Radiocarbon dating, also referred to as carbon dating or carbon-14 dating (C14), is a method that can calculate the age of materials of plant or animal origin, by measuring the amount of the radioactive isotope of carbon (C), the 14C isotope. An isotope is one of two or more species of atoms of a chemical element with the same atomic number but different atomic masses, the quantity of matter contained in an atom of an element [1]. Each atom has a nucleus, containing protons and neutrons, and a number of electrons orbiting around it. Since the matter of electrons is extremely small, the mass of an atom is based on the matter within the nucleus, i.e. on the number of protons and neutrons present. Isotopes of an element have the same number of protons but different numbers of neutrons [2]. Carbon has three isotopes, they all have 6 protons, but each isotope has 6 (12C), 7 (13C) or 8 (14C) neutrons. The heaviest carbon isotope, 14C is constantly created in the Earth’s atmosphere, and thus replenished in living organisms, like plants by photosynthesis, and animals through the food chain [3]. Once a living organism stops growing (plants or hair in animals or silk filaments produced) or an animal dies, replenishment of 14C stops. Since 14C decays at a constant rate, calculations can be made to estimate when replenishment stopped, by measuring the residual amount of 14C left in the organic matter, thus enabling dating of an excavated find [3]. However, this date gives the time of harvesting the plant fibres or collecting the animal fibres or filaments. The fibers are spun and woven into cloth, which could be used for decades or generations before the textile was deposited in a grave or discarded. The C14 date of textiles therefore reflects the time of production and less the time of consumption or final use. C14 method was first developed for dating organic compounds in mid 1940s by the physical chemist Willard Frank Libby at the University of Chicago [4].
Radiocarbon (hereafter C14) dating has become an important dating method in cultural heritage science, especially for authentication and forgery detection. The wide-spread use of the method is mainly due to the ubiquity of the organic materials found in the artworks, e.g. wood, canvas, leather, vegetal fibers, etc. One of the most critical issues in C14 dating is the sampling process, the sample size and the effect of various contaminants. Therefore, for a reliable age determination one has to assure the necessary sample mass, to avoid unintentionally contamination, as well as to remove the possible preservative substances from the sample. At the end of the chemical pretreatment, the samples are dried and inspected under the microscope for purity check. Afterwards, they are graphitised using the fully automated graphitization system AGEIII [5] and pressed into the AMS cathodes. The samples are measured in the AMS spectrometer MICADAS, normalising them to the Oxalic Acid II [6] age standard, and subtracting the 14C blank levels using an unknown old charcoal. The raw data is reduced to radiocarbon ages [7] and further calibrated on IntCal13 [8].
Since archaeological and heritage textiles are generally made of organic material, like plant and animal fibres, they have potential for C14 dating, and have in the past been subjected to this analysis. However, there are many factors that may affect C14 results, such as the condition of the textile, the sample pretreatment and the interpretation of the results. A widely known example of C14 dating of a textile is that of the Turin shroud assumed to have belonged to Jesus of Nazareth, where analyses gave very different results to those expected, placing it in the Medieval period [9, 10]. The potential of textiles as suitable C14 samples has been disputed in the past, when C14 dating results of certain Coptic textiles in the Louvre collection, which took place in the 1950s, gave contradictory results to the stylistic dating of the finds. This C14 dating was revisited many years later to identify the mistakes in the past analysis and interpretation, and the results were still in contradiction to the relevant dating [11]. In another case, C14 dating of stylistically dated textiles, a Tibetan painting and an Islamic kaftan, gave C14 date estimates in support of the stylistic dates, indicating that textiles can be suitable for C14 dating [12]. This hypothesis was further corroborated by the C14 of numerous textile and skin samples from Danish bogs that actually showed many Late Bronze Age finds were down dated to Danish Early Iron Age, thus providing a new tool for the interpretation of textile and skin technology [13]. Very importantly, C14 dating of the textiles from mummy bundles of the Chancay Culture in Peru have shed light on the chronology of the Chancay and Huaura valleys that even lacked pottery seriation [14]. Similarly, in the case of Pre-Columbian and Egyptian textiles that more often than not lack provenance information, C14 dating was able to authenticate two pre-Columbian ponchos from the Quai Branly Museum (Paris) [15], to discover a mummy was kept in the wrong coffin at the Nicholson Museum (Sydney) [16], and to uncover the incorrect association of two Coptic textiles of the Museum of Modern Greek Culture (Athens) [17]. In addition, C14 dating placed two contemporary Iron Age textile finds from different areas in Greece (Lefkandi, Euboea and Stamna, Aitolia), more than 100 years earlier than relative dating, raising issues like the use of heirloom textiles in certain burials or disputing conventional chronologies [18, 19, 20].
Sample pre-treatment seems to be critical to the successful application of the technique. For example the Acid-Base-Acid (ABA) pre-treatment of textile samples that was developed to remove contaminants from the soil or humic acids in the case of excavated finds, was in the past shown to be detrimental for poorly preserved textiles [21]. However, in the case of traditional silk and cotton textiles from the Ryukyu Islands of Japan, acetone-ABA pre-treatment gave dates consistent to the historical record, indicating that this method of sample treatment is suitable at least for better preserved fibres [22]. Repeated analysis and advanced pre-treatment with hexane, acetone and ethanol solvents was necessary for dating excavated textiles that had previously been treated with adhesives [17]. Consequently, C14 can provide crucial and particularly interesting information to textile studies, and the more applications made on textiles, the better for the efficiency and reliability of the technique to adapt and improve.
Archaeological textiles have undergone burial, which is a highly destructive process for their organic matter, as it is an environment ideal for micro-organisms’ growth that feed off the organic material [23]. There are however certain conditions that prevent microbial growth, thus textile preservation is achieved. Four such conditions generally responsible for the preservation of textiles excavated in Greece are included in this paper: 1. the presence and gradual impregnation of the organic matter by metal salts, created by degrading metal objects in their proximity (mineralisation) [e.g. 24, 25]; 2. extremely low and constant moisture levels (dessication) [e.g. 26, 27]; 3. the establishment of a micro-climate mainly characterised by a low oxygen content, and usually encountered in sealed tombs in Medieval churches [27]; and 4. incomplete burning, that chemically alters the fibres to a degree that they are no longer an appropriate food source for micro-organisms (carbonisation) [e.g. 28, 29, 30].
C14 dating is based on measurements of residual 14C within the organic material analysed. The purpose of this paper is to present and discuss how C14 results might be affected by the condition of the textiles, when that condition has disturbed the C content of the material analysed or when provenance information did not exist to corroborate C14 results. The C of excavated textiles might also be affected at some point after excavation and during conservation treatment as their condition is often so poor that it necessitates the use of synthetic adhesives or consolidants to retain their structure [31]. To serve our purpose, six case studies of archaeological textiles that had undergone burial were selected and dated with the C14 method, namely two mineralised textile finds, two from inhumation burials (one preserved in an extremely dry environment, while the other in limited oxygen), and two carbonised, while one of the mineralised and one of the inhumation burials had received treatment with synthetic adhesives in the past (Table 1). Mineralisation is the gradual impregnation and replacement of the organic matter of fibres by metal salts, the degradation products of the metal artifacts in the surrounding environment. Factors like moisture content and water movement in the burial and the proximity of the textile to the metal, play a crucial role to the amount of metal salts deposited in the fibres, as well as when this process will stop, and consequently affect the degree of mineralisation [24]. Cellulosic fibres (like the mineralised ones selected in this study) are polymers made up of cellulose chains (of C, oxygen (O) and hydrogen (H) atoms), lying parallel to each other in crystalline regions (more robust) or tangled in amorphous regions (more prone to deterioration) [32]. A recent study showed that mineralisation primarily deteriorates the amorphous regions of cellulose but also affects the crystalline regions at a macromollecular level and even causes loss of crystallinity, and the more the fibres are mineralised the less organic compound (i.e., the matter containing the C) is preserved [32]. In inhumation burials, regardless of the conditions prevailing, both cellulosic and proteinaceous fibres undergo natural ageing, which is mainly manifested as depolymerisation or chain scission of the cellulose or fibroin (in silk) chains, not altering however the organic nature of the material [32]. In carbonisation, the increased temperatures to which the organic matter is exposed, transform the major constituents like cellulose, hemicellulose and lignin (in the case of plant fibres) into aromatic compounds or inert graphite-like carbon [e.g. 28, 29, 30]. Two of the case studies included in this study had been treated with synthetic adhesives in the past: one was consolidated with polyvinyl acetate (PVA) (no. 2 in Table 1) [34], while the other was treated with PVA as an adhesive to secure the fabric support added to the object (no. 3 in Table 1) [35]. PVA is a thermoplastic resin of polymerised vinyl acetate that contains carbon in its monomer unit [e.g. 36, 37].
Table 1
No. | Reference Name | Condition | Type of fibre | Relevant Dating | Lab Code | Pre-treatment | C14 Results |
1 | Odos Thevon (Nikaia) | mineralised in storage | cellulosic bast | Classical period Greece (490 − 338 BC) | RICH-27497 RICH-27498 | ABA | RICH-27497: 2427 ± 24BP 68.2% probability 540BC (68.2%) 410BC 95.4% probability 750BC (15.9%) 680BC 670BC ( 4.4%) 640BC 550BC (75.1%) 400BC RICH-27498: 2430 ± 24BP 68.2% probability 540BC (68.2%) 410BC 95.4% probability 750BC (18.0%) 680BC 670BC ( 5.3%) 640BC 560BC (72.1%) 400BC |
2 | Eleusis | mineralised adhesives on permanent display | cellulosic bast | Middle Classical period Greece (ca. 450 BC) | RICH-27499 RICH-27500 | hexane-acetone-ethanol -ABA | RICH-27499: 2445 ± 26BP 68.2% probability 740BC (20.9%) 680BC 670BC ( 6.2%) 640BC 550BC (41.1%) 420BC 95.4% probability 760BC (26.3%) 680BC 670BC (11.2%) 610BC 600BC (57.9%) 400BC RICH-27500: 2430 ± 26BP 68.2% probability 710BC ( 2.3%) 690BC 540BC (65.9%) 410BC 95.4% probability 750BC (18.8%) 680BC 670BC ( 5.7%) 640BC 560BC (70.9%) 400BC |
3 | MMGC Coptic tunic | inhumation burial/ dessication adhesives on display (?) | cellulosic bast | 300–600 AD | RoAMS 569.73 RoAMS 625.73 | RoAMS 569.73: ABA RoAMS 625.73: hexane-acetone-ethanol-ABA | RoAMS 569.73: 3169 ± 30BP 95.4% probability 1504BC (95.4%) 1396BC RoAMS 625.73: 1527 ± 41BP 95.4% probability 424AD (95.4%) 611AD |
4 | BXM excavated | inhumation burial/ sealed tomb (?) in storage | 1. silk 2. leather | Unknown, Byzantine (?) (330–1453 AD) | RoAMS 1104.73 RoAMS 1104.73 | ABA | 1064 ± 28BP 95.4% probability 949AD (77.2%) 1028AD 895AS (18.2%) 925AD 1027 ± 23BP 980AD (95.4%) 1040AD |
5 | Theva | carbonised in storage | cellulosic bast | Middle to Late Bronze Age Greece/ Mycenaean (?) (1750 − 1050 BC) | RICH-27522 | ABA | RICH-27522: 3460 ± 27BP 68.2% probability 1880BC (20.8%) 1840BC 1820BC ( 7.2%) 1800BC 1780BC (31.9%) 1730BC 1720BC ( 8.3%) 1690BC 95.4% probability 1880BC (81.9%) 1730BC 1720BC (13.5%) 1690BC |
6 | Amorgos | carbonised from the excavation to the laboratory | 1. cellulosic bast 2. wood | Late Roman/ Byzantine, pottery seriation 600–800 AD | RoAMS 566.73 RoAMS 567.73 | RoAMS 566.73: CHCl3 RoAMS 567.73: HCL-ultrapure water-base-acid-ultrapure water | RoAMS 566.73: 1257 ± 28 BP 95.4% probability 671AD (84.4%) 779AD 691AD (6.2%) 830AD 837AD (4.5%) 865AD RoAMS 567.73: 1480 ± 28 BP 95.4% probability 541AD (95.4%) 642AD |