Birch bark is a naturally water-repellent organic material used for containers, fishing equipment and mats from Neolithic times until nowadays by indigenous and hunter-gatherer populations.
Objects are in a perishable condition when found wet [1, 2] and are often misshapen [3-6] due to an inbuilt tension of the material [7, 8]. They might also show delamination and increased brittleness [1, 9, 10]. In order to preserve the artefacts preventive and curative conservation is needed. Preventive conservation involves the setting of relative humidity targets to avoid deformations, mould growth and chemical degradation. Curative treatments involve water in a number of cases. Birch bark objects may be humidified with water vapor at room or higher temperature in order to soften them during re-shaping . They might also be frozen for short-term preservation or dried with  and without a pre-consolidation. Drying is performed with a general hesitation as it known to cause shrinkage and distortion in waterlogged wooden objects [11, 12]. How far birch bark objects might swell, shrink and deform is expected to depend on the amount of moisture absorbed by the bark and on its preservation condition, namely brittleness and delamination. The amount of moisture absorbed or released by the bark is characterized by the sorption isotherm being the equilibrium moisture content at different humidity values and constant temperature. The sorption isotherm of birch bark has been measured on freshly harvested samples but never on archaeological samples and the swelling, shrinkage and deformation of outer birch bark during wetting and drying has never been published. In this article we fill this gap by presenting first measurements of the sorption isotherm, swelling and shrinkage of birch bark on a number of archaeological and ethnographic samples.
The material birch bark: macroscopic and microscopic structure and chemical composition
The portion of birch bark used to manufacture objects is the outer bark, in botany called the phellem layer, in plain language cork. The phellem prevents the birch tree from transpiration, it isolates it from heat, sun-radiation and cold  and protects it from the penetration of parasites . Both chemical composition and microscopic structure determine the moisture adsorption properties.
The phellem is made of cells formed each year from May to August  by a thin layer of active cells, the phellogen or cork cambium, present between the inner and the outer bark (Figure 1). These cells are organized in bands of thin-walled cells and thick-walled cells that differ in the filling material of the cell lumen (Figure 2, middle and right). Both thick-walled and thin-walled cell walls contain suberin, a biopolymer made of polyaliphatic and polyphenolic parts connected by glycerol via ester bonds. As a consequence of the suberization of the cell walls these become impermeable and the cells die. Suberin makes 36.2 wt% of the phellem total composition and together with the closed structure of the cells is responsible for the low water permeability of the phellem . Lignin accounts for 14.3 wt% of the phellem composition and is found in the middle lamella and between the suberin molecules in the secondary cell walls  (Figure 2, right). Polysaccharides account for 10.3 wt% of the phellem composition while extractives accounts for 32.2 wt% and are located in the cell lumen . In particular the thin-walled cells are filled with betulin, a triterpene, while the thick-walled cells are filled with phenolic components.
The number of layers in each band depends on the phellem age and on the species. The freshly formed phellem cells are pushed against the existing layers and, as the tree circumference grows, pushed outwards, stretch in tangential direction and compress in radial direction (Figure 2, middle and Figure 3) [7, 8] and eventually come off the tree. Both phellem thickness  and stiffness  increase with age: the thickest outer bark can be found on the bottom of a tree. Generally, birch bark objects are made only from the phellem layer, as this can be easily removed from the trunk when the bark is harvested in spring and summer, the time when the phellogen is active. In this case the phelloderm and the phloem remain attached to the trunk (Figure 4). Only if the bark is harvested in winter the phelloderm and the phloem stay attached to the phellem. Further, when manufacturing objects, a portion of the white, outlying, oldest, more brittle and more permeable to water and oxygen layers  is usually removed.
The phloem and the phellem differ radically in function, orientation, microscopic structure and chemical composition. The function of the phloem is to transport sugars from the leaves to the roots of the plant, whose constituent parts are therefore mainly aligned in longitudinal direction. It is produced not by the phellogen but by the vascular cambium, that builds the phloem outwards and the secondary xylem, in plain language the wood cells, inwards (Figure 1). The phloem is a complex tissue consisting of sieve tubes, open cells dedicated to the transport of nutrients, fibres, phloem parenchyma, sclerenchyma and rays parenchyma [22, 23]. Parenchyma are living thin-walled cells of various functions, sclerenchyma are dead cells with a thick secondary lignified wall and support function and rays are the continuation of rays of the xylem and can produce additional cells. Polysaccharides like cellulose constitute 43 wt% of the phloem chemical composition, lignin the 32.2 wt%, suberin the 13.2 wt% and various extractives the 8.1 wt% .
Gas and water exchange between the internal tissues of the stem and the environment is allowed by the presence of pores in the phellem called lenticels that characterize the birch bark surface. Lenticels are fanned out bands of cell layers made of continuous bands of thick-walled cells and disrupted bands of thin-walled cells.
Interaction birch bark with water
Different authors [24-29] investigated the sorption behaviour of freshly harvested birch bark, mostly in studies on the influence of the addition of bark material to the properties of wooden particleboards. Samples where the phloem is still attached to the outer bark are characterized by a high hygroscopicity, similar to wood , while samples where only the phellem is retained are characterized by a four times smaller moisture content that has little dependency on the specific birch species. In Figure 5 the adsorption isotherms are reported. Kajita  measured the sorption isotherm at two temperatures, 20°C and 30 °C, and confirmed the well-known slight decrease of the equilibrium moisture content by increasing temperature observed in hygroscopic materials. Holmberg et al.  measured scanning isotherms in adsorption and desorption for Betula Papyrifera samples. The time needed to reach equilibrium at each step of the sorption isotherm depends on the sample thickness, preparation method and presence of ventilation in the surrounding environment. This study showed that for cuboidal samples with size of ~ 2 mm in a ventilated environment it is of the order of 8 hours where half of the weight loss is attained in the first 30 minutes.
An understanding of which component in the phellem absorbs water is provided by the study of Schönherr and Ziegler  on samples of the bark of Betula Pendula Roth. Thin samples of the bark were clamped between two permeability cups filled with a 1% AgNO3 water solution and infiltrated for 12 hours. Silver was then precipitated with hydrochloric acid and the samples embedded. Electron micrographs revealed silver in the middle lamella and in the primary but not in the secondary, suberized, cell wall. It was significantly more concentrated in the radial middle lamellae than in the tangential ones. Further it was found in the lenticels, which have intercellular cavities in the tangential direction. This shows that the radial middle lamella and the lenticels are the components that mostly absorb water and build the pathway for the diffusion of water in the phellem.
Moisture adsorption is therefore expected to increase if the phloem is retained on the sample, if the sample contains lenticels, if the number of cavities increases as it occurs in brittle degraded samples and by decreasing temperature.
The adsorption and desorption of water is expected to cause swelling and shrinkage of birch bark. Gilberg  analysed microtome samples of the outer bark of Betula papyrifera Marsh exposed for 24 hours to water vapor and found no radial and tangential swelling, while Groh et al.  analysed discs of 1 cm diameter from the outer bark of Betula potaninii and detected an increase of 4 % of the surface when exposed to 100 % RH. Bhat  investigated the shrinkage of Betula pendula and Betula pubescens inner and outer bark and found that it is anisotropic, being higher in radial then in tangential direction. The same anisotropic behaviour is documented for Douglas fir cork  and oak cork . The high radial swelling is attributed by both authors to the unfolding of corrugated lateral walls of thin-walled cells taking place upon moisture adsorption. The stretching of the radial cell walls is retained also upon drying while further adsorption of water vapor or liquid water causes a radial expansion of much smaller magnitude . No data are available on swelling and shrinkage of outer birch bark neither for contemporary nor for archaeological and ethnographic material.
Beside swelling and shrinkage, water adsorption may lead to bending of the bark. Water acts as a plasticizer allowing the phellem’s outermost cells, that in the tree have been stretched tangentially to accommodate the growth of the stem and the creation of new layers, to return to their original dimension. This causes a contraction of the outer layer and therefore a bending or rolling of the material with the outer side of the bark inwards [7, 8]. 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 humidification and drying.
The aim of the present research is to quantify moisture content, swelling, shrinkage and macroscopic deformation of archaeological and ethnographic birch bark samples upon adsorption and desorption of water vapor in order to provide conservators with indications on the risks related to procedures that involve humidification and drying.
 Natalia Vasiljeva, personal communication, September 2018.
 Natalia Vasiljeva, personal communication, September 2018.