The conservation of a group of waterlogged neolithic bark bowls

2021

Herit Sci

The aim of this study is to gain specific information on the water vapour interaction with archaeological and ethnographic birch bark. Water is involved in a number of curative and preventive conservation measurements e.g. when re-shaping or drying objects and when defining climate directives for long-term storage. We measured the sorption isotherm of archaeological, ethnographic and contemporary birch bark at different temperatures and analysed the moisture-induced size and shape changes (swelling, shrinkage, deformation) during humidification and drying. The analysis revealed that, compared to other organic materials like wood, the moisture uptake 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). The equilibrium moisture content is higher if lenticels or inner bark are present. The extent of brittleness and delamination of the sample influences the sorption behaviour: the less brittle and delaminated archaeological birch bark is, the lower the equilibrium moisture content (EMC). Since the moisture uptake is modest, the related swelling of the outer bark is also modest, but anisotropic due to the cellular arrangement. Swelling is largest in the radial direction, smaller in longitudinal and negligible in tangential direction. Water vapour can plasticize birch bark and as birch bark becomes flexible, it bends towards the outside of the bark. This deformation takes place at high moisture contents and the adsorption process is slow. Based on these results recommendations on how best to perform treatments involving moisture and on relative humidity ranges for birch bark objects are provided.

Section: Conservation Relevancementioning

confidence: 99%

Section: Conservation Relevancementioning

confidence: 99%

The interaction of water with archaeological and ethnographic birch bark and its effects on swelling, shrinkage and deformations

2021

Herit Sci

“…Degradation of archaeological birch bark artefacts causes macroscopic colour fading [23], discoloration [24], delamination [25,26], deformation-distortion [27] and increased water absorption [28], but the connection between the burial conditions of archaeological bark and the dominant degradation pathways has never been investigated in detail. Different authors have shown that when Mesolithic and Neolithic archaeological birch bark artefacts are recovered from wetlands [29], caves [30,31] or waterlogged environments [32], they can be brittle and lose substance. This increased brittleness can have chemical and/or biological origins.…”

Section: Degradation and Preservation Condition Of Archaeological Birch Barkmentioning

confidence: 99%

Atlas of Micromorphological Degradation of Archaeological Birch Bark

2021

Applied Sciences

In this paper we present an atlas of micromorphological degradation of archaeological birch bark for the first time. We analysed the morphology of 13 samples extracted from ice-logged, waterlogged and cave-retrieved objects dated from the Neolithic to the Middle Age by means of light microscopy (LM) and transmission electron microscopy (TEM). We then compared their morphology to that of a contemporary sample, both intact and decayed. In all samples, 13 morphological characteristics that can be associated with fungal, bacterial, chemical, mechanical and light degradation are defined and described, and example LM and TEM images are provided. This novel atlas provides conservator-restorers a much-needed tool to relate the macroscopic appearance to the microscopic structure of birch bark objects. The most important macroscopic features allowing estimation of the state of preservation at the cell level are colour changes, loss of pliability, presence of delamination and increased brittleness. Colour change and delamination can be connected to microscopic features, and microscopic analysis can trace whether they were caused by biotic, chemical or physical decay. However, increased brittleness cannot be connected to a specific microscopic feature.

“…Birch bark objects may be humidi ed with water vapor at room or higher temperature in order to soften them during re-shaping [7]. They might also be frozen for short-term preservation or dried with [2] and without a pre-consolidation [1].…”

Section: Conservation Relevancementioning

confidence: 99%

“…Deformation can also lead to delamination related to failures in the thin-walled cell layers that are intrinsically weaker than the thick-walled cell layers [21,32]. To avoid such deformation conservators may block the artefact in the desired shape using specially made capsules during both humidi cation and drying [2].…”

Section: Interaction Birch Bark With Watermentioning

confidence: 99%

The interaction of water with archaeological and ethnographic birch bark and its effects on swelling, shrinkage and deformations

2020

Preprint

The aim of this study is to gain specific information on the water behaviour of archaeological and ethnographic birch bark. Water is involved in a number of curative and preventive conservation measurements e.g. when re-shaping or drying objects and when defining climate directives for long-term storage. We measured the sorption isotherm of archaeological, ethnographic and contemporary birch bark at different temperatures and analysed the moisture-induced size and shape changes (swelling, shrinkage, deformation) during humidification and drying. The analysis revealed that, compared to other organic materials like wood, the moisture uptake 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). The equilibrium moisture content is higher if lenticels or inner bark are present. The extent of brittleness and delamination of the sample influences the sorption behaviour: the least brittle and delaminated archaeological birch bark is, the lower the equilibrium moisture content. As the moisture uptake is modest, the related swelling of the outer bark is also modest, but anisotropic being larger in radial direction and smaller in longitudinal and negligible in tangential direction. Water vapour can plasticize birch bark and as birch bark becomes flexible, it bends towards the outside of the bark. This deformation takes place at high moisture contents and the adsorption process is slow. Based on these results recommendations on how best to perform treatments involving moisture and on relative humidity ranges for birch bark objects are provided.