# Atlas of Micromorphological Degradation of Archaeological Birch Bark
## 4 Discussion  
The identification and systematic description of microscopic degradation features and the macroscopic characteristics such as colour, brittleness and pliability of the 15 birch bark samples analysed in this study allowed us to establish a correlation between some macroscopic and microscopic features. If delamination was macroscopically visible, fractures and detachments of thin-walled cell walls were found. If the sample surface showed discoloration, decay at a microscopic level was found. Bacterial decay was only found in the waterlogged sample, while fungal decay was common. However, if biotic decay is very localised, it might not lead to macroscopic colour changes. If the samples were exposed to sunlight, they lost their red-brown colour and appeared strongly weathered, and removal of the phenolic compounds from the cell walls and lumen could be found in the very outer layers. However, it was not possible to establish a correlation between macroscopic brittleness and microscopic degradation features. For example, the Lendbreen samples are macroscopically strongly weathered but microscopically intact, except for the outer layers. Similarly, the Kosackenberg sample is macroscopically extremely brittle, but microscopically all the cell wall layers have been preserved. Microscopic biotic decay might be present in well-preserved and pliable samples such as our contemporary reference sample. More importantly, the occurrence of microscopic degradation features is very local in birch bark and can vary within the same fragment. The next step in interpreting the collected microscopic data was to try to establish any causality for the identified degradation features. Clearly, the presence of hyphae as well as voids in the cell walls, and the reduction in cell wall thickness and the complete disintegration of cells are all connected to biotic decay, mostly fungal, except for the waterlogged sample, in which bacteria were found. Biotically produced enzymes can also lead to the swelling of cell walls, removal of phenolic compounds, reduction in wall thickness and detachment of cells from each other or from the compound middle lamella [77], but in this case, these features were restricted to areas close to the hyphae. When the loss of phenolic compounds is present overall but confined to the upper layers, and when macroscopically the surface has lost its brown-red colour, photocatalysis of the phenolic compounds due to sunlight has probably taken place [23; 78]. Irregularity of the cell structure and, consequently, the cell arrangement, swelling of the cell walls, increased toluidine blue staining and loss of birefringence can also be found in the absence of signs of biotic decay, and we suggest that they are due to chemical degradation of the cell walls. In particular, loss of birefringence can be connected to a loss of crystallinity of  
Figure 27: Sample 13. LM shows many fractures of radial thin-walled cells (fra) leading to delamination. Thin-walled cells are unfolded (un); thick-walled secondary cell walls reveal increased staining, especially close to the surface. Thick dark-coloured residue (re) covers the surface. An accumulation of a cloudy substance in the lumen of thin-walled cells (acc) is present; some lumens are completely filled. suberin [72; 79]. Toluidine blue staining, though, is not selective for suberin, and a thorough investigation of suberin degradation should be performed using different methods. Accumulation of foreign substances, probably gypsum, was only detected in the fragments recovered from the cave environment and might be due to the deposition of salts dissolved in the water the samples were exposed to. Finally, fracturing and unfolding of thin-walled cell walls and elongation of thick-walled cell walls are mechanical degradation features that might have chemical, biotic or physical causes. In particular, unfolding of thin-walled cells has been detected in combination with every degradation feature and is a reliable microscopic degradation marker.