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The find of a yet unknown type of birch bark container in the site of Schnidejoch (Bernese Alps, Switzerland) was a surprise, for in all the wetland or lakeshore sites of Europe no parallels to this unique object are known to date. First, when only a small part was discovered, it was supposed to have been part of an arrow quiver. However, with the final appearance of the ca. 170 cm long container, it needed a new interpretation. The yew bow found at the same site, 160.5 cm long, would fit perfectly inside. Two silex arrowheads found in the bottom confirm an association with archery. The find could be identified as the first Neolithic bow case.

In this paper we present results of water sorption of archaeological, ethnographical and contemporary birch bark and of the water-induced size and shape changes during humidification and drying. The analysis revealed that the equilibrium moisture content is higher if lenticels or inner bark are present and that the burial context influences the sorption behavior: the better the preservation condition of the archaeological birch bark, the lower the equilibrium moisture content. Compared to other organic materials like wood, the water uptake and the related swelling of outer birch bark is modest. This can be attributed to the cell structure and composition: outer birch bark is composed of closed cells made to a large extent of hydrophobic components (suberin, lignin). We show that warping of the bark takes place at high moisture content. This deformation is related to the plasticization effect of water and to the release of the built-in tension related to the stretching of the cells during the tree’s growth. Our results provide a first guidance to conservators in the decision of water treatments on birch bark objects.

Archaeological birch-bark artefacts from ice patches are rare and little knowledge about their conservation exists. The degradation mechanisms are unknown and it is uncertain how they affect the mechanical properties and the cell structure. Due to this lack of knowledge, the treatments for archaeological birch-bark artefacts usually mimic those for waterlogged wood, which are tuned to the preservation condition of the object. This is assessed by measuring the maximum water content and, in some cases, the basic density and by microscopic examination of microscopic examination. In this paper, it is explored whetherthese parameters and techniques can be used to characterise the degradation of archaeological birch bark. Light microscopy examinations showed that cell wall deformations and fractures were present in both unaged reference material and archaeological birch bark and are not a distinct attribute of degradation. Cell collapse was not detected in ice-logged samples, while loss of birefringence is a potential tool to characterise degradation. Birch bark cells cannot be saturated with water, not even in the case of waterlogged archaeological samples. The authors conclude that maximum water content is not a diagnostic tool to quantify degradation.

Introduction Since ancient times, the unique properties of birch barks (Betula genus) have made them a material of choice for producing both everyday-life and artistic objects. Yet archaeological birch bark artefacts are rare, and little is known about the chemical transformations undergone by bark (chemically composed mainly of suberin and triterpenes) in archaeological contexts. Understanding the chemical modifications induced by ageing is essential for selecting suitable preservation and conservation approaches. Thus, the main aim of this research is to assess the preservation and state of degradation of archaeological findings made of birch bark: a Neolithic bow case recovered from a melting ice patch in the Bernese Alps (Switzerland) and a waterlogged birch bark vessel discovered at Moossee Lake (Canton of Bern, Switzerland). Scanning electron microscopy (SEM) and gas chromatography/mass spectrometry (GC/MS) were used to obtain information at micro-morphological and molecular levels on the state of degradation of the birch bark findings. GC/MS analysis followed two different sample preparations, alkaline hydrolysis and solvent extraction, in order to investigate respectively the hydrolysable and soluble constituents, and to test whether part of the suberin structure was depolymerised by the long period of burial. Results and conclusions SEM investigations on archaeological birch bark samples have shown that the extent of degradation of the microstructure is much higher in waterlogged birch bark than in birch bark preserved in ice. GC/MS analysis revealed that at a molecular level, the birch bark was quite well preserved. In both the archaeological environments, ice patch and lake water, various reactions had taken place leading to the depletion of reactive and sensitive compounds such as unsaturated acids and epoxy-compounds. In addition, archaeological birch bark had undergone depolymerization and oxidation reactions leading to the appearance of free suberin monomers and of oxidised triterpenes (betulone and lupenone). GC/MS data also seems to suggest that the birch bark preserved in the waterlogged site had a more pronounced degradation both in terms of oxidation and depolymerisation.