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Environmental Threats to Buried Archaeological RemainsAuthor(s): Anders G. Nord, Kate Tronner, Einar Mattsson, Gunnar Ch Borg, and Inga UllénSource: AMBIO: A Journal of the Human Environment, 34(3):256-262. 2005.Published By: Royal Swedish Academy of SciencesDOI: http://dx.doi.org/10.1579/0044-7447-34.3.256URL: http://www.bioone.org/doi/full/10.1579/0044-7447-34.3.256

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Anders G. Nord, Kate Tronner, Einar Mattsson, Gunnar Ch. Borg and Inga Ullen

Environmental Threats to BuriedArchaeological Remains

The last century’s environmental pollution has createdhealth problems, acidification of ground and lakes, andserious damage to our cultural heritage. Outdoor monu-ments suffer from this pollution, but so do buried archaeo-logical remains. However, research on the deterioration ofarchaeological artifacts underground has so far been limit-ed, and it is important to draw attention to this neglected field.This article presents results obtained at the SwedishNational Heritage Board on the degradation of archaeolog-ical objects of bronze and iron and of bones from prehistoricgraves, materials of which seem to be most affected by pol-lutants. The investigation methods, which were employed,are described. Other relevant studies are briefly reviewed. Itis obvious that the deterioration rate of archaeologicalartifacts, especially of inorganic materials, has acceleratedin recent years, and that this increased deterioration toa large part can be attributed to anthropogenic pollution.Regions that might be endangered are exemplified.

INTRODUCTION

The previous century has witnessed increasing pollution prob-lems in the industrialized part of the world because of extensivechanges in industry, technology, and agriculture. Although theeconomic and social benefit from this process is obvious, thereare many negative effects on the environment. Acidification oflakes and ground, as well as serious decay of outdoor monu-ments, has clearly been attributed to anthropogenic pollutants.Severe damage has been observed on the Acropolis of Athens,

the dome of Cologne, the Egyptian pyramids, Scandinavian runestones, and so forth. This damage aroused the suspicion thatarchaeological artifacts underground also might be affected.European archaeologists have, in fact, observed an accelerateddecay of artifacts, but research in this field has only started quiterecently. In particular, this environmental threat is a seriousproblem in regions underlain by noncalcareous bedrock, leavingthe ground very sensitive to acidification (1, 2). Such conditionsare common, for instance, in parts of northern Europe,Scotland, and the Alps, where acidic deposition affects the soilproperties. Various archaeological materials will react in dif-ferent ways. To determine whether the buried artifacts will be atdanger, it is important to gain knowledge about the depositionof atmospheric pollutants in relation to what the ground cantolerate (the critical load; cf. below).

Against this background, the National Heritage Board ofSweden (NHB) has studied the environmental effect on buriedarchaeological metal artifacts. Well-preserved bronze finds fromthe Viking Age town of Birka were excavated around 1870 byHjalmar Stolpe (3). A recent excavation in the 1990s producedpoor finds, with no single object as fine as those from the olderexcavation (Fig. 1). Naturally, pollutants were suspected to bethe major cause of this increasing degradation. A pilot study wasstarted by Gunnel Werner. After her death, her colleagues con-tinued this work in a national project, with experts on airpollution, corrosion, chemistry, geology, conservation, archae-ology, and osteology, and some preliminary results werepublished (4–6).

NHB has also been a partner in an international researchproject within the European Commission (Frame WorkProgramme FP4), with the aim of studying the degradation ofarchaeological bone in soil (7). Results for the two studies aresummarized in this article. The methodology used is exemplifiedby the investigation of recently excavated bronze artifacts.Relevant results published elsewhere are briefly reviewed.Together, the studies may help to avoid irretrievable loss ofarchaeological material, which constitutes an important part ofour mutual cultural heritage, and predict the risk of damage toburied artifacts. Although most of the present knowledge isbased on European studies, the problem also may exist in nationslike India and China, with unique archaeological treasures andincreasing pollution problems. The outlined methodology maywell be applied in other countries.

MATERIAL AND METHODS

There are many factors that may influence the deterioration ofarchaeological artifacts in soil: external regional factors,including primarily global and local pollution from agriculturaland other anthropogenic sources, but also climate, geology, andtopography; external effects from the nearest surroundings,including soil chemistry, hydrology, vegetation, microorgan-isms, and the archeological context; and the composition of theartifact.

The material of an artifact may be inorganic, organic, ora mixture of both (e.g. bone). Materials rich in silicates, like flintstones, ceramics, and glass, show comparatively good re-sistance, whereas metals (excepting gold) more or less rapidly

Figure 1. Above: equal-armed brooch mainly consisting ofcorrosion products excavated at the Swedish Viking Age town ofBirka in 1993. Below: For comparison, a similar object excavatedaround 1870 is shown.

256 Ambio Vol. 34, No. 3, May 2005� Royal Swedish Academy of Sciences 2005http://www.ambio.kva.se

Report

corrode in soil. Iron, bronze, copper, and lead objects areseriously affected, but silver also may corrode under certaincircumstances. Organic materials like wood, leather, skin, andtextiles are mainly degraded by oxygen and microorganisms,but may be affected by extreme pollution (cf. below). Bone isa porous composite material, and its degradation is a complexprocess. This study concentrated on archaeological bronze,iron, and bone, which seem to be most affected by pollutants.

There are various methods to determine which factors causecorrosion in soil. For instance, specimens may be buried indifferent environments for (usually) 5–10 y and then examined(8). However, the large time spans occurring in archaeologynecessitate that the problem also be tackled in other ways thanthose conventionally used in corrosion science. Three methodswere used at NHB: studies of finds from excavations inprogress—a close examination of objects, surrounding soil,environment, and polluting sources; museum collection stud-ies—examination of the preservation status (cf. below) of a largenumber of museum objects (relevant data on time since burial,excavation, location, and the environment in general areretrieved from the archives); and survey of excavationreports—applicable whenever the preservation status and otherrelevant data are specified in the reports.

The principal aim of the studies was to identify the mainparameters affecting the deterioration. In comparison withother studies with the same aim, the following differences inapproach may be mentioned: samples from a large number ofsites were used; not only were artifacts and soil analyzed, butdata for anthropogenic pollution and the environment ingeneral were also included; and statistical multivariate analysiswas used to evaluate the relation between the preservationstatus and variables characterizing the exposure conditions.

There are two main parts of the national project on metalobjects: a detailed study (including chemical analyses) ofrecently excavated bronze and iron finds, and an overallinvestigation of many thousand objects in museum collections.In the first part of the project, 300 prehistoric and Medievalobjects were examined from 47 Swedish sites with differentconditions, excavated during the latest decade (see Table 1).Thin-walled objects with a weight of 4–20 g were preferablyselected. For each object, soil closely surrounding the artifactwas sampled, and data on the environment in general werecollected. The degree of deterioration, henceforward calledFdet, was determined by X-ray radiography and visual in-spection and classified on a scale from 1 (well preserved) to 5(heavily corroded): 1, almost no corrosion, very fine object,good surface; 2, minor corrosion, fine object; 3, considerablecorrosion, moderate preservation; 4, major corrosion, badobject; 5, almost no metal core, extremely bad condition.

The metal and corrosion products were analyzed by scanningelectron microscopy with X-ray microanalysis (SEM/EDS) and

X-ray powder diffraction (XRD). The analysis of soil wascarried out by conventional methods (4, 9). The grain sizedistribution and the soil type are static variables, as they havenot changed much during thousands of years. The chemicalvariables, however, are dynamic, which means that their valueshave changed over time, and usually only recent data areavailable for pH value, humidity, mass loss on ignition (forestimation of organic contents), electric conductivity, and somespecific anions and cations (4).

One advantage with recently excavated objects is that mostdata can be obtained. However, the number of artifacts islimited. Therefore, a complementary study of metal objectsfrom museum collections was carried out to compare finds andgive a survey of the state of deterioration as a function of dateand place of discovery, soil sensitivity, environment, andarchaeology. In this case, the number of finds was large, butnaturally only data included in the museum archives could beused. Three districts in Sweden with different environmentalconditions were selected: the Swedish west coast, with heavypollution and a sensitive soil; Uppland at the east coast (northof Stockholm), with clayey soil; and the calcareous island ofGotland in the Baltic Sea (cf. Fig. 2).

About 2800 Swedish and 400 Norwegian museum bronzes,as well as 1350 iron artifacts, were examined. They originatefrom more than 700 sites. The objects mainly date from theBronze and Iron Ages (1500 BC to 1000 AD). To achievecomparable results for the much-corroded iron objects, weselected nails and rivets. The study was based on findsretrieved between 1860 and 2000. The museum collectionshave microclimates with relative humidity ,50% and atemperature constantly around 208C. The preservation statusof the objects was classified by visual inspection and (for theiron objects) by X-ray radiography. As much backgroundinformation as possible was retrieved from the archives(archaeological age, time of excavation/discovery, soil type,context, and local environment in general). An estimate of thesoil sensitivity toward acidification was obtained for each sitefrom special geological maps for the superficial part of theground where archaeological objects are usually found inSweden (4, 9).

The FP4 project on bone degradation included 300 bone and600 soil samples. Bones (mostly human) from excavations inprogress in Sweden, the Netherlands, England, Italy, andTurkey were used, and 134 variables were recorded for eachbone sample, including data for the surrounding soil and theenvironment in general (cf. ref. 7). The bone preservation statusand alterations were defined using three basic variables: themacroscopic (visual) appearance, the collagen contents, and thehistological microstructure.

The most important aim of the two investigations was toestimate the effect of global and local pollution. Multivariatestatistical analysis was undertaken using SIMCA (Umetri AB,Umea, Sweden) and SAS (SAS Institute Inc., Cary, N.C.,USA). SIMCA handles a large number of variables and data,using principal component analysis (PCA) or partial least-squares projections to latent structures (PLS). The systemexcludes variables or observations with many missing—or toosimilar—values. With PLS, the crucial parameter Fdet wasexpressed as a function of all other variables, giving therelations as regression coefficients (rc). Attention should only bepaid to values j rc j . 0.03. A positive rc indicates that thevariable in question has a detrimental effect, whereas a negativerc indicates a preserving effect. Evaluations were also performedwith SAS. The linear model was chosen to minimize the sum ofthe squared distances between observed and predicted values.Pearson correlation coefficients (cc) were calculated for allvariables. Note that the SAS cc values should be distinguished

Table 1. Survey of the recently excavated bronze and ironartifacts.

RegionNumberof sites

Total numberof artifacts studied

The Malaren valley (west of Stockholm)Rich region with many ancient remains(moraine, sand, clay)

15 112

Southern Sweden(sand, partly calcareous ground)

18 123

Swedish west coast, vulnerable soilwith heavy air pollution (sand, gravel)

7 41

Gotland (sand, calcareous ground) 3 12Northern Sweden, comparatively cleanair and soil (sand, moraine)

4 12

Total 47 300

Note: The four largest sites (Birka, Fresta, Valsta, Haggvik) are described in ref. (4).

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from the SIMCA regression coefficients (rc). The former shouldhave a numeric value j cc j . 0.3 to be considered as having anysignificance at all.

RESULTS OF THE STUDIES AT THE NATIONALHERITAGE BOARD (NHB)Recently Excavated Bronze Artifacts

The bronze objects displayed a large variation in composition,probably because of the assiduous recycling of metal. Thewell-preserved objects showed an average corrosion depth of0–0.2 mm, the moderately preserved had a corrosive depthof around 0.4 mm, and the heavily corroded bronzes had aneven greater corrosion depth. Naturally, the corrosion has beenaffected by the soil chemistry. Brochantite, a copper hydroxidesulfate with the chemical formula Cu4(SO4)(OH)6, was quite

common as a result of deposition of acidic sulfur compounds.(It is also the dominating basic copper salt on outdoor bronzemonuments). Copper chlorides were found to be frequentlyoccurring on artifacts found in marine surroundings or onobjects found near roads salted during wintertime. Copperphosphates were identified in graves, where phosphate fromdegrading bone had reacted with the bronze. Inversely, thecorroded finds have produced an increased concentration ofpoisonous copper ions in the soil, where they act as biocides.

Each find was related to 90 variables describing the artifactitself, the soil, and the environment. The first letter of a variablename (F, S, A, or E) indicates the parameter type (Find, Soil,Archaeology, or Environment, cf. Table 2). All data wererecorded in an EXCEL file containing 15 000 entries, with only3% missing values.

Table 2 shows the SIMCA regression coefficients (rc:s) thatmight be regarded as significant. For obvious reasons, a largepositive rc value was noted for the corrosion depth (Fdep). Incontrast, the bronze composition seems to have minor impor-tance. The soil acidification is the most important deterioratingfactor, responsible for about 50%–90% of the bronze corrosion. Itis evident that a large deposition of sulfur compounds (ESrat) isdetrimental to bronze; that is, an effect of (mainly) globalpollution (Table 2). The effect of local pollution from industries,traffic, and agriculture is also obvious (Epoll). The deposition ofsulfur (and also nitrogen) compounds was, in vulnerable regions,ten times that of the critical load—a measure of what the groundcan tolerate (10). The worst parts in Scandinavia have very lowcritical loads of acidity, usually below 200 eq ha�1 y�1, whereascorresponding values for calcareous parts of central Europe arearound 1000–2000 eq ha�1 y�1 (2, 11, 12). The relation between

Figure 2. Map of Sweden showing the three selected regions: W = thewest coast, U = Uppland, G = Gotland.

Table 2. Regression coefficients with j rc j . 0.03 obtained froma multivariate analysis using the SIMCA system. The largestnumeric j rc j values identify the most significant variables.

VariableRegressioncoefficient Explanation of the variable

Deteriorating factorsESrat 0.104 Sulfur deposition versus

critical loadSCl 0.091 Chloride in the soilFdep 0.073 Corrosion depthAdepth 0.064 Depth below the groundSg2 0.060 Grains 2–4 mm

(% of distribution)Edep 0.056 Thickness of soil

from groundto solid rock

Epoll 0.055 Influence from localpolluting sources

Ewc 0.048 Find from the SwedishWest Coast

Ssoot 0.047 Soot present in the soilSexch 0.046 Exchangeable acidityShum 0.043 Humidity in the soilSphos 0.039 Phosphate in the soilScond 0.034 Electric conductivity

of the soil(salt contents)

Ssand 0.030 Sandy soil or sandEpine 0.030 Site in a pinewood

Preserving factorsEslop �0.032 Site on a hillsideSfs �0.037 Fine sand or siltSpH �0.042 pH value of the soil

(high pH = preserving)Sg0.25 �0.045 Grains 0.25–0.50 mm

(% of distribution)Ecalc �0.051 Calcareous groundEvegm �0.055 Soil moisture as judged

from the vegetationEec �0.083 Find from the Swedish

East CoastEdec �0.085 Site in a deciduous forest

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deterioration and variables characterizing the soil acidity (SexcH,SpH) is also obvious. In forest soil from SW Sweden, pH values(measured by means of water leaching) lower than 4.5 were oftenobserved in the podsolic B horizon. The degradation is alsoenhanced for a site located in a pine forest (Epine), which by itselfimplies a high acidity, whereas a deciduous forest (Edec) orcalcareous ground (Ecalc) is preserving. Consistently, the findsfrom the pollutedwest coast weremore degraded than those fromthe east coast (cf. Ewc and Eec in Table 2).

Presence of salt (Scond, SCl, Sphos) or soot (Ssoot) in the soilwas also found to be detrimental. The salt may originate from airpollution, fertilizers or road salting, the soot from cremation, orhousehold ashes. Soot layers contain carbon, which in contactwith metal may cause galvanic corrosion. A soil with medium-sized grains giving both oxygen (air) and water access to theburied metal was found to enhance corrosion (Sg2, Ssand),whereas fine-grained soils are less detrimental (Sg0.25, Sfs). Thedepth below the ground surface (Adepth) also appears to beharmful (i.e. the greater the depth, the more corroded is theobject, probably because of the soil pores near the groundwatertable being partly filled with water). Among the otherarchaeological variables, cremation layers (with soot and ashes)and open (permeable) constructions like stone settings seem tohave enhanced bronze corrosion. The SAS results were inaccordance with those of the SIMCA evaluations.

Recently Excavated Iron Artifacts

As expected, the iron findswere generallymore corroded than thebronzes, and usually they had a thick layer of rust. The result ofthe study is similar to that of the recently excavated bronzeartifacts (13). The statistical evaluation showed harmful influencefrom chlorides in the soil and from deposition of nitrogen andsulfur pollutants greater than the critical load. Fine-grained sandimplied a deteriorating effect. Among the preserving factors,deciduous forest and calcareous ground were again noted.

Bronze Objects from Museum Collections

In the three selected regions (the west coast, Uppland, andGotland), the archaeological excavations have been undertakenwith professional documentation, including the recovery ofa large amount of small metal fragments. This allowsa comparison to be made between excavations undertakenfrom around 1900 to the present. Although no analyses werepossible, two striking results were obtained. The first is a strongrelationship between deterioration and the sensitivity towardacidification as obtained from the special geological maps madewithin the project (cf. above). The second is that objects foundearly were, in general, better preserved than those recentlycollected. This is illustrated in Figure 3, in which the objects aresorted according to the decade when they were collected and theaverage deterioration for each group given. The accelerateddecay is clearly indicated. There might be a suspicion that single(stray) finds and hoards were better preserved than the average,and therefore might invalidate the result. Quite likely thefinders, usually farmers, only collected the best objects.However, excluding these finds, which were few, gave verysimilar results, as shown in Figure 3.

The three geographic regions were compared (Fig. 4). Thebest-preserved bronze objects were found on Gotland (withcalcareous ground), and the worst at the polluted west coast,whereas the Uppland finds form an intermediate group. TheNorwegian samples, all from the Stavanger area, showed thesame tendency as the Swedish west coast objects, which is notsurprising because the two regions have similar geology andpollution problems. Another interesting result was thatarchaeologically ‘‘young’’ or ‘‘old’’ finds from the same area

had about the same degree of preservation; that is, it was almostirrelevant whether a bronze object had been buried for 400 or4000 y. Obviously a great part of the corrosion has occurredduring the latest century, indicating a strong influence fromanthropogenic pollution. Soot (with carbon) was again found tobe a corrosive factor. The effect of the archaeological contextand more details are given in a paper by Ullen et al. (14).

Iron Objects from a Museum Collection

Only iron objects from theMuseum of National Antiquities wereincluded in the study. The results are analogous to those of thebronze museum objects. However, the accuracy of the studysuffers from the fact that part of the deterioration may beattributed to postexcavation corrosion. It is clear from the study,however, that the increasing air pollution has had a significantdeteriorating effect (cf. ref. 13).

Other Metals

Many rich silver treasures have been found in Sweden from theViking Age and the Middle Ages. Usually the finds are wellpreserved. However, silver objects in a very poor state of preser-

Figure 3. Average deterioration (Fdet) in scale from 1 (wellpreserved) to 5 (heavily corroded) for 2800 bronze artifacts foundin Sweden, sorted with respect to the decade when the excavation/discovery was made. The white columns include all finds, the blackcolumns only those from scientific excavations (i.e. excluding singleand hoard finds).

Figure 4. Distribution (in percent) of the average deterioration (Fdet)for prehistoric bronze artifacts from the three regions, sortedaccording to the five classes (class 1 = well preserved, class 5 =heavily corroded). The west coast is represented by the county ofBohuslan.

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vation have been found, for instance, at a farm called Spillings(parish of Othem) on the island ofGotland. The treasure weighedabout 70 kg, and the finds were heavily corroded, with silverchloride as themain corrosion product. The surrounding soil wasanalyzed, and very high chloride concentrations were observed,often greater than 1000 ppm. This may be compared with typicalvalues around 10–50 ppm in Gotland soil. The chloride probablyoriginates from artificial fertilizers. Although not included in theinvestigations described here, lead is another metal used forthousands of years. Nitric acid and organic acids are corrosive tolead. Sulfuric acid is not corrosive because the almost insolublelead sulfate will form a protective surface coating.

Bone Material

Fresh bone consists of inorganic calcium phosphates, mainlycalcium hydroxide apatite (about 70% by weight), proteins(22%; mainly collagen), and water. The EC-FP4 project showedthat many factors affect the deterioration of bone (7, 15, 16).The results of two multivariate analyses, performed with thesystems SIMCA and SAS, were in good agreement. Theenvironment was found to affect the three preservationvariables in different ways. The chemical environment (primar-ily soil acidity) mainly affects the macroscopic appearance,whereas microorganisms (mainly bacteria and fungi) havea major influence on the organic contents and the histologicalmicrostructure (7, 15–17). The inorganic material is clearlyaffected by soil acidity (low pH values), whereas proteinsdegrade at high pH values. Calcareous soil is favorable for themacroscopic appearance; bones from Italy were well preserved,whereas the Swedish bones usually showed bad preservation.

DISCUSSION

Most research concerning decay of our cultural heritage has sofar been concentrated on outdoor monuments, while investiga-tions of unexcavated archaeological artifacts underground arelimited. Some of the latter studies, usually European, will besurveyed in this section. Work on human-induced soil degrada-tion has indicated, not surprisingly, that Europe is so far themostpolluted continent (18).

Corrosion of Metal Artifacts in Soil

It is well known from corrosion science that less noble metals(iron, lead) corrode more than metals like copper or silver.Metal corrosion requires an oxidizing agent (usually oxygenfrom the air) and an electrolyte (e.g. a water solution) (e.g. ref.19). Accordingly, a well-aerated and moist soil, such as sandjust above the groundwater table, should be most detrimental tometals, giving access to oxygen at the same time as the soil poresare partially filled with capillary water. An oxygen-free peat hasinstead a preserving effect. Acids, salt, and soot (carbon)enhance corrosion because of increased electric conductivity.

Other authors support the NHB results. For instance,Geilmann (20) and Klas and Steinrath (21) have shown thatbronze objects corrode in aerated, sandy soils, whereas a poorlyaerated, water-logged soil instead has a preserving effect.Tylecote (22) reported that acid soils are aggressive to copperalloys, and alkaline soils and peat are benign. Wagner et al. (23)have studied the situation at eighteen European sites andsuggested suitable countermeasures. Gerwin and Baumhauer(24) found strong correlation between iron corrosion and sandysoil, acidic soil, soil with high salt contents, and urban soil ingeneral. Scharff (25) has examined thousands of iron objects inGerman museum collections. He observed a significantly worsepreservation status for objects excavated after 1960. Someauthors have emphasized that chlorides are indeed disastrous to

iron artifacts (26–28). All these results for metals are well inagreement with those found in the NHB investigation.

Degradation of Bone Material in Soil

Investigations have shown that the environment affects theinorganic bone material, the collagen content, and thehistological microstructure in different ways (7, 29). Apparent-ly, acidic soil primarily affects the inorganic bone componentdecisive for the macroscopic appearance, as pointed out earlier(30), whereas calcareous soil is favorable (31). However, acidicsoil does not affect the collagen nor the histological micro-structure, and microorganisms, mainly bacteria and fungi, havean important detrimental influence on the organic contents andthe histological microstructure (7, 15–17, 32). Soil with a highorganic content seems to be slightly protective, which probablydepends on the low oxygen content (7, 33). The influence of soiltype, however, is not clear. The hydraulic conditions of the soilaffect the supply of reactants to and the removal of reactionproducts from the bone surface (34).

A deep grave or a grave with a large covering superstructure(e.g. a barrow) was usually found to offer protection towardspollution (7). The bone decay usually increases with the time sinceburial (cf. ref. 35). Microorganisms cause degradation of organicbone material and the histological bone structure, but the statis-tical analysis did not provide much insight into which other fac-tors have influenced this degradation. Suchunknown factorsmaybe related to taphonomic processes, which have affected the re-mains between death and the burial (season, climate, burial tra-ditions, etc.). In addition, living conditions or diseases are likelyto have had some influence on the bone degradation (36–38).

Degradation of Some Other Archaeological Materials in Soil

Although bronze, iron, and bone are considered the bestindicators for damage to buried archaeological artifacts causedby pollution, other materials will very briefly be surveyed.Archaeological glass degrades in acid as well as in alkaline soil(39–43). Soda glass is more stable than potash glass. Accordingly,archaeological glass from the Mediterranean area, where sodalakes were common, is more stable than glass made north of theAlps, where potash was instead used for the manufacturing. Forthe organic objects, microorganisms such as bacteria and fungiare the main factors causing degradation (44–47). In addition,oxygen and sunlight may be detrimental. Quite few organicobjects are recovered from archaeological sites, although well-preserved finds have been found, for example, in bog or peat inDenmark, where the oxygen-free environment has created goodpreservation conditions. The Viking ship of Oseberg (southernNorway) has been well preserved by embedment in waterloggedclay with low oxygen content. Cellulose fibers like wood, flax, orcotton degrade in an acidic surrounding (48–50), whereasprotein-rich fibers like wool or silk are more stable in the acids.

Acidification of Soil by Air Pollution

Sulfur and, under certain circumstances, nitrogen species causeacidification of soil. These compounds are mainly formed byburning and can be transported long distances via theatmosphere. Ammonia from manure may locally inducenitrogen pollution, which might contribute to ground acidifi-cation, depending on the conditions.

The emission of SO2 in Europe has been reduced by 50%between 1980 and 1994 (51). The emission of sulfur in Europewas reduced by 37% from 1990 to 2000, and in the United States,the lowering was 17% (52), but sulfur emissions increased by30% in China, according to a compilation by Kuylenstierna et al.(53). However, corresponding reductions of NO2 emissions are

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indeed modest (51); the overall reduction for Europe is only 8%,with Germany as an exception reporting a reduction of 21% (51).Recent measurements of atmospheric deposition in Sweden (54)show that the trend of deposition of SO2 is diminishing, but fornitrogen it is difficult to measure trends.

In western Sweden, a 5-y study (The Gardsjon Roof Project)has been performed with the aim to experimentally investigatethe response of a forested catchment to a reduction in sulfate,nitrate, and mercury deposition (55). Because of the existingstorage of sulfur species in the soil, and the fact that adsorptionof sulfur compounds is a reversible process, the acidificationprocesses in soil at Gardsjon will continue for 25–45 y. Theorganic sulfur compounds are assumed to be negligible,although mineralization of organic sulfur could prolongacidification processes for centuries. Hultberg and Skeffington(55) has called this delay ‘‘the memory effect of the soil.’’

The damage to archaeological remains caused by soilacidification depends on the deposition of acidifying substancesin relationship to the critical load of the region in question. Inthis paper, only the critical load for acidity is discussed; that is,the acidity below which significant harmful effects do not occur(10, 56, 57). Critical loads for acidity in Europe have been deter-mined (2, 11, 12). The sensitivity toward acidification in manyScandinavian areas, dominated by slowly weathering magmaticbedrocks, is at least 10 times that of the regions in southernEurope, while most terrestrial ecosystems in central Europe areintermediate (2, 11, 12). In southern Europe, calcareous bed-rocks are common, and in addition, the winds transportcalcareous particles from the Sahara desert (53). Posch et al.(12) compare the critical load and the atmospheric acidifyingdeposition in continental Europe. Because of large emissions, thecritical load is clearly exceeded in parts of England, Germany,the Czech Republic, and Poland. Many parts of Scandinaviareceive large amounts of global pollution, implying depositionslarger than the critical load. In other parts of Europe, thedeposition also exceeds the critical load, but to a lower degree. Insouthern Europe, eastern Ukraine, and Russia, the acidifyingdeposition is usually lower than the critical load (12).

Kuylenstierna et al. (53) quote several European investiga-tions, concluding that the most important soil propertygoverning the sensitivity to acidifying deposition for terrestrialecosystems is the weathering rate, assuming that soil buffering isthe primary factor determining the response of the ecosystem.The differing tolerance of plant and vegetation species isregarded to be important, but less so than the weathering rate.Weathering rates are difficult to determine, especially at a globalscale. Kuylenstierna et al. (53) have therefore recommendedthat base saturation and cation exchange capacity measured atfield pH be used for the determination of relative sensitivityclasses. This is valid, all other things being equal (53). Theadvantage of using these two parameters is that they are easy tomeasure, and the digital FAO soil maps can be used (58).

On a local scale (e.g. for some counties in Sweden), soilsensitivity classes (SSC) in relation to archaeological objects havebeen produced (4, 9). The SSC classes are based on the rathersuperficial ground properties, as artifacts are often located in theshallow ground. Important factors determining SSC are, forexample, geology, soil type, land use, vegetation, topography,and climate. In the study of metal objects from museumcollections (see above), a strong relationship was found betweenSSCmaps and the deterioration of artifacts from the same region.

CONCLUSIONS

This study has shown that the deterioration rate of buriedarchaeological artifacts, especially inorganic materials, hasaccelerated in recent years. The objects have rested in ‘‘soil

archives’’ for hundreds or thousands of years under stableconditions, only suffering from a slow corrosive decay, but theyare now endangered. The earlier assumption that archaeologicalartifacts are best preserved when allowed to remain un-derground has turned out to be doubtful, at least in regionswith serious soil acidification. A major part of the increaseddeterioration of metal artifacts may clearly be attributed toanthropogenic pollution. As the soil has been affected by largedepositions of pollutants during the latest century, but notearlier, the major part of the corrosion attack seems to haveoccurred then. This also means that it is almost irrelevant forthe corrosion status whether a metal object has rested in soil for400 or 4000 y. It seems that acid and salt are the main factorscausing the deterioration. A soil allowing access of air (oxygen)and water at the same time enhances metal corrosion. Thepresence of soot (carbon) is an additional detrimental factor.Extremely well preserved metal objects are usually found only inarid or richly calcareous regions, or in peat or bogs.

Organic materials are less affected by air pollutants thanmetals, although acidic soil is detrimental to cellulose molecules.Instead, microorganisms, such as bacteria and fungi, are themain factors causing the decay of buried organic objects. Asmentioned, bone is a complex composite material, and itsdegradation depends on a manifold of factors, some detrimentalto the inorganic component and others to the organic one.

Because of the soil acidification, many unexcavated archae-ological artifacts are endangered, especially in parts ofScandinavia, which in addition to unfavorable geologicalconditions receive a substantial amount of global pollution. Infact, acid soil conditions and high salinity may explain thelimited number of archaeological metal finds from the Swedishwest coast. This observation has earlier led to the questionableconclusion that in comparison with the rich districts of southernScandinavia, the west coast was a remote region with few buriedbronzes. Instead many artifacts may have disappeared becauseof heavy pollution.

Areas sensitive to acidic deposition are found in Scandinavia,central Europe, and northern Russia (53). Outside Europe, theacidifying deposition exceeds the critical load in the eastern partof North America, southeast China, the Malay Peninsula, partsof Indonesia, and mining districts in southern Africa (53). Inareas with heavy pollution (e.g. south China), the peril to buriedartifacts may be considered to be highest.

Badly corroded objects may receive treatment in the form ofrestoration and conservation. These measures may strengthenand reinforce a fragile object. Various nations have different ethicprinciples as regards what should be allowed to do. Theconservation work aims at reducing further degradation as muchas possible. For instance, harmful salts like chloride on metalsshould be removed by water leaching (26, 27, 59). Dirt andcorrosion products may be removed mechanically. Stabilizingmeasures should be carried out in case the artifact is very fragile.

In conclusion, the results obtained will hopefully be of valuefor the prediction of damage to buried artifacts caused byenvironmental threats, such as increasing air pollution, new largeindustries, new highways, road salting, afforestation, substantialchanges in land use, or lowering of the groundwater table. Theresults also form a basis for possible measures in situ tocounteract the corrosion of buried metal objects, for example,by raising the groundwater table or by lime treatment. Suchmeasures, however, require careful consideration not to causenegative secondary effects on the environment. Further, corro-sion science in general may benefit from the results by a betterunderstanding of the long-term corrosion of metal objects in soil.The results will help the cultural authorities in various countriesto meet the need for appropriate preservation methods. They willalso improve the basis for cultural heritage management when

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decisions have to be made concerning the preservation of thecultural landscape or concerning excavations in connection withlarge-scale changes of the nearby landscape.

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project and to all field archaeologistswhohave helpedwithmaterial for this study.MonikaFjaestad is cordially thanked for valuable contributions to this project. Generous grantsfrom theWallenberg Foundation for analytical instruments are gratefully acknowledged.

61. First submitted 3 Feb. 2004. Revised manuscript received 4 May 2004. Accepted forpublication 10 May 2004.

Anders G. Nord is employed as a conservation scientist atNHB. He is associate professor in inorganic chemistry and haspublished more than 100 scientific papers. His address:National Heritage Board of Sweden (Riksantikvarieambetet),P. O. Box 5405, SE-114 84 Stockholm, Sweden.anders.nord@raa.se

Kate Tronner is a senior chemist at the National HeritageBoard. She has for many years performed research related tothe deterioration of cultural objects. Her address: NationalHeritage Board of Sweden (Riksantikvarieambetet), P. O. Box5405, SE-114 84 Stockholm, Sweden.kate.tronner@raa.se

Einar Mattsson was for many years head of the SwedishCorrosion Institute in Stockholm and was appointed professorin corrosion science. After retirement he is still active in manyfields. His address: Einar Mattsson Korrosionskonsult, Apelva-gen 26, SE-182 75 Stocksund, Sweden.emk.mattsson@telia.com

Gunnar Ch. Borg graduated in quaternary geology at theUniversity of Uppsala, and was appointed associate professorat the Chalmers University of Technology in Goteborg. Hisresearch is focused on hydrological and chemical processes.His address: geoBorg Consulting, Lilla Tulteredsvagen 42, SE-433 31 Partille, Sweden.gunnar.ch.borg@telia.com

Inga Ullen is a senior curator at the Museum of NationalAntiquities. She has a wide experience in field archaeologyand cultural heritage management. Her address: The Museumof National Antiquities, P. O. Box 5428, SE-114 84 Stockholm,Sweden.inga.ullen@historiska.se

262 Ambio Vol. 34, No. 3, May 2005� Royal Swedish Academy of Sciences 2005http://www.ambio.kva.se