highly coloured mottles and patches of a blue c o m p o u n d closely resembling in appearance the mineral vivianite, Fe3(PO4) 2. 8H20. In fact, X-ray examination of scrapings from a R o m a n knife revealed that the corrosion products were a mixture of ferric phosphate hydrate and ferrous phosphate hydrate, in approximately equal proportions. Chemical examination of the product also revealed the presence of traces of tannate. Farrer et al. (1953) considered that the presence of large quantities of wood had probably increased the concentration of tannate in the surrounding soil which, being peaty by nature, would also have a natural content of tannins. The burial of scrap leather would, however, greatly increase the concentration oftannate locally. Animal and especially dog refuse (blackened dog bones were found in the peat) is a rich source of phosphate, and this would locally increase the concentration of phosphate. The authors stated that these two factors would appear to have been of primary importance in the preservation of iron specimens on the site. The phosphate had functioned by providing a protective deposit upon the iron objects, and the tannate has e n h a n c e d this protection by suppresing bacterial activity. It is well known that the acid, anaerobic character of most peat deposits ensures that microbial degradation of buried organic materials is extremely slow. Thus, organisms or their parts, which are trapped in the peat, do not decompose and may be excavated at a later date, beautifully preserved. This is dramatically emphasized by the 700 or so Iron and Bronze Age h u m a n corpses which have been recovered from Scandinavian peat bogs (Richardson, 1981). Furthermore, the complex mixture of organic substances present in peat usually includes phenols and polyphenols. A correlation between phenolic structure and microbial inhibitory activity is evident (Fuchsman, 1980). Therefore, inactivation of bacteria involved in corrosion processes could be considered as the main cause of preservation of ancient buried iron objects in York. Farrer et al. (1953) demonstrated the inactivation o f Desulfovibrio desulfuricans by addition of 0-5% of soil from the excavations to culture media. Similarly, archaeological materials recovered from wrecks have been found in different preservation conditions, in which tannins were also
372 A. S6nchez deI Junco, D. A. Moreno. C Ranninger. J. J. Ortega-Ca&o, C. Sdiz-Jim¢;nez p r e s u m e d to be protective for metallic structures. A group of Greek a n d Punic wrecks from the 3rd century BC containing wood, lead sheet, bronze, copper a n d iron nails were recovered off the Sicilian coast near Marsala. Some of the nails attached to the timbers of the vessel were f o u n d to be of leaded bronze, a n d claimed to be possibly protected by tannins from corrosion. The nails released from the wood structure had suffered considerable corrosion and, by some m e c h a n i s m , the surface h a d been depleted in tin and lead, which is the reverse of what might have been expected from the relative insolubility of tin c o m p o u n d s . The corrosion conditions were anaerobic, caused by the growth a n d decay of P o s i d o n i a grass, which, at the time of excavation of the wreck, was giving offlarge a m o u n t s of hydrogen sulphide. There was also m a r k e d evidence of SRB, which were responsible for the conversion of the metal corrosion products to lead and copper sulphides (Tylecote, 1977). M a c L e o d (1982) distinguished three types of concretions on copper objects recovered from wrecks sites. The first appears as a cemented mixture o f c o r a l l i n e material a n d shell fragments, which is fairly porous a n d is stained with green copper corrosion products. The second type of aerobically formed copper concretions are often dense, thin layers of c e m e n t e d calcareous material, which give fairly adventitious iron corrosion products. The third type of concretion is associated with material which has been in an anaerobic environment. The source of sulphide ions is the activity of naturally occurring bacteria which are f o u n d in anaerobic m a r i n e sediments. T h e colour is that of the d o m i n a n t copper sulphide that form the bulk of the concretion. In cases of disturbance of the wreck site, a mixture of types of concretion can be found, shown by the appearance of green (aerobic) a n d deep blue-grey black (anaerobic) copper corrosion products. More recently, Daldorff (1987) also considered the MIC of iron objects. Different ions a n d total iron were analysed in the extracts obtained during conservation procedures at the Tromso M u s e u m (Norway), during the period 1982-1985, of 2500 to 3000 iron objects excavated from the soil. Organic matter was also determined for about 50% of the objects. Based on these analyses, the a u t h o r c o n c l u d e d that all elements necessary for the growth of micro-organisms such as bacteria a n d fungi were present. The presence of sulphide in all of the 50 objects tested was indicative of a contact with SRB in at least one stage of the deterioration process. Nevertheless, no microbiological studies were performed a n d the discussion of M I C in those objects was merely speculative. D u n c a n & Ganiaris (1987) studied the corrosion products of excavated metal objects from L o n d o n waterfront sites. Some of these
MIC of metallic antiquities and works of art: review
373
objects, made of copper alloys and lead alloys, showed a 'dull-gold' coloured layer which occasionally obscured the underlying alloy. Black corrosion products were also present on copper alloy objects. The analysis of such corrosion products revealed that they consisted of sulphides such as chalcopyrite, pyrrhotite, chalcocite and covellite. The anaerobic soils where those objects were excavated were typically black, wet and cohesive, with a foul sulphurous smell. The authors attributed the formation of the sulphide corrosion products to the SRB prevalent in those soils, through the generation of hydrogen sulphide. Oxidation of the sulphides may form sulphuric acid, which causes further corrosion. The stability of sulphide layers of the copper alloy objects was also studied by D u n c a n & Ganiaris (1987) in recently excavated material and compared with objects of similar origin that had been acquired by the Museum of London since the late 19th century. In both cases, a green corrosion product sometimes occurred associated with black corrosion layers. Experiments with recently excavated waste fragments showed that high relative humidity accelerated the colour change of sulphide surfaces. Mechanical and/or chemical cleaning was proposed for the removal of sulphide corrosion products. The deterioration of metallic works of art is not restricted only to buried or watery environments--incorrect preservation treatment or adverse environmental conditions leads to corrosion of m u s e u m objects. Brinch Madsen & Hjelm-Hansen (1979) observed that bronzes from Copenhagen, Stockholm, Oslo and London museums showed black spots on their surfaces. Chemical and X-ray diffraction analyses revealed that the black spots were copper sulphide. Because the spots resembled small mould spot, they were examined for micro-organisms. Three isolates from culture media containing increasing amounts of copper were obtained: one of them was probably a Cladosporium sp. Evidence for a chemical reduction of sulphur dioxide to hydrogen sulphide in the object's surroundings was lacking; sulphur-reducing micro-organisms are anaerobic, and museums do not provide ideal conditions for such growth. Therefore, a precise explanation of the origin of the black spots developed on the m u s e u m exhibits was not advanced. Makes (1985) reported that a 16th century chased helmet placed in a closed glass show-case was attacked by micro-organisms when the relative air humidity and temperature changed accidentally. The helmet was preserved with a mixture of lanolin, vegetable fats and water, which formed an emulsion favourable for microbial growth. The microorganisms produced lipolytic enzymes which hydrolysed the fats into fatty acids. The acids lowered the pH, which in turn increased the
374 A. S(mchezdel Junco, D. A. Moreno. C. Ranninger. J. J. Ortega-Cairo. C. S(fiz-Jimdnez corrosion of the helmet. The changes caused damage to the tin surface coating and the underlying steel, both acting as a galvanic cell, with the steel functioning as an anode and the tin coating as a cathode.
CONCLUSIONS The corrosion of archaeological metallic artifacts has usually been regarded as a chemical process, in which microbiological agents were rarely thought to be involved. However, bacteria have an important role in m a n y corrosion processes in nature, particularly u n d e r anaerobic conditions. In most cases it was only inferred that such a process could occur after burial or sinking of the object. This has resulted in a scarcity of papers on this topic, and those available are generally devoid of experimental and conclusive data on MIC. The intervention of MIC in the deterioration of metallic antiquities and works of art is today taken into consideration. This will undoubtedly allow the analysis, from both biological and metallurgical points of view, of the state of conservation of metallic works of art exposed to environments which will possibly contain microorganisms. Information and knowledge has been passed from the technical and industrial field, where economic and efficiency motives have pushed the study of MIC, to the area of cultural heritage. This could speed the adoption of measures to avoid the deterioration and facilitate the conservation of works of art for future generations.
REFERENCES Atlas, R. M. & Bartha, R. (1987). Microbial Ecology. Fundamentals and Applications, Benjamin/Cummings, Menlo Park, CA. Brinch Madsen, H. & Hjelm-Hansen, N. (1979). Black spots on bronzes -- a microbiological or chemical attack. Proceedings of the Symposium on the Conservation and Restoration of Metals, Scottish Society for Conservation & Restoration, Edinburgh, pp. 33-9. Daldorff, S. A. (1987). Microbial corrosion and museum iron objects. 8th Triennial Meeting, ICOM Committee for Conservation, Sydney, Australia. pp. 1063-6. Duncan, S. J. & Ganiaris, H. (1987). Some sulphide corrosion products on copper alloys and lead alloys from London waterfront sites. In Recent Advances in the Conservation and Analysis of ArtOeacts, ed. J. Black. Summer Schools Press, London. pp. 109-18. Farrer, T.W., Biek, L. & Wormweli, F. (1953). The role of tannates and phosphates in the preservation of ancient buried iron objects. J. Appl. Chem.. 3, 80-4.
MIC of metallic antiquities and works of art: review
375
Fellers, B. D. (1989). An interdisciplinary perspective on microbiological influenced corrosion. Corrosion 89. NACE, Houston, TX, Paper 510. Fuchsman, C. H. (1980). Peat. Industrial Chemist~ and Technology. Academic Press, New York. Hamilton, W. A. (1985). Sulphate-reducing bacteria and anaerobic corrosion. Ann. Rev. Microbiol.. 39, 195-217. Hamilton, W. A., Moosavi, A. N. & Pirrie, R. N. (1988). Mechanism of anaerobic microbial corrosion in the marine environment. In Microbial Corrosion L eds C. A. C. Sequeira & A. K. Tiller. Elsevier, Amsterdam. pp. 13-19. Iverson, W. P. (1987). Microbial corrosion of metals. Advances in Applied Microbiology. 32, 1-36. Lynch, J. M. (1988). The terrestrial environment. In Micro-organisms in Action." Concepts and Applications in Microbial Ecology. eds J. M. Lynch & J. E. Hobbie. Blackwell, Oxford, UK, pp. 103-31. MacLeod, I. D. (1982). Formation of marine concretions on copper and its alloys. Int. J. Nautical Archaeol. and Underwater Explor.. 11,267-75. MacLeod, I. D. (1989). Marine corrosion on historic shipwrecks and its application to modern materials. Corrosion Australasia, 14, 8-14. Makes, F. (1985). Preservation ofa 16th century Italian helmet. Livrustkammaren. 17, 79-92. Richardson, D. H. S. (1981). The Biology of Mosses. Blackwell, Oxford, UK, pp. 194-5. Royuela, J. J. (1991). Corrosirn en suelos. In Corrosirn y Protecci6n Metdtlicas. Vol II. Coleccirn Nuevas Tendencias. eds S. Feliu & M. C. Andrade. Consejo Superior de Investigaciones Cientificas, Madrid. pp. 3-21 Stambolov, T. (1985). The Corrosion and Conservation of Metallic Antiquities and Works of Art. CL Publication, Central Research Laboratory for Objects of Art and Science, Amsterdam. Tylecote, R. F. (1977). Durable materials for sea water: the archaeological evidence. Int. J. Nautical Archaeol. and Underwater Explor., 6, 269-83. Tylecote, R. F. (1979). The effect of soil conditions on the long-term corrosion of buried tin-bronzes and copper. J. Archaeol. Sci.. 6, 345-68. Videla, H. A. (1991). Corrosirn microbiol6gica. In Corrosirn y Proteccirn Metdllicas. Vol I. Coleccirn Nuevas Tendencias, eds S. Feliu & M. C. Andrade. Consejo Superior de Investigaciones Cientificas, Madrid. pp. 117-37. Walker, R. (1980). Corrosion and preservation of bronze artifacts.J. Chem. Educ.. 57, 277-80.