The interaction of water with archaeological and ethnographic birch bark and its effects on swelling, shrinkage and deformations (original) (raw)
Related papers
Heritage Science, 2021
The aim of this study is to gain specific information on the water vapour interaction with archaeological and ethno-graphic birch bark. Water is involved in a number of curative and preventive conservation measurements e.g. when reshaping 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.
2020
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 humidi cation and drying. The analysis revealed that the equilibrium moisture content is higher if lenticels or inner bark are present and that the burial context in uences 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 rst guidance to conservators in the decision of water treatments on birch bark objects.
Atlas of Micromorphological Degradation of Archaeological Birch Bark
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 brittle...
Sorption of moisture and dimensional change of wood species used in historic objects
Joint meeting of COST …, 2008
General' moisture sorption and swelling/shrinkage patterns which would apply as a first approximation to any wood species constituting cultural objects were proposed. They were obtained from the experimental data measured for 21 historically important wood species used in the past for panel paintings and woodcarving. Information on further wood species of interest to the wood conservation community can be added to the database and used to constantly improve the general relationships.
Heritage Science, 2015
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.
Journal of Archaeological Science, 2014
The state of preservation of waterlogged archaeological wood was evaluated on the basis of the maximum moisture content (MMC), the basic density (BD) and the wood substance density (WSD) determined in water and helium. The degree of wood degradation was compared under the criteria: the loss of wood substance (LWS) and the loss of wood substance density (LWSD). Studies were conducted on the wood samples differing in species, degree of degradation, age and place of origin. The physical properties of wood were determined for the material containing mineral compounds and the material without minerals. The properties of the latter, in which the minerals are replaced by water, were calculated from the mass and volume of the wood containing minerals as well as the content and density of the ash obtained after burning the sample. The study revealed the effect of minerals on the tested parameters and wood degradation indices. A strong relationship between BD and MMC was confirmed for both the wood containing minerals and without them, by contrast a substantially weaker correlation between BD and WSD was observed. It was found that the assessment of the state of wood preservation conducted on the basis of LWS and LWSD yielded different results. In addition, it was revealed that both indices of wood degradation might be unreliable. The main drawback of the LWS-based assessment is associated with a wide range of basic density of fresh wood. In turn, the LWSD mainly indicates the changing ratio of the carbohydrates/lignin content, but fails to provide information on the loss of wood substance. This may hinder the comparison of the wood sampled from different sites and subjected to different decay mechanisms. Nevertheless, the WSD-based assessment of the state of preservation of waterlogged archaeological wood might be a valuable complementary method to the BD-and/or MMC-based assessment, which is routinely carried out in many conservation centers.
2000
Ogilvie, Ticca Margaret Alison (2000) Water in archaeological wood : a critical appraisal of some diagnostic tools for degradation assessment., Durham theses, Durham University. Available at Durham E-Theses Online: http://etheses.dur.ac.uk/1528/ Use policy The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:
Study in the changes of the moisture content in wood
IOP conference series, 2021
The presence of water in wooden elements is an important factor in the understanding of the evolution of this material used for a structure. This happens because wood has the ability to change its volume due to climate variations; it can shrink or it can double its size depending on the season. This can influence the mechanical properties of the wooden piece and introduce in the structure deformations and changes in sectional efforts that can be very dangerous. For this study, environmental conditions have been tampered with so that wood that has moisture content of 12% will intake or output water, in order to arrive at a moisture content of 8% respectively 16%. In this way, the moisture content of untreated Spruce (Picea abies) and Chestnut (Castanea Sativa) has been modified in a climatic chamber at certain conditions of temperature and relative humidity. The wooden pieces have been verified for their moisture content in an oven with a temperature of 105℃ after a complete dry. The purpose of the study is to follow the behaviour of wooden pieces when found in other environments.
Towards a description of the degradation of archaeological birchbark
ICOM-CC 18th Triennial Conference 2017 Copenhagen, 2017
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 whether these 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.