The Degradation of Archaeological Bronzes Underground: Evidence from Museum Collections

Article excerpt

Introduction

During the twentieth century, modern society has transformed Scandinavia's old cultural landscape. Rational large-scale production methods have been introduced in agriculture and forestry, and technology and industry have grown very fast. While the economic and social benefit from this process is obvious, the negative effect on the preservation of the cultural heritage has only recently started being taken into account. Since 1988 the National Heritage Board in Sweden has initiated several projects to study the effects of air-borne pollution on buildings and rock-carvings. Several of these studies have verified that anthropogenic pollutants are responsible for the accelerated deterioration of rune stones, rock carvings and rock paintings in various parts of Scandinavia (Lindborg 1990; Bertilsson & Lofvendahl 1992). The results led to the suspicion that pollution might also be affecting the decay of archaeological finds in the soil. Archaeologists in Scandinavia had observed anecdotally that artefacts excavated today in some regions are more deteriorated than those found around 50 years ago.

Against this background, the National Heritage Board in Sweden launched an interdisciplinary project which was carried out between 1994 and 2001. Archaeologists, chemists, conservators, corrosion experts, geologists and statisticians got together to determine the main factors affecting the deterioration of archaeological metal artefacts in soil. The basis for the project and choice of methods was described in detail in Mattsson et al. (1996). There were two main parts to the programme: a detailed study (including chemical analyses) of recently excavated bronze and iron objects (Nord et al. 2002, 2004) and an overall investigation of earlier museum collections. It is the second of these two projects which forms the subject of this paper.

The aim of the study was to provide an overall survey of the state of deterioration of objects as a function of their date of discovery, and to supply a comparison in this respect between different regions in Sweden. The survey was also expected to shed light on the importance of grave type and burial customs as regards the state of the preservation of the finds. Three counties in Sweden with different soil conditions were selected for the study (Figure 1): Bohuslan situated on the northern part of the West Coast, Uppland on the East Coast, and the calcareous island of Gotland in the Baltic Sea. In these three areas archaeological excavations have been undertaken since the nineteenth century with professional documentation, including the recovery of a large amount of small metal fragments. This allows a comparison to be made between excavations undertaken from around 1900 to the present.

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Studying acidification

The soil in Bohuslan is very sensitive to atmospheric acid deposition because the crystalline bedrock weathers very slowly and the cover of Quaternary deposits comprising tills are often very thin and poor on nutrients. The area is also affected by severe global pollution. The buffering capacity towards acidic atmospheric deposition is low in comparison with the county of Uppland, where the quaternary deposit comprises clay calcite originating from the calcareous bedrock in the Baltic Sea and in the Bothnian Bay, transported by the inland ice. Finally, the island of Gotland in the Baltic Sea has a very calcareous subsoil with a high buffering capacity towards acidic deposition.

Generally speaking the environment in Sweden most vulnerable to acidification is coniferous forest on moraines. Many ancient monuments are found in such areas, predominantly in podsol soil. This soil type is in itself acid and vulnerable to acidic pollutants. Very low pH values, even as low as pH=4, have been recorded in forested inland regions. The degree of pollution is measured in terms of 'critical load' (CL), and the following definition has been agreed upon: A quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of environment do not occur according to present knowledge (Grennfelt & Thornelof 1992; Hettelingh et al. …