# Atlas of Micromorphological Degradation of Archaeological Birch Bark
## 3 Results
### Archaeological Samples
#### 3.3.4 Sample 4  
A further degradation feature is the detachment of the secondary cell wall from the compound middle lamella and the tertiary wall. This is visible in sample 4. Unfolding of thin-walled cells and stretching of thick-walled cells is also present. Sample 4 was taken from the bow case inner lining, but the original position on the object is unknown, possibly the lid. The birch bark is slightly pliable and retains some strength. The surface is compact but shows cracks and delamination in the top half (Figure 4c). The bark is somewhat brittle, especially the lenticel. The outer side of the bark is grey, the inner brown. In the lower area, a black cloudy discoloration is visible that is also present on the edges. Red-brown colours are strongly reduced.  
Thin section 4a shows a higher degree of decay than 4b. This sample is characterized by unfolding of thin-walled cells, elongation in the transverse direction of thick-walled cells and increased staining. Along with this, the structure becomes irregular. While in sample 4b the cell structure is regular, the walls of the thin-walled cells are still slightly folded and there is only slight light blue staining of the secondary cell walls, an indication of low water sorption.  
The detachment of the secondary cell wall from the compound middle lamella and the tertiary wall affected large areas of thin section 4a and only slightly affected thin section 4b (Figure 20). TEM images (Figure 21, bottom right) show that due to this separation, voids and threadlike structures appear between the wall layers. The very thin, delaminated tertiary cell wall layer is present as a ring within the lumen. The swelling of the secondary cell wall causes a wave deformation of the secondary wall layer. The compound middle lamella is broken, and interruptions are present. The original structure of the suberin lamella has disappeared and broad light and dark bands with the same orientation as the former suberin lamella have formed. The degradation of the compound middle lamella led to a separation of the cells and a degradation of the phenolic content, visible as reduced cell lumen content.  
Figure 19: Sample 3. **Left**: LM image. Secondary cell walls of thick-walled cells are swollen (swe-ov). Deformation of cells has also taken place, leading to irregular structure (ir). Parts of tertiary cell wall of thin-walled cells are detached (det). **Right**: TEM image. Swelling of secondary cell wall of thick-walled cells is visible, together with detachment of parts of tertiary cell wall of thin-walled cells. In sample 3, this phenomenon is widespread and not localised.  
Figure 21: Sample 4a. Detachment of secondary cell wall from compound middle lamella is found throughout (de-ov-wl, **top left**). Elongation of thick-walled cells (un-st) is present (**top right**). TEM (**bottom left**) shows detachments of secondary wall from compound middle lamella and tertiary wall (de-ov-wl). Very thin, delaminated tertiary cell wall layer is present as a ring within the lumen. Compound middle lamella is broken and shows large interruptions (in). Swelling caused wave deformation of secondary wall layer. Separation and threadlike structures appear between secondary and tertiary wall (**bottom right**). Original structure of suberin lamella has disappeared and broad light and dark bands with the same orientation as the former suberin lamella are formed.  
Figure 20: Sample 4. Two thin sections, about 4 mm apart, were made from the embedded sample: thin section 4a (**left**) and thin section 4b (**right**). Thin section 4b reveals a much better-preserved cell structure than 4a: it is regular, walls of thin-walled cells are still slightly folded and there is only slight light blue staining of secondary cell walls, an indication of low water sorption; thin section 4a shows unfolding of thin-walled cells and stretching of thick-walled cells (un-st).