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Naturally Dyed Wool and Silk and Their Atomic C:N Ratio for Quality Control of 14C Sample Treatment

Published online by Cambridge University Press:  12 January 2016

Mathieu Boudin ,
Marco Bonafini ,
Ina Vanden Berghe  and
Marie-Christine Maquoi
Mathieu Boudin*
Affiliation:
Royal Institute for Cultural Heritage, Brussels, Belgium
Marco Bonafini
Affiliation:
Royal Institute for Cultural Heritage, Brussels, Belgium
Ina Vanden Berghe
Affiliation:
Royal Institute for Cultural Heritage, Brussels, Belgium
Marie-Christine Maquoi
Affiliation:
Royal Institute for Cultural Heritage, Brussels, Belgium
*
*Corresponding author. Email: mathieu.boudin@kikirpa.be.
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Abstract

Quality control of sample material (e.g. charcoal, collagen) is receiving considerable attention in the effort to obtain more reliable 14C dates. The atomic carbon to nitrogen (C:N) ratio is a useful indicator of contamination and/or degradation of bone collagen. Wool and silk are also composed of proteinaceous material such as bone collagen, and the C:N ratio may also be a useful quality indicator for archaeological wool and silk. Analyses of modern undyed, mordanted, non-mordanted, and naturally dyed silk and wool were done in order to determine a C:N range that indicates the sample quality. The C:N range can be different for every material as the amino acid composition of wool, silk, and bone collagen are distinct. The measured minimum and maximum C:N values were used to set up a C:N range of uncontamined and undegraded wool and silk. Then, the C:N ratio and 14C were analyzed of archaeological wool and silk samples. The applicability of the C:N ratio as a quality indicator for archaeological silk and wool was shown by the good agreement of the 14C dates with the presumed historical dates for the uncontaminated samples and the disagreement of the 14C dates with the presumed historical dates for contaminated samples.

Keywords

C:N ratiowoolsilkquality control

Information

Type
Research Article
Information
Radiocarbon , Volume 58 , Issue 1 , March 2016 , pp. 55 - 68
Copyright
© 2016 by the Arizona Board of Regents on behalf of the University of Arizona 

INTRODUCTION

Wool and silk, and textiles in general, are gaining more attention as suitable radiocarbon dating material due to their short lifespan, potentially presenting the true age of an object made of these materials (Rageth Reference Rageth2004; Van Strydonck et al. Reference Van Strydonck, De Moor and Bénazeth2004; van der Plicht et al. Reference van der Plicht, van der Sanden, Aerts and Streurma2004; Kim et al. Reference Kim, Southon, Imamura and Sparks2008; Mannering et al. Reference Mannering, Possnert, Heinemeier and Gleba2010; Kuzmin et al. Reference Kuzmin, Keally, Jull, Burr and Klyuev2012; Van Strydonck and Grömer Reference Van Strydonck and Grömer2013; Vedeler and Bender Jørgensen Reference Vedeler and Bender Jørgensen2013; Hajdas et al. Reference Hajdas, Cristi, Bonani and Maurer2014).

The quality control of sample material (e.g. charcoal, collagen) is receiving considerable attention in order to obtain more reliable 14C dates (DeNiro Reference DeNiro1985; Alon et al. Reference Alon, Mintz, Cohen, Weiner and Boaretto2002; Van Strydonck et al. Reference Van Strydonck, Boudin and Ervynck2005; Boudin et al. Reference Boudin, Boeckx, Vandenabeele, Mitschke and Van Strydonck2011). Boudin et al. (Reference Boudin, Boeckx, Vandenabeele, Mitschke and Van Strydonck2011) demonstrated that humic substances are a major contaminant in archaeological wool and silk samples by nondestructive fluorescence spectroscopy analyses. However, degraded wool and silk may contain other contaminants such as mold, fungus, dirt, or other carbon-containing materials (Kim et al. Reference Kim, Southon, Imamura and Sparks2008). Therefore, the atomic carbon to nitrogen (C:N) ratio could serve as a quality control indicator of archaeological wool and silk as it does for bone collagen to indicate contamination and/or degradation (DeNiro Reference DeNiro1985; Schoeninger et al. Reference Schoeninger, Moore, Murray and Kingston1989; Ambrose Reference Ambrose1990). The C:N range can be different for every protein-containing material as the amino acid composition of wool, silk, and collagen are distinct. Wool consists mainly of the protein keratin, which is a α-helix structure with cystine, leucine, glutamic acid, arginine, and serine as the most abundant amino acids (Sibley and Jakes Reference Sibley and Jakes1984). However, wool proteins are composed mainly of 21 amino acids, with variations in amino acid content due to breed of sheep, diet, climate, and other external influences (Gillespie et al. Reference Gillespie, Broad and Reis1969; Maclaren and Milligan Reference Maclaren and Milligan1981; Leeder and Marshall Reference Leeder and Marshall1982; Jones et al. Reference Jones, Rivett and Tucker1998). Silk protein (fibroin) is a polypeptide polymer comprising 15 amino acids; the largest percentage (80%) in the fiber consists of the amino acids glycine, alanine, and serine in an approximate 3:2:1 ratio. These substances are small in size, with no large side groups. Therefore, the polypeptide chains can pack together closely as β-sheets (Sibley and Jakes Reference Sibley and Jakes1984; Becker et al. Reference Becker, Magoshi, Sakai and Tuross1997). For archaeological human and animal hair isotopic analysis, the C:N ratio can be used to indicate if the hair keratin is contaminated or not. Analysis of modern human hair defines a C:N range for uncontaminated archaeological hair between 2.9 and 3.8 (O’Connell and Hedges Reference O’Connell and Hedges1999a, Reference O’Connell and Hedges1999b; O’Connell et al. Reference O’Connell, Hedges, Healey and Simpson2001). Taylor et al. (Reference Taylor, Hare, Prior, Kirner, Wan and Burky1995) conducted 14C, C:N, and amino acid composition analyses on hair. The glycine/glutamine and glycine/aspartic acid ratios indicate well-preserved chemical structures of the hair. The C:N ratios of the archaeological hair fall within the range of 2.9 and 3.8, determined by modern human hair analyses. The 14C dates of the hair were also in agreement with the archaeological expectations. This suggests that the C:N ratio may be a useful indicator for quality 14C control of human hair and undyed wool (animal hair). However, mordanting and/or dyeing may alter the C:N ratio. Boudin et al. (Reference Boudin, Boeckx, Vandenabeele and Van Strydonck2013) proposed a C:N range for modern undyed, mordanted, non-mordanted, and naturally dyed silk (all Bombyx mori). Based on their analyses of a small data set, the value was found be between 2.9 and 3.4.

Dyeing is an ancient art, which predates written records. It was practiced during the Bronze Age in Europe. Primitive dyeing techniques included sticking plants to fabric or rubbing crushed pigments into cloth. The methods became more sophisticated with time and techniques were developed using natural dyes from crushed fruits, berries, and other plants, which were boiled into the fabric and gave light and water fastness (resistance). Until the mid-19th century all dyestuffs were made from natural material, mainly vegetable and animal matter. Synthetic organic dyes were introduced in the mid-19th century, with mauveïne as the first synthetic organic dye produced in AD 1856 (Herbst and Hunger Reference Herbst and Hunger1997).

Few natural dyes are color-fast with fibers. Mordants are substances used to fix a dye to the fibers (IUPAC 2006). Different types of mordants are available, including metals in the form of salts (chrome, cupper, tin, iron, aluminium), tannins, cream of tartar, baking soda, and vinegar. Different mordants will give different color with the same dye (Aspland Reference Aspland1997).

The classification of natural dyestuffs can be determined on the basis of the following:

  • ∙ Chemical constitution of the dyestuff molecule. This can be based on the chromophore structure, as with anthraquinones (e.g. madder) or flavonoids (e.g. weld) or the derivatives of one dye (Dedhia Reference Dedhia1998; Hofenk de Graaff 2004).

  • ∙ The application method of the dyestuff. In the case of natural dyestuffs, only three groups are of importance (Gulrajani and Gupta Reference Gulrajani and Gupta1992; Hofenk de Graaff 2004):

  1. 1. Dyestuffs that are soluble in water but form metal complexes on the fiber. They can only be applied with the help of water-soluble metal salts (mordants) and are therefore called mordant dyes.

  2. 2. Dyestuffs that are insoluble in water and therefore have to be first converted to their water-soluble form before application to the fibers. These are vat dyestuffs.

  3. 3. Dyestuffs that dissolve in water and can be applied as such. These are direct dyestuffs.

Mordanting and dyeing of wool and silk may alter the C:N ratio because most dyes and some mordants contain carbon and/or nitrogen atoms in their molecular structure, which are added to the textile fiber or fabric. Therefore, in this study, the C:N ratio of modern undyed, mordanted, non-mordanted, and naturally dyed silk (Bombyx mori) and wool were measured to determine a C:N range of uncontaminated and undegraded wool and silk. The viability of the established C:N range as a quality indicator for archaeological silk and wool was tested by comparing 14C analysis of archaeological wool and silk with the presumed historical dates.

MATERIALS AND METHODS

Sample Selection

Different modern wool and silk fibers were dyed in the Textile Laboratory of the Royal Institute of Cultural Heritage (RICH), Brussels, Belgium. Some of the most frequently applied dyestuffs as listed in Table 1 (kermes, cochineal, madder, brazilwood, red sandalwood, Persian berries, barberry root, safflower, turmeric, dyer’s broom, woad, indigo, orchil, galls) for wool and silk were used, but also less common dyestuffs were applied in this study as listed in Table 2.

Table 1

The most frequently used natural dyestuffs for wool and silk, the timescale of practice, and its practice location (Hofenk de Graaff 2004).

Table 2

Name (species) of dye source and its dye color of uncommon natural dyes used in this study.

Archaeological silk and wool samples from different geographical origins and with different color were selected to study (see Tables 4 and 5).

Design

First, C:N analyses of modern undyed, mordanted, non-mordanted, and naturally dyed silk and wool were conducted in order to determine their C:N range, by using the minimum and maximum value, for uncontamined and undegraded wool and silk. This study design is based on the papers of DeNiro (Reference DeNiro1985) and O’Connell and Hedges (Reference O’Connell and Hedges1999a) wherein modern bone collagen and hair were analyzed to define a C:N range for uncontaminated bone collagen and hair.

Secondly, the C:N ratio, dye, and 14C analyses were conducted of archaeological wool and silk samples. The viability of the C:N ratio as a quality indicator for archaeological silk and wool was tested by comparing the 14C dates with the presumed historical dates. If the C:N ratio of the bulk did not fall within the proposed C:N range in this study and indicated contamination, cross-flow nanofiltration on the bulk samples was performed to remove contaminants and improve the sample quality by observing a C:N decrease (bulk samples that underwent cross-flow nanofiltration are referred to as permeate). The efficiency of cross-flow nanofiltration to obtain better sample quality and the used protocol are described in Boudin et al. (Reference Boudin, Boeckx, Vandenabeele and Van Strydonck2013, Reference Boudin, Boeckx, Vandenabeele and Van Strydonck2014).

METHODS

Sample Pretreatment

Preliminary tests with the standard pretreatment protocol for archaeological wool and silk were performed on modern wool and silk. No C:N alteration of modern wool and silk was observed. Therefore, the modern undyed and dyed silk and wool samples were not pretreated prior to analysis. The archaeological wool and silk samples (also referred to as bulk samples) were pretreated with hexane, acetone, ethanol, Milli-Q™ water (Merck Millipore, Belgium), 1% NaOH, and 1% HCl for bulk 14C and C:N analyses as described in detail in Boudin et al. (Reference Boudin, Boeckx, Vandenabeele, Mitschke and Van Strydonck2011).

Atomic C:N Ratio

Analyses were performed in duplicate on a ThermoFlash EA/HT elemental analyzer. The standard used was acetanilide (0.5–1 mg was used).

14C Dating

14C dates were measured on the AMS at the Leibniz Labor für Altersbestimmung und Isotopenforschung in Kiel, Germany (lab code KIA) (Nadeau et al. Reference Nadeau, Grootes, Schliecher, Hasselberg, Rieck and Bitterling1998) or at the Royal Institute for Cultural Heritage, Brussels (lab code RICH) (Boudin et al. Reference Boudin, Van Strydonck, van den Brande, Synal and Wacker2015). CO2 was obtained by sample combustion in the presence of CuO and Ag. Graphitization was done with H2 over a Fe catalyst. Targets were prepared at the Royal Institute for Cultural Heritage in Brussels (Van Strydonck and Van der Borg 1990–Reference Van Strydonck and Van der Borg1991). 14C calibrations were performed using OxCal 3 (Bronk Ramsey Reference Bronk Ramsey1995, Reference Bronk Ramsey2001) and IntCal13 calibration curve data (Reimer et al. Reference Reimer, Bard, Bayliss, Beck, Blackwell, Bronk Ramsey, Buck, Cheng, Edwards, Friedrich, Grootes, Guilderson, Haflidason, Hajdas, Hatté, Heaton, Hoffmann, Hogg, Hughen, Kaiser, Kromer, Manning, Niu, Reimer, Richards, Scott, Southon, Staff, Turney and van der Plicht2013).

Dye Analysis

Each dye analysis requires a sample between 0.2 and 0.5 mg. After pre-examination with a binocular microscope to avoid visible contamination, the dyes were extracted from the yarn using the following extraction method. To the sample of yarn, 250 μL water/methanol/37% hydrochloric acid (1/1/2, v/v/v) is added. The mixture is heated for 10 min at 105°C in open Pyrex™ tubes in a heating block. After cooling, it is filtered through a porous polyethylene frit. The filtrate is dried in an evacuated desiccator over NaOH pellets. The dry residues are taken up in 50 mL of methanol/water (1/1, v/v) and 20 mL of this solution is injected for analysis. The high-performance liquid chromatography (HPLC) equipment consists of a high-pressure pump (Model M615, Waters, USA), a photodiode array detector (Model 996, Waters, USA), and a system for data storage, manipulation, and retrieval (Empower, Waters, USA). Chromatographic conditions were applied using a temperature-controlled (20–22°C) Lichrosorb RP-18 column (4.0×125 mm, 5 mm particle size, VWR, Belgium). Three solvents are used: water, methanol, and 5% (w/v) phosphoric acid in water. The elution program is 60A/30B/10C for 3 min, followed by a linear gradient to 10A/80B/10C for 26 min. A flow rate of 1.2 mL/min was used.

Identification of the dye components was done by comparison of the spectral data with the reference spectra in the RICH database (130 references) at the maximum absorbance wavelength of each peak (Wouters Reference Wouters1985; Van den Berghe et al. Reference Van den Berghe, Gleba and Mannering2009).

RESULTS AND DISCUSSION

C:N Analyses of Modern Wool and Silk

Comparing the C:N ratios of untreated textiles with mordanted and/or dyed textiles indicates that mordanting and/or dyeing may slightly change the C:N ratio (see Table 3). However, almost all mordanted and/or dyed textiles fall into the C:N ranges (see Table 3):

  1. 1. For hair/wool, proposed by O’Connell and colleagues (O’Connell and Hedges Reference O’Connell and Hedges1999a, Reference O’Connell and Hedges1999b; O’Connell et al. Reference O’Connell, Hedges, Healey and Simpson2001): between 2.9 and 3.8;

  2. 2. For silk, proposed by Boudin et al. (Reference Boudin, Boeckx, Vandenabeele and Van Strydonck2013): between 2.9 and 3.4.

Table 3

Atomic C:N ratio of undyed/dyed/mordanted modern wool and silk. The typical values are 3.0–3.2 for silk and 3.4–3.7 for wool.

This means that the C:N ratio may be a good indicator to control the sample quality of naturally dyed (until AD 1856) archaeological wool and silk for 14C dating. The minimum value for wool in this study is 3.4 and is in perfect agreement with the theoretical C:N value of keratine. This allows us to refine the C:N range proposed by O’Connell and colleagues (O’Connell and Hedges Reference O’Connell and Hedges1999a, Reference O’Connell and Hedges1999b; O’Connell et al. Reference O’Connell, Hedges, Healey and Simpson2001). The C:N range for uncontaminated wool should be between 3.4 and 3.8. The C:N range proposed by Boudin et al. (Reference Boudin, Boeckx, Vandenabeele and Van Strydonck2013) for silk is valid for dyed and/or mordanted wool or silk, based on this data set, and can thus serve as an indication if a sample is contaminated or not.

The only exceptions (marked bold in Table 3) are the textiles dyed with Miscanthus tinctorius, Sophora japonica, and gallic acid. Miscanthus tinctorius and Sophora japonica were only used in Eastern Asian textiles (Bechtold and Mussak Reference Bechtold and Mussak2009). Miscanthus tinctorius ranges in color from yellow to light brown depending on the mordant, while Sophora japonica is yellow to olive green depending on the mordant. Gallic acid was used globally. Although gallic acid was often applied as a mordant, it can also be used as a dye, but very rarely (like in this study). When applied as a dye, it is more concentrated and consequently increases the C:N more than when used as a mordant. The obtained color of the dye gallic acid varies from yellowish brown to dark brown depending on the mordant.

C:N Ratio, Dye, and 14C Analyses of Archaeological Wool and Silk

Table 4 lists all the archaeological wool and silk samples from which the C:N ratio falls within the proposed C:N range of 3.4–3.8 for wool and 2.9–3.4 for silk, indicating uncontaminated samples. The 14C dates of all the samples are in perfect agreement with the presumed historical date, suggesting the samples are not contaminated.

Table 4

Archaeological site, sample color (used dye shown in parentheses if dye analysis was conducted), laboratory code, 14C ages (BP), calibrated ages (2σ), presumed historical date, and atomic C:N ratio of analyzed archaeological uncontaminated wool samples.

The sample color was visually determined prior to analysis and dye analyses were conducted on some samples. The detected dyes are shown in parentheses next to the determined sample color in Table 4. The analyzed archaeological yellowish and brown silk and wool samples from Egypt, Austria, south Iran, Byzantium, and Central Asia have a C:N ratio lower than the upper boundary, respectively, 3.8 for wool and 3.4 for silk. This excludes exogenous contamination or the use of gallic acid as a dye (see Table 4).

The C:N ratios of the bulk samples (wool and silk) fall outside the proposed C:N ranges, indicating contamination (see Table 5). Moreover, the 14C ages of samples with untypical C:N ratios do not show a good agreement with the presumed historical date. Cross-flow nanofiltration decreased the C:N ratio and thus improved the sample quality. The obtained C:N ratio of the permeates falls within the proposed C:N ranges of uncontaminated wool and silk, indicating the absence of contaminants in the permeates. The 14C dates of the cross-flow nanofiltrated wool samples (permeate) are in perfect agreement with the presumed historical dates, suggesting uncontaminated wool samples after cross-flow nanofiltration. The 14C dates of the permeate of the silk samples (Baldwin 46 and Baldwin 43) are not in agreement with the presumed historical date, but this disagreement is probably due to the reuse of older (parts of) textiles as suggested by Van Strydonck and Bénazeth (Reference Van Strydonck2014).

Table 5

Archaeological site, sample color, laboratory code, 14C ages (BP), calibrated ages (2σ), presumed historical date, and atomic C:N ratio of analyzed archaeological contaminated wool and silk samples before (bulk) and after (permeate) cross-flow nanofiltration.

The brown color of the contaminated samples Beerlegem, Baldwin 46, and Baldwin 43 are likely due to the presence of humic substances, as shown by nondestructive spectrofluorescence analyses in Boudin et al. (Reference Boudin, Boeckx, Vandenabeele, Mitschke and Van Strydonck2011, Reference Boudin, Boeckx, Vandenabeele and Van Strydonck2014), and also demonstrated by the decrease of C:N ratio of the permeate after cross-flow nanofiltration as well as the good agreement of the 14C date with the presumed historical date.

CONCLUSION

The C:N ratio is a good indicator for sample quality (contamination) of naturally dyed (until AD 1856) archaeological wool and silk for 14C dating. The C:N range indicating good (uncontaminated) sample quality is between 2.9 and 3.4 for silk and between 3.4 and 3.8 for wool. If the C:N ratio of the archaeological sample is greater than the upper boundary of the C:N range of wool and silk, respectively, 3.8 and 3.4, then the following may be true:

  1. 1. The sample may be dyed with Miscanthus tinctorius or with Sophora japonica in the case of Eastern Asian textiles or with gallic acid (used worldwide). A dye analysis can reveal if one of these three dyes was applied on the textile. If the dye analysis is positive, the sample is not contaminated and can undergo 14C analysis.

  2. 2. However, if the dye analysis is negative (none of the aforementioned dyes detected), the sample is contaminated and should be rejected for 14C dating, with the too-high C:N ratio indicating contamination. An option to avoid dye analysis is taking samples for 14C dating with a different color than the color obtained by applying these three dyes.

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Figure 0

Table 1 The most frequently used natural dyestuffs for wool and silk, the timescale of practice, and its practice location (Hofenk de Graaff 2004).

Figure 1

Table 2 Name (species) of dye source and its dye color of uncommon natural dyes used in this study.

Figure 2

Table 3 Atomic C:N ratio of undyed/dyed/mordanted modern wool and silk. The typical values are 3.0–3.2 for silk and 3.4–3.7 for wool.

Figure 3

Table 4 Archaeological site, sample color (used dye shown in parentheses if dye analysis was conducted), laboratory code, 14C ages (BP), calibrated ages (2σ), presumed historical date, and atomic C:N ratio of analyzed archaeological uncontaminated wool samples.

Figure 4

Table 5 Archaeological site, sample color, laboratory code, 14C ages (BP), calibrated ages (2σ), presumed historical date, and atomic C:N ratio of analyzed archaeological contaminated wool and silk samples before (bulk) and after (permeate) cross-flow nanofiltration.