What factors contribute to the deterioration mechanisms for cedar bark? Peter McElhinney’s presentation on work completed as an Andrew W. Mellon fellow in object conservation at the National Museum of the American Indian in Washington D.C. addressed this question. His project was inspired by encountering labels on storage boxes for objects in the collection with the words “Inherently Fragile: Will Have Continued Loss.” Peter set out to better understand why cedar bark deteriorates so rapidly and dramatically.
Objects made from cedar bark come from Western Red Cedar trees that grow in the North West coast region of North America. Native groups in that region harvest and weave cedar bark to make baskets, hats, mats and other objects. Cedar trees can grow to between 65-70 m tall and 3-4 m in diameter. One of the unusual features of these trees is the way that the bark is made and the type of cells present on the exterior of the bark. Peter focused on four aspects of cedar bark that play a major role in the way it deteriorates: the disruption of the cells on the exterior bark, calcium oxalate crystals, dehydration of pectin, and phenols.
Peter demonstrated the changes to the bark’s cellular structure using diagrams and CT scans. Cedar bark, called phloem, is made up of sieve cells, parenchyma cells and fibers. Cross sections of bark examined with a Skyskan 1172 micro CT scanner from Micro Photonics Inc. enabled the differentiation of inner and outer phloem. The cells in inner phloem, the section of the bark closest to the tree, are orderly, more rectilinear, and less disrupted. As the cells are pushed towards the outside of the tree, they become outer phloem, and develop a more disordered, compressed, less rectilinear appearance. The fibers in outer phloem have stronger cell walls, whereas the parenchyma and sieve cells tend to be crushed or squished. These changes in the phloem relate directly to the shedding characteristic of objects made from cedar bark.
The CT scan also revealed the presence of a large bio-mineral crystal in the bark sample. These bio-minerals form as part of the normal function of cedar trees based on minerals absorbed from the soil. Scanning electron microscopy with energy dispersive spectroscopy identified small, shard-like crystals as calcium oxalate and the large particle as a silica aluminum crystal. The small shard-like crystals were most abundant in the cell walls in the middle and outer phloem. This corresponds with literature that cedar trees have 10-20 times as many calcium oxalate crystals as other trees. These crystals may cause cell wall abrasion during manipulation of the cedar bark, which could contribute to the bark’s rapid deterioration.
The dehydration of the pectin and phenols also affect the cells. Cedar bark used for objects loses moisture over time, which can cause the dehydration of the pectin in the bark. Dehydrated pectin may reduce the ability of cells to adhere together. Significantly higher numbers of phenols are present in the outer phloem than in the inner phloem. The phenols protect the bark from ultraviolet radiation damage. This characteristic could influence lighting requirements for objects made from cedar bark if we can determine whether they are made from inner or outer bark.
Conservation applications of these findings help to improve understanding of how cedar bark deteriorates. The cells in outer bark are already structurally compromised, which can contribute to the shedding associated with cedar bark objects. Calcium oxalate crystals can further damage cells during handling of the object. Dehydrated pectin reduces cell adhesion within the bark. Finally, phenols present in high quantities in the outer bark may project the material from damage due to Ultraviolet radiation. Overall, this talk applied complex information about cellular biology to develop a better understanding of cedar bark deterioration mechanisms. This information is essential for developing better preventive care handling procedures for these fragile objects. I’m looking forward to reading the post prints for this talk and studying the figures and images in more detail.