On the rolling and plasticization of birch bark

Freshly harvested outer birch bark is a pliable, leather-like material, but within a short period it loses flexibility and becomes rigid. Further, still during the removal from the tree, it starts to roll back upon itself with the inner side out. This is relevant to conservators as objects made of birch bark do often show this type of deformation and need to be re-shaped. In this paper methods used nowadays by craftspeople to avoid the rolling of the outer birch bark (phellem) and flexibilization methods used by both craftspeople and conservators are reviewed. Some of these methods like the use of solvent vapours have some major drawbacks as birch bark contains a high amount of soluble substances within the cell lumen that can be solubilized during the treatment. In order to optimize treatment procedures for birch bark artefacts in the future it is important to understand the rolling of birch bark. We provide for the first time an explanation of the rolling pattern based on the microscopical analysis of the phellem cell dimensions on 3 to 6 years old twigs of Betula pendula and mature barks. The cell size and number measurements showed that the tangential cell length in the outer phellem layer is larger than in the inner layers while their number is lower. The combination of these two factors lead to the rolling of the bark with the inner side out once the inner tension is released upon the removal from the tree.