Drying methods for archaeological birch cork

This paper discusses the development of a drying method for waterlogged and ice-logged archaeological artefacts made of birch cork. Four years of research on the structure and degradation of birch cork have shown that both structure and degradation differ drastically from wood. The challenges of finding an appropriate drying method are equally different. In wood the main issue is to prevent the collapse of the cells and cavities during the evaporation of water whereas in birch cork, deformation and increased brittleness are the main risks. We analysed the effects of four drying treatments, freeze-drying at -10°C under vacuum and atmospheric pressure and air-drying at 4°C and 20°C on unconsolidated sample materials both archaeologic and contemporary. The parameters to evaluate the effects of the drying methods were quantitative and qualitative and included shrinkage, sorption isotherm, macroscopic deformation, formation of cracks and brittleness. The more the artefacts are degraded, the larger the influence of the drying method. Atmospheric freeze-drying resulted in significantly increased brittleness and hygroscopicity of waterlogged archaeological samples. Vacuum freeze-drying caused deformations and delamination of the same samples. Air-drying above zero degrees has the least negative impact on the macroscopic appearance and condition of archaeological birch cork.