“Animal, Vegetable, Mineral?” – Identifying mystery fibers in the field

When conservators are working on archaeological excavations, their work often encompasses many different aspects of field conservation.  This can include materials identification and characterization, lifting fragile artifacts and aiding in archaeological research.  No matter what facet of the project they are involved in, the work can be challenging without the comforts of a well-stocked lab and requires lots of problem solving and improvisation.  Last summer while working on the Ancient Methone Archaeological Project, we were faced with the challenge of trying to identify an unusual looking fibrous material which required us to MacGyver a transmitted light microscope to aid in the examination and identification of the mystery fibers.

During the 2014 season, a team of geomorphologists working on the project were taking core samples in an area thought to be an ancient harbor.  In one of the cores, they pulled out a clump of fibers they thought might be cordage (fig. 1).  They brought the samples to the conservation lab to see if we could identify the fibers and determine if it was cordage or the remnants of woven fibers. Some samples were set aside for radiocarbon dating and the remainder of the sample, which was still bound in sediment, was examined.

Figure 1. Clump of fibers found in a core sample taken by the Geomorphology team. Photo: Ancient Methone Archaeological Project

The initial macroscopic examination revealed that the fibers appeared translucent (fig. 2).  They seemed to be grouped into bundles and some of these bundles initially appeared to cross each other, giving the impression of a woven structure.

Fgiure 2. Detail of the fibers encased in the sediment. The fibers are translucent and are grouped in bundles. Photo: Ancient Methone Archaeological Project

The fibrous material was encased in a gray, silty sediment, which appeared to include quartz, foliated phyllosilicates/sheet silicates (like mica, vermiculite, etc.) (fig. 3), as well as small shells, both fragmented and whole (fig. 4). The sample was initially wet, and was allowed to slowly dry out in the lab. The sediment was gently pushed away using a pin-vise under binocular magnification, to better define the structures and reveal diagnostic features of the material for its identification (fig. 5).  Photographs of the fibers were taken using the DinoLite USB microscope (7013MZT Series). During this examination and initial cleaning, the fibers were found to be very brittle.  Though they appeared to be in bundles, they were not actually bound to each other and could be easily separated.

Figure 3. During cleaning, plate-like inclusions were found in the soil and between the fiber bundles. These inclusions resembled sheet silicates like mica. Photo: Ancient Methone Archaeological Project

Figure 4. Small shells or shell fragments were also found in the deposit. Photo: Ancient Methone Archaeological Project

Figure 5. After some initial cleaning to remove soil, more of the fiber bundles and associated materials are visible. Photo: Ancient Methone Archaeological Project

Though examination with a stereomicroscope helped to reveal more about the fibers and the structure of the bundles, we were not able to clearly identify what the fibers were.  We felt that examination using transmitted light microscopy would be the most helpful since it could highlight any morphological features in the fiber that could aid in identification.  So we set out to make one armed with our DinoLite microscope and a flashlight. The set up turned out to be quite simple. We just needed to be able to shine a light through the fibers from below and examine the fibers at a high magnification using the DinoLite (fig. 6). We took a fiber bundle from the sediment and placed it on a multi-bulb LED flashlight (fig. 7).  This flashlight was flat and rectangular and the ideal shape for our light source since the fiber samples could be directly placed on the top surface of the flashlight.  The fact that the flashlight was flat also meant it was easy to position the light source under the microscope where needed (fig. 8).

Figure 6. We created a transmittled light microscope using the DinoLite USB microscope and an LED flashlight which acted as the transmitted light source. Photo: Ancient Methone Archaeological Project

Figure 7. We placed samples of the fibers directly onto the flashlight during examination. Photo: Ancient Methone Archaeological Project

Figure 8. Using our transmitted light microscope to examine the fibers. Macgyver would be proud! Photo: Ancient Methone Archaeological Project

Looking at the fibers in transmitted light, we observed a central void within some of the fibers.  Since we were considering the possibility of the fibers being organic in nature, we thought these central voids could be the medulla or lumen of an organic fiber (fig. 9). However, no other morphological features were present that helped us determine at this point what the fibers were.

Figure 9. Looking at the fibers under transmitted light, we could see they had a central void, which initially made us think this was the lumen of a plant fiber or medulla of an animal fiber. Photo: Ancient Methone Archaeological Project

We were also able to take a look at the cross-section of the fibers with the addition of a polarizing lens on the DinoLite (fig 10). Some fibers appeared hexagonal in section (fig. 11).  Some of the ends of the fibers ended in a point or were triangular in shape.

Figure 9. With the addition of a polarizing lens on the DinoLite we were able to see the cross-sections of some of the fibers. Some appeared hexagonal or triangular in section. Photo: Ancient Methone Archaeological Project

Figure 11. Details of the fibers showing their shape in section. Photo: Ancient Methone Archaeological Project

Further cleaning revealed a tiered growth structure that resembled the growth of minerals more than plant or animal fiber bundles (fig. 12).  The inclusion of sheet silicates in relation to the fibers, either located between bundles or within them further suggested these fibers were mineral.  In searching the literature we came across images of asbestos minerals which looked similar to our mystery fibers.  Several types of asbestos minerals are fibrous in appearance (fig. 13), and can occur near phyllosilicate deposits.  Armed with this information we concluded that the fibers were definitely mineral in nature and could possibly be asbestos.

Figure 12. Fiber bundle after cleaning. Photo: Ancient Methone Archaeological Project

Figure 13. An image of crocidolite, a fibrous form of the mineral riebeckite, and one of the 6 recognized forms of asebstos minerals. © Raimond Spekking / , via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/b/b3/Krokydolith_-_Mineralogisches_Museum_Bonn_%287385%29.jpg

Luckily we were able to bring a sample of the fibers back with us and conduct some analysis in the UCLA/Getty Conservation labs. And we were quite surprised by the results!  It turns out we were correct in deducing the fibers were mineral in nature, but we were incorrect about which mineral. XRF and XRD analysis did not find any asbestos minerals in the sample, but instead the fibers were identified as calcite (fig. 14).  Though we had never seen calcite that was fibrous in appearance, it is one of the mineral’s crystal forms. An example is shown in fig. 15 where you can see the SEM image of “lublinite”, a needle-type of calcite whose form is thought to be associated with the activity of microorganisms.

Figure 14. XRD analysis results showing the fibers were composed of calcite.

Figure 15. SEM image of lublinite, the fibrous form of calcite. Image taken from: http://www.speleonics.com.au/jills/bymineral/lublinite.html

Even though we were not able to identify the fibers as calcite in the field, the use of a stereomicroscope and our makeshift transmitted light microscope certainly helped distinguish their mineral nature and rule out plant or animal origins.  And now that we’ve figured out how to make a transmitted light microscope and tested it out, we’re ready for any future material ID questions that would require one.

MacGyver looks on and smiles at our ingenuity. Photo: http://macgyver.wikia.com/wiki/List_of_problems_solved_by_MacGyver

Written by the 2014 Ancient Methone Archaeological Project Conservation Team: Heather White (UCLA/Getty Program Grad Student, Class of ’16), Vanessa Muros (Conservation Specialist/Lecturer, UCLA/Getty Program) and Anna Weiss (Campus Art/Artifact Collections Coordinator, Conservator, Univ. of Chicago)

Examining Plant Fibers and Identifying Characteristic Features using Microscopy

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.

Image: Lesley Day

Image: William Shelley

Image: Betsy Burr

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.

Image: Lesley Day

Image: William Shelley

Image: Betsy Burr

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.

Image: Lesley Day

Image: William Shelley

Image: Betsy Burr

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.

Image: Lesley Day

Image: Lesley Day

Image: William Shelley

Image: Betsy Burr

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)