Grading the decay of waterlogged archaeological wood according to anatomical characterisation. The case of the Fiavé site (N-E Italy)

•New biological attacks on wood after transformation from a peat bog into a lake.•Micromorphological wood decay grading well correlated with Maximum Water Content.•Grading approach mostly targeted to multidisciplinary studies.•Numerical evaluation of wood state of preservation from the anatomical perspective.

The grading of the anatomical preservation of cells inside waterlogged archaeological wood can be obtained by linking qualitative information obtained by microscopes with quantitative results inferred from physical characterisation. This study shows an approach to grading based on the microscopic analyses of 81 cores coming from poles of the pile-dwelling of Fiavè (IV-II millennia B.C.). Observations and physical analyses were undertaken on three different sectors of each core, taking into account their different state of preservation. In total 239 samples were analysed. The analyses identified 38 cores of larch (Larix decidua Mill.), 24 of spruce (Picea abies Karst.) and 19 of silver fir (Abies alba Mill.). Based on the morphology of decay, clear signs of biological attacks were identified: soft rot cavities, pointed ended, moving along the micro-fibrillar orientation of the cell wall, and “V”-shaped notches of eroded cell wall beyond the granular aspect in cross sections of heavily degraded cells. Results of classification (5 classes) according to micromorphological observations were compared to MWC values, for validation. This way we could verify an increase of average MWC with increasing micromorphological decay class, thus proving the reliability of the grading process. However, the micromorphological grading appeared more informative about the first stages of the attack.