A colloidal suspension becomes unstable when a sufficient amount of non-ionic surfactant is added, undergoing a segregation process ascribable to depletion forces induced by the surfactant micelles. The concentrated phase which separates out is, however, less compact than theoretically expected and shows stable gel-like features. This anomaly can be imputed to a dynamic freezing of the segregation process due to the interconnection of transient particle clusters. The results are compared with the predictions for the percolation threshold of Baxter adhesive hard-sphere model. Small-angle light scattering measurements show that the < phase is fractally organized over large spatial scales.

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.

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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.