Degradation of Archaeological Wood Under Freezing and Thawing Conditions—Effects of Permafrost and Climate Change

The degradation of archaeological wood at freezing and thawing temperatures is studied at the site of Qajaa in West Greenland through a combination of environmental monitoring, measurement of oxygen consumption and microscopy of wood samples. Permanently frozen wood is still very well preserved after 2–4000 years, while wood samples that thaw every summer show attack by soft rot and an average density loss of 0.1 g cm^–3 (corresponding to 25% of the dry mass) over the past 27 years. Future increases in temperature may increase the decay rate significantly (Q₁₀ = 4.2 at 0–10°C) but the effects on site depend on local hydrology.     

EVALUATION OF THE APPLICABILITY OF CONVENTIONAL METHODS FOR THE CHEMICAL CHARACTERIZATION OF WATERLOGGED ARCHAEOLOGICAL WOOD

A series of 112 waterlogged archaeological wood samples of different provenances, wood species, burial times and states of preservation were analysed with the aim of assessing the validity (in terms of limits and reliability) of procedures conventionally used in the pulp and paper industry, to evaluate the state of preservation of decayed wood. In particular, the extent of any damage wrought on decayed wood tissue by the repetitive cycles that characterize these procedures was evaluated as a function of the degradation of the samples. The results indicated that standard methodologies give reliable measurements regardless of the extent of wood decay. However, it is essential to compare these data with those of non‐decayed wood that has been measured using the same procedures to take account of the errors that are inherent in these methodologies and that affect measurements.     

Reasoned use of chemical parameters for the diagnostic evaluation of the state of preservation of waterlogged archaeological wood

Waterlogged archaeological wood undergoes decay processes that depend on both the burial conditions and the constituting species, and which cause the depletion of the structural components of wood cells. To quantitatively assess the state of preservation of the decayed material, specific parameters are usually measured by means of both chemical and physical analyses. In this paper an innovative approach in the use of the data obtained from these kinds of measurements is developed. A series of 132 archaeological wood samples of different wood species, burial times and states of preservation, and coming from different sites in Italy, was analysed. Their residual chemical composition, maximum water content and basic density were measured, and a reasoned use of these parameters was carried out through their elaboration, with the aim of both evaluating eventual incongruence or anomalies in the raw physical and chemical data (which has never been accomplished so far) and directly comparing in a reliable way the analytical results obtained from archaeological samples with very different states of preservation. This approach allowed defining the effective values of chemical parameters related to wood decay according to a same reference basis of calculation among the various data. By this way, it was possible to state that lignin can be also attacked by the agents causing biotic decay, and that in hardwoods its decay is more related to the burial conditions than to the wood species. Instead, the mechanism of polysaccharide depletion is diversified: conifers showed a uniform behaviour whereas hardwoods were more species-dependent. Moreover, in addition to the chemical composition, also anatomical factors influence the carbohydrate rate of decay in waterlogged wood.     

The Future Preservation of a Permanently Frozen Kitchen Midden in Western Greenland

Abstract

Archaeological materials may be extraordinarily well preserved in Arctic areas, where permanently frozen conditions in the ground slow down the decay of materials such as wood, bone, ?esh, hair, and DNA. However, the mean annual air temperature in the Arctic is expected to increase by between 2·5 to 7·5°C by the end of the twenty-?rst century. This may have a signi?cant warming effect on the soil and could lead to permafrost thaw and degradation of currently frozen archaeological remains. Here we present a four-year monitoring and research project taking place at Qajaa in the Disko Bay area in West Greenland. Qajaa is a large kitchen midden, containing frozen remains from 4000 years of inhabitation, from when the ?rst Palaeo-Eskimos entered Greenland, until the site was abandoned in the eighteenth century. The purpose of the project is to investigate current preservation conditions through ?eld and laboratory measurements and to evaluate possible threats to the future preservation.

Preliminary results show that the archaeological material at Qajaa is still very well preserved, but some microbial decay is observed in the exposed wooden artefacts that thaw every summer. Maximum temperatures are above 0°C in the upper 40–50 cm of the midden and between 0 and ?2°C down to 3 m depth. Thereby the permafrost may be vulnerable to quite small increases in air temperatures. Laboratory measurements show that the decay of the archaeological wood in the midden is temperature-dependent, with rates increasing 11–12% every time the soil temperature increases 1°C. Moreover, the soil organic material produces heat when decomposed, which could have an additional warming effect on the midden. At the moment the water or ice content within the midden is high, limiting the subsurface oxygen availability. Threats to the future preservation are related to further thawing followed by drainage, increased oxygen availability, microbial decay of the organic material, and heat production.     

Quantifying woodland resource usage and selection from Neolithic to post Mediaeval times in the Irish Midlands

The antiquity and context (site or feature type) are the two principal factors that act as a framework for interpreting charcoal and wood data from archaeological sites in the recreation of past woodland environments. This research addresses these parameters through the analysis of over 18,000 charcoal fragments from 57 archaeological excavations carried out along a 61?km road scheme in the midland region of Ireland. Observations on spatial vegetation dynamics and woodland resource patterns are also reported on. The data were analysed through percentage frequency and multivariate analyses. The results reveal woodland resource usage in terms of wood types selected for use as fuel or as structural wood as well as patterns of change and variability in taxonomic composition in relation to site types, feature types and time periods. Selection was found to be low in prehistoric times and in particular at burnt mound sites/fulachta fiadh but became increasingly important as time progressed. This has important implications when charcoal from archaeological settings is used in environmental reconstructions.     

Modeling wood acquisition strategies from archaeological charcoal remains

Archaeological charcoal remains are often used to reconstruct local woodland composition in the past, but rarely address how and why people may have selected specific woody taxa for particular purposes. Models from the field of human behavioral ecology predict that people forage for wood resources by taking into account the relative usefulness, abundance, and handling time related to procuring different wood types. Archaeological and ecological data from the site of Gordion, in Central Anatolia (modern Turkey), were used to test expectations associated with such models. Results suggest that inhabitants of Gordion used wood types for fuel in proportion to their local availability, but that they selected specific, more distant woods for construction. In most occupation periods pine was preferred for construction, perhaps because it produces long, straight timbers for roofing, despite the distance at which it grows from the site. This case study demonstrates that behavioral ecology modeling can help to distinguish between multiple wood acquisition strategies potentially used in the past and improve our understanding of wood use from archaeological charcoal remains.     

Morphological,chemical and physical changes during charcoalification of wood and its relevance to archaeological contexts

Wood exposed to a heat source can be transformed into charcoal if subject to conditions of carbonisation (in the absence of air) or charring (in restricted air). Charcoal recovered from archaeological sites can yield fundamental information to our understanding of human economic and cultural development over time and (ecological) setting. This work describes the morphological (anatomy, degree of shrinkage), physical (reflectance) and chemical (elemental, molecular composition) properties of charcoal in relation to heat source and wood variables. In this study charcoal and charcoal fuel were experimentally produced whereby temperature (160–1200 °C), time of exposure (2–1440 min), heating rate (high and low) and wood type (angiosperm and conifer) were varied. The results show that charcoal, often described as an inert, black material, has different chemical and physical properties in relation to the investigated variables. By using these different properties it is possible to distinguish between the different types of fires (domestic and industrial) exploited by humans in the past. Morphological analyses and reflectance measurements are effective tools for this purpose and can be used in wood exposed to temperatures of 300 °C and above—temperatures which are relevant to archaeological research. Angiosperm and conifer wood react in different ways when exposed to heat and thus the taxonomic identity of archaeological material needs to be known. Chemical analyses can be used for wood exposed to temperatures below 400 °C whereas elemental analyses of the carbon content can be used for wood exposed to temperatures up to a maximum of 650 °C.     

Seasoned or green? Radial cracks analysis as a method for identifying the use of green wood as fuel in archaeological charcoal

For several years, sociological questions have been central in anthracology. The development of socio-economic approaches based on the recognition of anatomical signatures in wood has made it possible to focus on topics related to firewood use and its management. The presence of radial cracks (RC) on archaeological charcoal is generally interpreted as the result of the burning of green wood. The present study proposes a verification of this theory by the experimental combustion of seasoned and green wood of Scots Pine (Pinus sylvestris L.). Experimenting with this species was a research priority since it was the dominant taxon (representing 80-100% of the charcoal remains) identified in the Palaeolithic European sites under consideration.The experiments were conducted using two different methods: (i) inside a muffle furnace that allowed us to control the combustions, but in which the combustion process is quite different from most archaeological situations and (ii) in an open fireplace, less controllable but closer to the archaeological conditions. The systematic quantification of the number, length and width of the radial cracks (RC) measured on the transversal sections of the experimental samples demonstrates that (i) radial cracks occur independently of the moisture content of the wood before the combustion (on green and seasoned wood); (ii) however, the average number of radial cracks (RC/cm²) allows distinguishing seasoned from green wood; (iii) in the muffle furnace, the size of the RC appeared to be a good criterion for discriminating seasoned and green wood, but this observation was not confirmed by the open-air combustions. Our results clearly show that the appearance of radial cracks is not diagnostic of the combustion of green wood. Nevertheless, the number of radial cracks (RC/cm²) could represent a new method that might help identifying the combustion of green wood in archaeological charcoal samples.     

Archaeobotanical inference of Bronze Age land use and land cover in the eastern Mediterranean

Charcoal and charred seeds at five Bronze Age archaeological sites discern ancient land use in the eastern Mediterranean. Seed frequencies of orchard crops, annual cereals and pulses, and wild or weedy plants are used to characterize plant utilization at different archaeological sites on the island of Cyprus, in the Rift Valley of Jordan, and in the Jabbul Plain and along the upper Euphrates River valley in Syria. Seed to charcoal ratios provide proxies to determine the relative usage of dung versus wood for fuel across the ancient Mediterranean landscape. Greater charcoal and lower charred seed values are interpreted to represent a wooded environment, while higher amounts of charred seeds and minimal wood charcoal suggest a much great use of dung as a fuel source. Interestingly, Politiko-Troullia (Cyprus, Cypriot archaeological sites are, by convention, named for the nearest modern village (Politiko), followed by an italicized toponym (Troullia) referring to the plot of land that incorporates the site) has the lowest seed to charcoal ratio, suggesting its residents primarily burned wood and that the landscape surrounding Troullia remained relatively wooded during the Bronze Age. In contrast, villagers at Tell el-Hayyat (Jordan) utilized a mixture of wood and dung, in contrast to Tell Abu en-Ni’aj (Jordan), and especially Umm el-Marra and Tell es-Sweyhat (Syria), where inhabitants relied solely on dung fuel. Comparative analysis and interpretation of seed and charcoal evidence thus illustrates the variety of fuel use strategies necessitated by the dynamic and diverse Bronze Age landscapes of the Eastern Mediterranean.     

Identifying archaeological wood stack charcoal production sites using geophysical prospection: magnetic characteristics from a modern wood stack charcoal burn site

Charcoal making was a common process in the woodlands of Britain for many centuries. However, historic ‘wood stack’ production sites are difficult to identify. This paper tests the hypothesis that geophysical survey is an appropriate method for the discovery and identification of archaeological charcoal making sites. A traditional wood stack charcoal kiln was constructed in Low Staindale, Dalby Forest, near Pickering, North Yorkshire, to investigate the charcoaling process and the effect of low-temperature carbonisation on the magnetic properties of the soil underlying the kiln. The results from temperature monitoring (within and beneath the charcoal stack) throughout the charcoaling process, geophysical surveys across the charcoal kiln platform, and laboratory analysis of soil and ash samples, are presented. The degree to which the magnetic properties of the ground beneath the charcoal kiln had been enhanced as a consequence of this low-temperature process is discussed. Whilst some magnetic enhancement to the soil beneath the kiln platform was recorded, enhancement appears to be the result of contamination from the wood stack sealing material rather than the effect of heat transfer resulting from the charcoaling process. The results suggest that whilst historic wood stack charcoal production sites are likely to remain an enigmatic and under-reported feature in the archaeological landscape, geophysical prospection does have the potential for identification of these sites.     

A preliminary study on using enzymes in cleaning archaeological wood

The main goal of this study was to study and evaluate the effect of hydrolyzing enzymes on the anatomical structure and chemical composition of archaeological wood samples. Pine and beech wood samples, which were taken from anonymous mashrabia, have been cleaned with three types of enzymes then a scanning electron microscopy (SEM) and infrared spectroscopy (FTIR) study were undertaken, to see if any significant structural or chemical differences could be detected between “untreated” and “treated” wood. No dramatic changes in functional groups on the wood surface, as monitored by infrared spectroscopy, occurred in the samples before and after enzymatic cleaning. SEM data, however, show that protease and lipase enzymes may give good results in cleaning wood surface, but the enzyme residues remaining on the cell walls of archaeological wood are another point to consider.     

Timber exploitation during the 5th–3rd millennia BCE at Arslantepe (Malatya,Turkey): environmental constraints and cultural choices

A considerable amount of charcoal remains from the archaeological site of Arslantepe (Eastern Anatolia) has been analysed. The anthracological assemblage comes from seven archaeological periods, ranging from the Late Chalcolithic 1–2 (mid-5th millennium BCE) to the Early Bronze Age III (late 3rd millennium BCE). The woody taxa exploited by the local communities appeared to have only minor changes throughout the investigated periods. For the evaluation of wood use practices, charcoal was chronologically grouped according to depositional context. The categories of depositional context identified differentiate between the uses of wood for structural parts of buildings, object manufacture, fuel, refuse, and wood found in outdoor areas or in burial contexts. Communities at Arslantepe, characterized by different cultural and socio-economic traits, appeared overall to select timber depending on its use: hydrophilous plants prevail in building material, with the exception of the 2900–2500 BCE period when environmental constraints probably motivate the dominance of woodland-steppe plants. The differential occurrence of taxa in the diverse depositional contexts highlighted cases of under/overestimation of remains, in particular in relation to the woods for construction. Finally, taxa have been attributed to different ecological groups. The interpretation of results and the comparison with other available palaeoenvironmental data point out that climatic factors play only a secondary role in the choice of wood exploitation in the area. Human choice may vary even with constant environmental records.     

Ashes to ashes, charcoal to dust: micromorphological evidence for ash-induced disintegration of charcoal in Early Neolithic (LBK) soil features in Elsloo (The Netherlands)

Charcoal and other forms of charred organic material - an important part of the archaeological record - consist of benzenoids. Such components are unstable in basic or alkaline conditions. Since ashes are alkaline, this means that archaeological charcoal may have been disintegrated and lost if they were buried together with ashes, e.g. as in fireplaces. Ash may also cause clay translocation in decalcified loess because of the disaggregating effect of K⁺ ions in the soil solutions. We investigated the interplay of these two processes, using micromorphological samples from the Early Neolithic site at the Joannes Riviusstraat in Elsloo. Evidence for charcoal disintegration was found in the form of cavities in charcoal fragments, most commonly filled in with thick reddish limpid clay coatings. The combination of cavities and clay coatings are evidence for the disintegration of charcoal under the influence of ash. Ash may also have been instrumental in preserving small bone fragments in these decalcified well-drained loess soils. The evidence of ash-induced charcoal disintegration implies that charcoal preservation in the archaeological record is dependent on (1) whether or not is was buried alongside with ashes, and (2) on various soil characteristics that determine that determine how quickly the ash-derived alkalinity and potassium ions are leached.     

New trials in the consolidation of waterlogged archaeological wood with different acetone-carried products

Some acetone-carried consolidants for waterlogged archaeological wood were tested in order to evaluate treatments able to save time and energy. In details, colophony (rosin), two esterified colophonies (Rosin 100^® and Rosin 459^®), a mixture of colophony with PEG 3400 and a vinyl acetate - vinyl versate copolymer (Vinavil 8020S^®) were tested. The treatments were carried out at temperatures of 20 and 35 °C on waterlogged maritime pine, elm, oak and beech. The materials came from the archaeological site of the ancient ships of Pisa (Tuscany, Italy) and were dated back to VII cent. BC – II cent. AD. To evaluate the processes, equilibrium moisture content and dimensional stability of treated wood samples at different relative humidity, and retention of impregnating products were measured; moreover macroscopic and microscopic examination were also run to respectively assess the shape and appearance of treated wood and the way of deposition of consolidants. The results highlighted that natural and modified colophony treatments gave the most satisfactory results both in the maintenance of shape and dimensions of samples and in the stabilization with respect to RH variations. Moreover, the equilibrium moisture contents of samples treated with R100 and R459 were much reduced in comparison to the other consolidants and to untreated archaeological wood. This fact was related to the high retention values of those products that occluded most of the porosity including the microporosity of cell walls. Therefore, in terms of higroscopicity treated wood was more similar to impregnating substances rather than to decayed wood. This fact was considered helpful in contrasting the moisture-related negative effects in cases of eventual faults in the climate control during e.g. exhibition and in protecting treated wood from the risks of new fungal attacks.     

Preservation potential of charcoal in alkaline environments: an experimental approach and implications for the archaeological record

Charcoalified plant material (usually wood) has been described as being inert with a high preservation potential. Although usually black on both the outside and inside, the physical and chemical properties differ as a function of temperature and time of exposure to a heat source. Post-depositional processes however may exert additional affects on charcoalified material and introduce bias into the archaeological record. This work explores the influence of alkaline conditions, often encountered in the soil environment, on the preservation potential of charcoalified material. Charcoal, experimentally produced at different temperatures, was exposed to a number of solutions of potassium hydroxide and calcium hydroxide with different pHs, to simulate exposure to alkaline soil conditions. Under these conditions chemically mediated physical interactions occur resulting in fragmentation of the charcoal samples into (very) small pieces the size of which depends upon the initial temperature of charcoalification. Moreover further interactions result in a decrease in reflectance relative to samples not exposed to alkaline conditions. Although experimentally driven the potential for physical loss of charcoal from a particular site coupled with the reduction in (expected) reflectance of any resident material, has important implications for any conclusions drawn regarding the archaeological record.     

饱水木质文物的细菌病害及其诊断技术的进展

为了促进微生物学理论与技术在我国文物保护领域中的应用,使我国的文物保护与国际先进水平接轨,本文通过对国内外有关文物保护微生物学研究进展的综述,阐述了饱水木质文物细菌性病害的成因和机理.侵蚀细菌、钻管细菌和空化细菌是饱水木质文物细菌性病害的病原体,但它们的作用方式有一定的区别.在饱水木质文物细菌性病害诊断上,国外已较为普遍地使用了光镜技术、电镜技术、细菌培养技术以及多种分子生物学技术.根据众多样本分析的结果,国外学者已建立评价饱水木质文物腐蚀状态的定性指标体系.文中还结合目前我国有关饱水木质文物保护的现状,阐述了文物保护微生物学在我国饱水木质文物保护中的应用前景,并建议尽快开展相关的研究工作.     

X‐RAY SUB‐MICRON TOMOGRAPHY AS A TOOL FOR THE STUDY OF ARCHAEOLOGICAL WOOD PRESERVED THROUGH THE CORROSION OF METAL OBJECTS

Wood preserved in the corrosion layer of two early medieval iron objects was examined using X‐ray tomography. A state‐of‐the art multi‐resolution X‐ray tomography set‐up ( http://www.ugct.ugent.be ) provided virtual cross‐sections of the archaeological wood samples at sub‐micron resolution. These were compared with scans of samples of similar modern wood. These scans demonstrate the power of sub‐micron X‐ray tomography for wood identification, although the process of mineralization pushes this technique to its limits. Furthermore, this technique facilitated appraisal of the mineral content of the archaeological wood, which is useful in selecting the most appropriate strategy for the (preventative) conservation of the archaeological object.     

Modern and fossil charcoal: aspects of structure and diagenesis

The structures and compositions of modern and fossil charcoal samples were compared in order to evaluate charcoal degradation processes in archaeological sites. Modern charcoal samples produced in campfires contain two major phases: graphite-like microcrystallites and a non-organized phase. These phases create a mosaic-like structure with differing relative proportions depending on the taxonomic source of the wood used. Fossil charcoal samples (Tel Dor, Israel: 3000 years BP and Kebara Cave, Israel: 40,000 years BP) also contained the graphite-like microcrystallites and the non-organized phases, but were clearly altered compared to modern charcoal. The graphite-like phase of the fossil charcoal has much higher electrical resistivity, and its ESR properties show that it has markedly altered surface electronic states. Infrared spectra show the presence of additional carboxylate groups. Oxidation has therefore altered the structure. This appears to be a “self-humification” process that affects the graphitic component, and probably the non-organized phase as well.     

Dating Ancient Burial Mounds in Denmark – Revealing Problematic Ancient Charcoal

The National Museum of Denmark and the Department of Geography at the University of Copenhagen have collaborated on a project investigating burial mounds near early Medieval churches. The aim was to identify a possible continuity in cult sites across the shift to Christianity in the late Viking Age. Charcoal samples from 18 mounds were radiocarbon dated but the results showed they were far older than expected. Control dating undertaken on burial mounds of known age confirmed that charcoal in the mound fill can at least be up to 3000 years older than the mound itself. As charcoal can survive in the surface soil layer for millennia, in spite of ploughing, bioturbation and frost, it may also dominate the charcoal pool of the grass or heather turfs used in the mound construction. Therefore, the article concludes, charcoal cannot be used to securely date archaeological features built with turfs and it is important to be aware of the possible presence of very old charcoal when selecting material for dating archaeological features, even those which otherwise would be judged unaffected by material from earlier archaeological periods.     

Temperature,compression and fragmentation: an experimental analysis to assess the impact of taphonomic processes on charcoal preservation

This paper explores how mechanical properties of different South Asian tree taxa might influence charred wood fragmentation and the composition of charcoal assemblages retrieved from archaeological contexts. The dataset is composed of selected plant species that were collected in Gujarat (Northern India) in September 2007 as part of the North Gujarat Archaeological Project. The taxa analysed represent the most common wood species found in the arid thorn scrubland formation in South Asia and are among the most abundant identified in Neolithic and Chalcolithic archaeological sites in this area. The specimens have been measured and subsequently charred at three different temperatures, under constant conditions. Afterwards, their resistance to compression has been measured on two planes, parallel and perpendicular to the direction of fibres, using a Hounsfield 5-kN machine. The tests were performed in order to understand the different response to compressive stress of wood that has been subject to a range of thermal degradations. The standardised treatment applied to the samples has permitted the comparison of results and the delineation of simple correlations and divergences among the species analysed. Furthermore, the utilisation of a relatively simple operating protocol easily allows the addition of further data in the future. The applied protocol was specifically designed to answer archaeological questions. Therefore, even though from a material science point of view some of the measurements were not taken according to the prevailing standard procedures, it offers valuable data for anthracological research applied to archaeology in arid zones.