Birch bark is a natural water-repellant organic material used for containers, fishing equipment and mats both in Neolithic times and nowadays by indigenous and hunter-gatherer populations.
Conservation treatments for birch bark objects involve water in a number of cases. Objects may be humidified in order to soften them during re-shaping. Craftspeople use mainly warm liquid water [1, 2] while conservators use mainly water vapour [3-6]. They might also be frozen for short-term preservation or they might be dried for exhibition and long-term preservation.
These treatments are known to possibly cause in archaeological wooden objects shrinkage and distortions and therefore there is a general hesitation in performing them on archaeological birch bark. How far these objects might swell, shrink and deform is expected to depend on the amount of water absorbed by the bark and on its preservation condition. The quantification of birch bark’s sorption behavior and of the connected swelling and deformation is necessary therefore to provide guidance to conservators in the decision of water treatments.
The sorption isotherm of birch bark has been measured on freshly harvested samples but never on archaeological samples and the swelling and shrinking of outer birch bark during wetting and drying has never been published. In this article we fill this gap by presenting novel measurements of the sorption isotherm of birch bark on a number of archaeological and ethnographic samples and at different temperature. We also estimate the swelling and shrinkage during absorption and desorption processes and further deformations connected to an air-drying at room temperature.
Based on these results we provide recommendations for the performance of water treatments and for the relative humidity ranges for long-term preservation.
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. The outer bark prevents the birch tree from transpiration, it isolates it from heat, sun-radiation and cold [7, 432] and protects it from the penetration of parasites [8, 85f]. Both chemical composition and microscopic structure determine moisture absorption properties.
The phellem is made by cells formed each year from May to August [9, 27] by a thin layer of active cells, the phellogen or cork cambium, present between the inner and the outer bark. 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 1). Both thick-walled and thin-walled cell walls are composed of suberin, a biopolymer made of polyaliphatic and polyphenolic parts connected by glycerol via ester bonds. As 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 water low 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 [11, 86f]. Polysaccharides account for 10.3 wt% of the phellem composition while extractives accounts for the remaining 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 specie. The freshly formed phellem cells are pushed against the existing layers and, as the tree circumference grows, migrate outwards, stretch in tangential direction and compress in radial direction  and eventually come off the tree. Both phellem thickness [14, 209] and stiffness [15, 468f] increase with age: the thickest outer bark can be found on the bottom of a tree. When manufacturing objects, a portion of the white outlying, oldest, more brittle and more permeable to water and oxygen layers [16, 351] is usually removed.
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.
The phellem detaches easily in spring, the typical season when birch bark is harvested. If the bark is harvested in winter a part of the phloem, the layers of living cells internal to the phellogen, might be retained and used to manufacture objects with decorations carved in the inner bark surface. The phloem and the phellem differ radically in function, microscopic structure and chemical composition. The function of the phloem is to transport sugars from the leaves to the roots of the plant. It is produced not by the phellogen but by the vascular cambium outwards. The vascular cambium produces inwards the secondary xylem, in plain language the wood cells. The phloem is a complex tissue consisting of sieve tubes, open cells dedicated to the transport of nutrients, phloem parenchyma, sclerenchyma and rays parenchyma . 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. 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% .
Interaction of water with birch bark
Different authors [18-23] investigated the sorption behavior 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 specie (Figure 2). We report here only the adsorption isotherm as the authors reporting desorption results  did not let the samples saturate at 100% RH, measuring therefore scanning isotherms whose hysteresis cannot be univocally interpreted . The difference in sorption behavior between samples with or without phloem can be explained by the fact that the secondary phloem is formed structurally by open cells, the sieves tubes, and chemically mostly by polysaccharides that are hygroscopic due to the large number of hydroxyl and carboxyl polar groups that can form hydrogen bonds with water molecules. The phellem on the contrary is formed by closed cells and mostly by suberin, a lipophilic biopolymer.
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 absorption 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. Holmberg et al.  showed that for cuboid samples of 3 mm side 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 [16, 387f] on samples of Betula Pendula Roth. The samples were infiltrated with water containing as indicator silver nitrate that was subsequently precipitated with hydrochloric acid. Electron microscopy 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.
The sorption isotherm is expected to increase therefore if the phloem is retained in the sample, if the sample contains lenticels, if the amount of cavities increases as it occurs in brittle degraded samples and by decreasing temperatures.
The adsorption and desorption of water is expected to cause swelling and shrinking of birch bark. While Gilberg  found no radial and tangential swelling on previously dried contemporary microtome sections of outer birch bark exposed for 24 h to water vapor, Groh et al. [18, 799] detected 4 % swelling at 100 % RH of the surface of round birch phellem samples. Bhat  investigated the shrinkage of the inner and outer bark together and showed that it is anisotropic, being higher in radial direction then in tangential direction. The same anisotropic behavior is documented for Douglas fir cork [26, 98] and oak cork [11, 198]. The high radial swelling is attributed by both authors to the unfolding of corrugated lateral walls of thin-walled cells taking place upon moisture absorption. The stretching of the radial cell walls is retained also upon drying while further absorption 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 ethnographical material.
Beside swelling and shrinking water absorption may lead in birch bark to the warping of the bark with the outer side in. Water acts as a plasticizer allowing the phellem outer cells, that in the tree have been stretched tangentially to accommodate the growth of the stem and the creation of new cork layers, to return to their original dimension. The tangential length of the outer phellem cells is shorter than the tangential length of the inner phellem cells, as these have been produced when the stem had a larger diameter. The release of the tension through moisture plasticization cause a contraction of the outer layer and therefore a warping or rolling with the outer side of the bark inwards . Deformations can also lead to delaminations related to failures in the thin-walled cell layers that are intrinsically weaker than the thick-walled cell layers [16, 348, 28, 251]. To avoid such deformations conservators may block the artefact in the desired shape using specially made capsules during both humidification and drying.
A clearer understanding of the swelling, shrinkage and of the deformations of birch bark upon moisture absorption and desorption can support conservators in their choices of conservation procedures.