# Atlas of Micromorphological Degradation of Archaeological Birch Bark
### Degradation and Preservation Condition of Archaeological Birch Bark  
Degradation of archaeological birch bark artefacts causes macroscopic colour fading [23], discoloration [24], delamination [25,26], deformation-distortion [27] and increased water absorption [28], but the connection between the burial conditions of archaeological bark and the dominant degradation pathways has never been investigated in detail. Different authors have shown that when Mesolithic and Neolithic archaeological birch bark artefacts are recovered from wetlands [29], caves [30,31] or waterlogged environments [32], they can be brittle and lose substance. This increased brittleness can have chemical and/or biological origins. Concerning the chemical origins, gas chromatography/mass spectrometry (GC/MS) examinations of waterlogged Neolithic alpine artefacts [25] detected depolymerization and oxidation, attributed to higher hydrolysis of susceptible groups, such as epoxy linkages. Concerning the biological origins, investigations of artefacts recovered from the wetland site Star Carr dating from the ninth millennium BC revealed fungal hyphae within the bark structure, and pyrite and gypsum crystals on the surface [29]. The fragility of the artefacts was attributed to both the mineral deposits and the microbial attack. Gypsum crystals within the bark structure were also found in birch bark torches from the Kosackenberg caves dated 2300-1800 BC and Iron Age [30], in a quiver from Xinjiang (Northwest China) dated 618-907 AD [33] and in Bronze Age banks from salt mines [34]. Specific investigations of the biotic decay of birch bark could not be found in the literature. Tree phenlems that consist of suberized cells, such as birch cork, are understood to be very resistant to biological decay [35,36] because they are constructed to protect the plant/tree from infection by pathogen microorganisms [37]. Studies performed in the 1970s on the degradation of contemporary barks under natural burial conditions and in the laboratory reported that the phelem was the last component affected by fungi [38]. The decay was reported to start after 3 years of exposure, and microorganisms grew within the thick secondary tangential suberized cell walls. The growth of hyphae mechanically damages the cells by creating oval cavities, and this might lead to weight loss. Early studies on conifer barks showed that the weight loss caused by fungi in bark was five times smaller than that in wood, and that despite a weight loss of up to 10%, the physical properties may remain unchanged [39]. Several authors have shown that enzymes produced by fungi can degrade suberin [40,41,42,43,44], but the detailed pathways are still largely unknown, because suberin is strongly cross-linked and an unchanged extraction was not possible until new extraction techniques were developed [45,46]. [PERSON] et al. [47] revealed that cutinasen, lipases and long-chain alcohol-modifying enzymes released by _Aspergillus nidulans_ fungus depolymerises suberin. The long-chain fatty acids are broken down via peroxisomal \(\beta\)-oxidation. A very recent publication found that auxin-regulated GDSL lipases could degrade suberin [48]. Studies of enzymes from cold Arctic [49] and Antarctic [50,51] environments show that enzymes are active at temperatures close to 0 \({}^{\circ}\)C. Some enzymes have limited function, but activity at 0-15 \({}^{\circ}\)C is common for many of them [52]. Regarding archaeological wood, it is known that when it is recovered from low-oxygen sediments or under water, bacteria are the main cause of slow degradation [53,54,55], and when it is exposed to air, fungi lead to a very fast metabolism controlled by temperature and humidity [56; 57]. The absence of water and oxygen and low temperatures inhibit biotic decomposition, and changes due to chemical and physical processes may become evident [58]. However, very extreme conditions are required for microbial degradation to be completely suppressed [59].