Investigation of Fiber Mineralization Using Fourier Transform Infrared

Chapter 14

Investigation of Fiber Mineralization Using Fourier Transform Infrared Microscopy 1

R. D. Gillard and S. M. Hardman

Downloaded by UNIV OF GUELPH LIBRARY on July 1, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0625.ch014

Department of Chemistry, University of Wales, Cardiff, P.O. Box 912, Cardiff CF1 3TB, United Kingdom The mineralization of cellulose and protein fibers has been simulated in the laboratory using oxygenated aqueous solutions. The mechanism peculiar to a given solution has been shown to depend on the initial metal-ligand bonds formed and on kinetic factors relating to the mineral product formation. These experiments rationalize both the occurrence and the comparative rarity of mineralized organic fibers from archaeological deposits. FTIR microscopy has revealed that traces of organic component can survive long-term burial and, in appropriate circumstances, permit their identification even in highly mineralized samples. Remnant dye on excavated textile has also been identified using FTIR microscopy on a scale far below that which is possible by extractive techniques. A spectral data library offibers,dyes and mordants has been built into the computer analysis program. The mechanisms offiberdecay and mineralization are poorly understood. The relative rarity of mineralized material limits extensive analysis. We have successfully reproduced mineralization in the laboratory for copper and copper alloys. Samples from these experiments have been analyzed and the results compared with those on archaeological material. FTIR microscopy was used extensively for this work. This paper presents results and discusses implications. Background Well-preserved textiles are rare except in certain environments, for instance peat bogs (anaerobic waterlogging), desert (virtual desiccation) and permafrost (extreme cold) (7, 2). Much information about textiles is obtained indirectly from sources such as tools of manufacture, impressions on pottery, paintings, drawings, ancient texts and mineralized fibers. 1

Current address: School of History and Archaeology, University of Wales, Cardiff, P.O. Box 909, Cardiff CF1 3XU, United Kingdom 0097-6156/%/0625-0173$12.00A) © 1996 American Chemical Society

In Archaeological Chemistry; Orna, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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Definition. Mineralization is the combination and/or replacement of the organic matrix with an inorganic matrix. Mineralized textile fragments are normally small and fragile, generally occurring where the textile is in direct contact with a metal artifact, e.g., coins, knives or cloak pins. They are most frequently found with copper alloy and iron objects. Lead and silver corrosion products have also been reported to participate in the mineralization process (3-5). Early silver artifacts, however, often contain considerable quantities of copper. Where mineralization is observed on such objects, copper may instigate mineralization. Calcium carbonate has also been reported to preserve fiber information (6). Types of Mineralization. Mineralized information occurs in two forms. In positive casts (which generally provide more information) metal ions penetrate the fiber and coordinate with the organic matrix. This mechanism provides sites for nucleation allowing further metal ions to "lock on". The corrosion products then gradually replace the fiber as it decays giving a positive three-dimensional fiber impression composed primarily, if not totally, of metal corrosion products. Such casts can provide information on the fiber type, yarn (e.g., the direction of spinning and evidence of plying) and weave pattern. Negative casts form when corrosion products deposit on the surface of the fiber which then decays, leaving behind a negative impression in the corrosion products. The extent of mineralization can vary considerably even within the same piece of textile. Variables involved include: (1) The different chemical and physical properties of protein and cellulosefibers;(2) The type of metals/alloys present, their respective corrosion mechanisms and the corrosion products produced; (3) The possible use of dyes, mordants and metal-wrapped yarns; (4) The burial conditions, e.g., soil type, redox potential (Eh), pH and microbial activity. "Mineralization" Versus "Fossilization". In the past, the term "fossilization" has been wrongly interchanged with "mineralization" to describe textile remains on metal artifacts (7). "Fossilization" a specific geological term, refers to organic material preserved in rock strata. In contrast "mineralization" is a much more general term encompassing all preservation of organic information by an inorganic mineral. FTIR Microscopy Infrared analysis was performed using a Nicolet 510 Fourier transform infrared (FTIR) spectrometer in conjunction with a 620 processor and Spectra-Tech FTIR research microscope. FTIR microscopy enables spectra to be obtainedfromsamples as small as ΙΟμπι χ ΙΟμπι (i.e., a single crystal or fiber). Analysis can be performed using either transmitted or (if necessary) reflected radiation. Transmission microscopy is preferred whenever possible because the energy throughput is greater. By reflection from its surface, non-destructive analysis can be performed on an artifact when samples cannot be taken.


14.

GILLARD & HARDMAN

Fiber Mineralization and FTIR Microscopy

Sample preparation. All results herein were all collected in transmission mode. Each sample was run for 250 scans at a resolution of 4 cm . Fibers were selected and prepared using a low power optical microscope (10X - 30X magnification). Using a clean scalpel and dissecting pin, a small sample was removed and placed on a microscope slide. To prevent diffraction of the IR beam, the sample was slightly flattened using a small roller directly applied to its surface, then placed on a small (13mm χ 2mm) NaCl plate and transferred to the FTIR microscope stage. After focusing the fiber at 150X magnification, shutters above and below the field of the sample stage were used to select visually the area for analysis (generally an area of 40μιη χ ΙΟΟμιη was chosen for fibers). The shutters blocked off the IR beam to the rest of the sample and ensure that the spectrum produced is only that of the area of interest. This technique ("Redundant Aperturing and Targeting") also makes possible analysis of the least contaminated area of the sample.


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Textile Chemistry In order to understandfibermineralization, the chemistry of thefibersthemselves must be considered. Protein and cellulose fibers consist of crystalline and amorphous regions. The degree of crystallinity is important to thefiberssurvival: crystalline regions resist penetration and attack owing to their strong intermolecular bonds and tighter packing. This resistance reduces chemical accessibility and the number of reactive sites available. Cellulose Fibers. The building block of cellulose comprises two anhydroglucose units joined via an ether link to form cellobiose (Figure 1). In the burial environment, cellulosefibersare sensitive to acidic conditions: hydrolysis occurs and the ether links are broken.

Figure 1. Cellulose consists of repeating units of cellobiose. Degree of polymerization = 2n+2. Protein fibers. Proteinfibersare much more complex than cellulosefibers.Each type is built up of a characteristic sequence of amino acids joined together by peptide linkages. The general structure is shown in Figure 2 where R R and R define the amino acid radicals: -H for glycine, -CH for alanine, - C H C H for phenylalanine, etc.. l 5

3

2

2


3

6

5

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aCO.NH^HCO.NH

Investigation of Fiber Mineralization Using Fourier Transform Infrared

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