On the deterioration of archaeological iron artefacts in soil

On the deterioration of archaeological iron artefacts in soil

Nord, Anders G.

Fornvännen 2002(97):4, s. 298-300

http://kulturarvsdata.se/raa/fornvannen/html/2002_298

Ingår i: samla.raa.se

298 Korta meddelanden

On the deterioration of archaeological iron artefacts in soil

Archaeologists have observed that metal arte-facts excavated today are more corroded than finds recovered 50-100 years ago. Acidification of lakes and ground can now be attributed to anthropogenic pollutants. Accordingly, they were also suspected to be a major cause of the increasing deterioration of buried remains. An interdisciplinary project was started at the National Heritage Board to study these pro-blems. O u r first study c o n c e m e d bronze arte-facts and showed that the deterioration has ac-celerated during the last century, and that pol-lution is o n e of the most important deteriora-ting factors. Iron artefacts are still more endan-gered. T h e present paper describes investiga-tions of archaeological iron artefacts, from re-cent excavations as well as from museum col-lections. The results are important for the ma-nagement of the archaeological cultural keri-tage.

Study of recently excavated iron artefacts 145 recently excavated iron artefacts and rela-ted soil samples kave been examined. T h e d e gree of deterioration for eack object was referred to as Fdet. T h e same persons dassified all ob-jects on a scale from 1 (very fine object) to 5

(mainly rust) by X-ray radiographs and visual inspection. These objects were usually in bad condition, with a poor metal core (if any) sur-rounded by a thick layer of rust Each soil sample taken around the find was dassified, and the grain size distribution determined. These vari-ables are static; i.e. they have not changed sig-nificantly since the artefact was buried. On the other hand, the chemical variables are dyna-mic: addity, humidity, mäss loss on ignition, dectric resistivity, and salt contents (cf. Mattsson et al. i g g ö ) . Data on the arckaeological con-text, geograpky and environment were inclu-ded. Statistical evaluations were undertaken by means of multivariate analysis (Eriksson et al. i g g g ) . T k e karmful influence of cklorides in tke soil and large polluting deposits of nitrogen and sulphur c o m p o u n d s (in relation to what

the ground can tolerate) was obvious. Fine-grained sand implied a strong deteriorating ef-fect. Among the preserving factors we noted si-tes in d e d d u o u s forests or calcareous surroun-dings. Influence from tke arckaeological con-text was, kowever, weakly indicated. Tkis is one of tke reasons wky an examination of museum objects was also undertaken. We are aware of tke fact tkat information on internal structure, carbon content and chloride content would have been of interest, but we were not allowed to carry out such analyses of the artefacts.

Results of the museum collection study

1350 objects from the Museum of National Antiquities in Stockholm were studied. The linds date from the Early Iron Age to around AD 1600. In order to achieve an objective das-sification of the deterioration, we have basically ckosen nails and rivets for tkis study. Docu-mentation for tke museum objects was obtai-ned from the archives. T h e information on artefacts found before i g o o AD was rather scarce, but as many data as possible were retrie-ved concerning type of object, locality, year of acquisition or exeavation, arckaeological con-text, and environment. T k e objects were dassi-fied as described in the previous section. No soil samples were available. In order to include some information on tke soil sensitivity towards aci-dification, special soil sensitivity maps were pre-pared (cf. Mattsson et al. i g g ö ) . T k e multiva-riate evaluation skowed tkat tke west coast witk its vulnerable soil is indeed disastrous to ar-chaeological iron. For other parts of Sweden, the effect of tke soil sensitivity was ratker weak. Finally, as regards conservation treatments, water leacking came out as a strong preserving factor.

Conventional statistical metkods were used to evaluate tke arckaeological and environ-mental variables. Table 1 shows the average de-terioration {Fdet) for finds sorted into four pe-riods after year of exeavation. Tkere is a trend of a slightly acederated deterioration for more

Korta meddelanden 299 Period of exeavation acquisition i 8 2 Ö - i 8 g g

1900-1949

•95°-»969

3.66

3-9«

Table 1. The average degree of deterioration (Fdet) versus exeavation year.

recent finds, although less p r o n o u n c e d than for the museum bronzes (cf. Ullen et al. Ms.). For the earliest period there is a risk that the archaeologists of the time mainly collected tlie best artefacts, thus falsifying the average Fdet. However, our selection of artefacts has been made so that the best-preserved iron finds were compared for eack time period and region. Part of tke deterioration may also be attributed to post-excavation factors like unsuitable con-servation methods or museum conditions. It is clear from the study, however, that the increa-sing air pollution has had a major deteriorating effect

Iron artefacts found in cremation graves were generally in a worse condition than those from inhumation graves. Graves in cairns, i.e. without any protecting soil layer, have been particularly exposed to acid rain, with Fdet (ave-rage) as high as 4.2g, indicating severely cor-roded objects mainly consisting of r u s t Finally, for the finds from the Black Earth of Birka, the-re was a striking diffethe-rence in pthe-reservation for various exeavation dates. Finds excavated during tke igth century showed an Fdel( ave råge) of 2.23, while the corresponding average value for tke 2()tk century finds was 4.60.

Discussion

Tke results agree witk tkose obtained for bron-ze artefacts, although the iron artefacts were generally more corroded, in agreement with corrosion science. Many technical corrosion studies have been undertaken, i n d u d i n g iron in soil. The duration of these experiments has, however, been very brief in comparison with the time spän for arckaeological objects. Among the few investigations undertaken on

archaeo-logical iron, Scharff ( i g g s ) has examined iron objects in German museum collections. He ob-served a significantly worse preservation status for objects excavated after 1 g6o. Like North & Pearson ( i g 7 8 ) and Mattsson & Norlander ( i g g ö ) , he emphasised that chlorides are dis-astrous to iron artefacts. Gerwin et al. ( i g g 8 ) also found tkat sandy soil, salt, and acidic soil kave a strong corrosive effect on iron artefacts. Treatment of keavily corroded iron kas been dis-cussed by Hjelm-Hansen et al. (1998).

It is well known from corrosion science that corrosion requires an oxidizing agent (usually oxygen from the air), and an dectrolyte (e.g. a water solution). Accordingly, a moderately well aerated and moist soil suck as sand, ratker tkan clay or gravel, should be most detrimental to metals, giving access to oxygen while the soil pores are partially filled with water. Peat and certain culture layers instead have a preserving effect. The detrimental effect of salt in soil or cremation layers containing soot and askes is due to increasing electric conduetivity. It should finally be mentioned that a similar study of the degradation of archaeological bone, another important material, has been carried out as an EU project in co-operation between the Netherlands, Sweden (i.e. the National Heritage Board), Great Britii n and Italy (Kars et al. 2002).

Conclusions

T h e present study has shown that the deterio-ration råte of archaeological iron objects has acederated in recent years, and that it is increas-ed by acidic soil, salt and soot. T k e total age since burial has not shown up as a significant deteriorating variable. However, the soil has only been affected by the deposition of various pollutants d u r i n g tke last 50-100 years. Ac-cordingly, tke corrosion in recent years may partly be attributed to anthropogenic pollution. The study kas also shown tkat soil conditions allowing access of air (oxygen) and water at the same time will increase the corrosivity. This must be taken into consideration when plan-ning large-scale environmental changes which may affect the water table. Archaeological para-meters found to affect tke corrosion are cre-mation layers and »open» construetions like

3 0 0 Korta meddelanden c a i r n s . M a n y u n - e x c a v a t e d a r c k a e o l o g i c a l i r o n a r t e f a c t s a r e e n d a n g e r e d , especially o n t h e west c o a s t of S w e d e n . Arknowledgements We a r e v e r y g r a t e f u l t o all a r c k a e o l o g i s t s w k o k a v e k e l p e d us witk m a t e r i a l for tkis study. We a r e i n d e b t e d t o P r o f e s s o r E i n a r M a t t s s o n , for-m e r h e a d of t h e S w e d i s h C o r r o s i o n I n s t i t u t e , for v a l u a b l e d i s c u s s i o n s a n d g u i d a n c e t h r o u g h -o u t t h e p r -o j e c t , a n d t -o A g n e t a Å k e r m a r k Kraft, G u n n a r C h . B o r g a n d H e n r i k R u n e s s o n f o r v a l u a b l e h e l p . References

Eriksson, L., Johansson, E., Kettaneh-Wold, N. & Wold, S. 1999. Introduction lo multi- and megava-riale dala analysis using projection methods (PCA åf PI.S). Umeå.

Gerwin, W., Scharff, W. Se Baumhauer, R. 1998. Corrosive decay of archaeological metal finds from different soils, and effects on environmen-tal pollution. MEA1AL g 8 - Proceedings of lhe International Conference on Metals Conservation, Draguignan, France, May i g g 8 . London. Hjelm-Hansen, N., van Lanschout, (., Szalkay, C D .

& Turgoose, S. 1993. Electrochemical assess-ment and monitoring of stabilization of heavily corroded archaeological iron artefacts. Corro-sion Science 35. Oxford.

Kars, H. & Kars, E. (eds). 2002. The degradation of bo-ne as an indicator in Ihe deterioration of lhe European archaeologicalproperty. Final report fm lhe EU project ENVj-CTgS-oy 12. Amersfoort

Maltsson, E., Nord, A.G., Tronner, K, Fjaestad, M., Lagerlöf, A., Ullen, I. & Borg, G.Ch. 1996. De-terioration of archaeolngical material in soil. Results on bronze artefacts. Konserveringstekniska Studier 10. National Heritage Board. Stockholm.

Mattsson, E. & Norlander, Ä. 1996. Bevarande av arkeo-logiskt järn efter utgrävning. Konserveringstekniska Studier 13. National Heritage Board. Stockholm. North, N.A. 8c Pearson, C. 1978. Methods for

trea-ting marine iron. ICOM Committee for Conser-vation, rjlh Triennal Meeting, Zagreb.

Scharff, W. 1993. Gefährdung archäologischer Funde durch immissionsbedingte Boden-versauerung. För-st hungsbericht des Landes-denkmalamt Baden-Wurllemberg.

Ullen, L, Nord, A.G., Fjaestad, M., Mattson, E., Borg, G.Ch. & Tronner, K. 2003. Environmental influ-ence on the degradation of archaeological bron-zes in Scandinvia (in manuscript).

Anders G. Nord a n d Kate Tronner National Heritage Board Box 5405 SE-i 14 84 Stockholm, Sweden anders.nord@raa.se, kale. t r o n n e r ® raa.se Inga Ullen Museum of National Antiquities Box 5428 SE-i 14 84 Stockholm, Sweden inga.ullen@historiska.se