Last quarter in the class Structure, Properties and Deterioration of Organic Materials (CAEM 262), we completed practical assignments in order to understand how to identify and approach organic materials found within and comprising cultural objects. Each week the class studied a different category of organic materials including wood, paper and bark cloth, other plant materials, skin and leather, bone and ivory, plastics, and hair, quills and feathers.
During our study of plant fibers, we utilized light microscopy, both with transmitted and polarized light, to view and identify fibers and diagnostic features of plants that could help in identification and documentation of materials used in the manufacture of objects. Different types of plant tissues and fibers can be used to produce a range of materials including cordage, baskets, paper, and native or conservation mends. The processing method of fibers may also be seen, and commercial fibers can usually be identified by their absence of impurities or extraneous material. An important reason for identifying plant materials is to understand their current state and how they may deteriorate in the future due to their inherent properties. By studying and identifying the materials of which an object is made, as conservators, we will be to make more informed decisions about conservation treatment, repairs, stabilization and storage (Florian et al. 1990: 29).
Using the UCLA/Getty Program’s vast reference collection of plant materials, each student chose and mounted on a microscope slide a surface section of a seed hair (fibers that surround plant seeds, like cotton or kapok), a surface section of a bast fiber (fibers harvested from woody stems, like flax or hemp), a cross-sectional sample of a monocot leaf (like palm) and a cross-sectional sample of a monocot stem (like a grass). We then took photomicrographs of our samples and labeled the features that could be identified. Features that we were hoping to observe included tissue organization, cellular structure and details, and birefringent patterns and colors. Below are the photomicrographs obtained using and Olympus BX51 microscope with transmitted and cross polarized light by Lesley Day, William Shelley and Betsy Burr.
Seed Hair Surface Sections
Polarized Light Microscopy (PLM) is very useful in identifying cotton and other seed hairs. Cotton characteristically shows ribbon-like twists and birefringence. Cotton hairs are single cells that originate from the fruit or boll of the cotton flower. Other seed hairs such as kapok and cattail characteristically show few to no features, and lack of features can help in their identification (Florian et al. 1990: 40). Samples were prepared by teasing out a small amount of material and placing it on a glass microscope slide. A drop of water was placed on the fiber and covered with a plastic cover slip.
Bast Fiber Surface Sections Bast fibers are long thick-walled cells harvested from the inner bark of hardwood trees. They are commercially and culturally important because they are used in cordage, basketry and paper. Flax and hemp are examples of bast fibers and are harvested from the stems of dicots. Salient features used to identify bast fibers are dislocations (also called kinks), long length, tapered ends and narrow lumens. The fibers usually occur as clusters, and the presence of single fiber ultimates may indicate more extensive processing (Pearlstein, lecture 28 January 2014). Samples were prepared in the same manner as above. In some instances, the samples were macerated in order to separate fiber bundles for better viewing of fiber ultimates, by gently applying pressure in a circular motion to the coverslip with a pencil eraser.
Cross Sectional Sample of a monocot leaf Monocot leaves from palms and grasses are commonly used culturally in the weaving of baskets and other structures. The leaves are quite strong due to the parallel veins and the strengthening sclerenchyma and vascular bundles corresponding to them. A waxy cuticle is present on the exterior, which contains natural, water repellent waxes, and is often removed during processing. Observable characteristics of monocot leaves include the epidermis, vascular bundles (which include xylem and phloem cells), sclerenchyma bundles, and occasionally stomata, which are tiny mouths that permit transpiration (Pearlstein, lecture 28 January 2014)! Samples were prepared by cutting a very thin slice from the end of a monocot leaf using a scalpel and carefully placing the sample on a glass microscope slide so that the cross section faced up. Once the sample was in the correct orientation, a drop of water was placed on the area around the sample and a plastic cover slip was placed on top.
Cross-sectional sample of a Monocot Stem Monocot stems are also used in basket weaving and stems are obtained from grasses, sedges, rushes, and palms. Monocot stems do not develop bark on their exteriors and do not exhibit growth rings. Identifying features observable in a cross-sectional sample include the outer cortex, vascular bundles (in which phloem, xylem and cambium may be visible), epidermal cells, and sclerenchyma bundles (Pearlstein, lecture 28 January 2014). Samples were prepared by cutting a very thin slice from the end of a monocot stem using a scalpel and carefully placing the sample on a glass microscope slide, followed by a drop of water and a cover slip.
In addition to taking really beautiful and informative photomicrographs, students gained valuable experience with sampling techniques. The challenges of isolating single fiber ultimates or obtaining good cross sections on the reference materials, illustrated that much experience and care are necessary when taking samples from actual artifacts. Once students were able to see the differences between features, locating them without sampling was possible in some cases!
References Florian, Mary-Lou E., Dale Paul Kronkright, and Ruth E. Norton. The Conservation of Artifacts Made from Plant Materials. Getty Publications, 1991. Pearlstein, Ellen (2014). Other Plant Materials, Lecture in Structure, Properties and Deterioration of Organic Materials, UCLA, Los Angeles, CA, January 28th , 2014.
by Lesley Day (’16)