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.     

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.     

Charcoal reflectance measurements: implications for structural characterization and assessment of diagenetic alteration

Charcoal is a valuable source of archaeological and palaeoenvironmental proxy data. However growing evidence suggests that production conditions can strongly influence post-depositional alteration of charcoal. Consequently, both reconstruction of production temperature and understanding of the potential for diagenetic alteration are of great interest. Here, we use mean random reflectance (Ro_mean) in conjunction with other chemical characterization methods to address these questions. Ro_mean was obtained for a suite of modern analogue charcoal, produced under controlled conditions, and for a series of natural charcoal samples, obtained from archaeological and palaeoenvironmental deposits. Ro_mean proves to be a robust measure to assess formation temperature for samples produced at 400 °C and above, even after exposure to highly oxidizing conditions. Ro_mean is also useful for samples formed between 300 °C and 400 °C. However, if an assemblage of charcoals has been exposed to oxidizing conditions, lower temperature charcoals may be preferentially lost. It is apparent that charcoal produced at lower temperatures is more highly susceptible to chemical oxidation, and that there is a continuum in charcoal degradation potential, dependant upon fuel material and production conditions.     

The use of reflectance values for the interpretation of natural and anthropogenic charcoal assemblages

Charcoal assemblages occur in both natural and archaeological settings. Cell wall reflectance values of charcoal in polished blocks under oil provide a proxy for temperature of formation. This paper aims to (1) determine whether wildfire charcoals and anthropological charcoals from various pyrotechnical activities can be distinguished using reflectance data and (2) establish if re-charring (i.e. use of charcoal fuel) can be recognised in the archaeological record through analysis of laboratory-produced re-charred charcoals and charcoals from an experimental iron smelt and traditional bronze casting which utilised charcoal fuel. Reflectance frequency data from assemblages representing burning of charcoal, in this case of iron smelting and bronze casting, indicates temperatures from above the mean value of charcoal production (>475°C) up to the maximum temperature reached in the subsequent process (i.e. >475 to >1,100°C). In contrast, wildfire charcoals showed a range of values including material with barely measurable reflectance (minimum values from 0.06% to 0.56%Ro) to maximum reflectance values varying from 1.65%Ro (Tilford) to 3.8%Ro (Zacca). The mean wildfire reflectance indicated temperatures in the range 325–400°C, which can therefore clearly be distinguished from that of the charcoal burning processes. The laboratory-produced re-charred charcoals take on the reflectance value of the highest temperatures experienced; reflectance values were not constrained by the original temperature of formation. High temperatures are most easily achievable by the burning of charcoal fuel, and hence high reflectance charcoals are likely to represent re-charred charcoal. Therefore, this quantitative reflectance method can be used in archaeology to determine the minimum temperature of formation of charcoals in anthropological processes which involve fire, can indicate the likelihood of use of charcoal or wood as fuel and can distinguish between an assemblage of high temperature anthropogenic charcoals and charcoals formed from natural wildfire.     

Reburial in the context of development: Approaches to reburial in the English planning process

Abstract

Rescue archaeology in England is now firmly embedded in the planning process, following the introduction by the UK government of Planning Policy Guidance Note 16 in 1990 (PPG 16; Departmentof the Environment. Planning Policy Guidance Note 16 — Archaeology and Planning. HMSO, London (1990)). The basic premise of PPG 16 is a strong preference for in situ preservation of nationally important archaeological remains. This often includes reburial of archaeology prior to construction activity, once it has been exposed in test-pitting (‘evaluations’) or full excavation. Reburial strategies are often prepared by civil engineers and there appears to be an overemphasis on consideration of the potential physical impacts on the archaeology. These strategies usually include the use of geotextiles and sand, or other graded material, chosen for its particular particle size/shape characteristics. A greater awareness of the chemical and physical characteristics of a burial environment is now required to ensure that reburial is effective. Archaeologists and conservators are beginning to consider both the natural and, increasingly, the ‘man-made’ (industrial, domestic and agricultural) changes to the soil chemistry of an archaeological site. More use needs to be made of the potential information ‘locked up’ in the artefactual material retrieved from evaluations and excavation nearby. An assessment of the degree of preservation of material should be an integral component for the design of a reburial strategy.     

Fires at Neumark-Nord 2, Germany: An analysis of fire proxies from a Last Interglacial Middle Palaeolithic basin site

Few sites with evidence for fire use are known from the Last Interglacial in Europe. Hearth features are rarely preserved, probably as a result of post-depositional processes. The small postglacial basins (<300?m in diameter) that dominate the sedimentary context of the Eemian record in Europe are high-resolution environmental archives often containing charcoal particles. This case study presents the macroscopic charcoal record of the Neumark-Nord 2 basin, Germany, and the correlation of this record with the distinct find levels of the basin margin that also contain thermally altered archaeological material. Increased charcoal quantities are shown to correspond to phases of hominin presence—a pattern that fits best with recurrent anthropogenic fires within the watershed. This research shows the potential of small basin localities in the reconstruction of local fire histories, where clear archaeological features like hearths are missing.     

Alkali extraction of archaeological and geological charcoal: evidence for diagenetic degradation and formation of humic acids

Charcoal forms a crucial source of archaeological and palaeoenvironmental data, providing a record of cultural activities, past climatic conditions and a means of chronological control via radiocarbon (¹⁴C) dating. Key to this is the perceived resistance of charcoal to post-depositional alteration, however recent research has highlighted the possibility for alteration and degradation of charcoal in the environment. An important aspect of such diagenesis is the potential for addition of exogenous “humic acids’ (HAs), to affect the accuracy of archaeological and palaeoenvironmental reconstructions based upon chemical analyses of HA-containing charcoal. However the release of significant quantities of HA from apparently pristine charcoals raises the question whether some HA could be derived via diagenetic alteration of charcoal itself. Here we address this question through comparison of freshly produced charcoal with samples from archaeological and geological sites exposed to environmental conditions for millennia using elemental (C/H/O) and isotopic (δ¹³C) measurements, Fourier Transform Infrared Spectroscopy (FTIR) and proton Liquid-State Nuclear Magnetic Resonance (¹H NMR). The results of analyses show that the presence of highly carboxylated and aromatic alkali-extractable HA in charcoal from depositional environments can often be attributable to the effects of post-depositional processes, and that these substances can represent the products of post-depositional diagenetic alteration in charcoal.     

Characterisation of microbial attack on archaeological bone

As part of an EU funded project to investigate the factors influencing bone preservation in the archaeological record, more than 250 bones from 41 archaeological sites in five countries spanning four climatic regions were studied for diagenetic alteration. Sites were selected to cover a range of environmental conditions and archaeological contexts. Microscopic and physical (mercury intrusion porosimetry) analyses of these bones revealed that the majority (68%) had suffered microbial attack. Furthermore, significant differences were found between animal and human bone in both the state of preservation and the type of microbial attack present. These differences in preservation might result from differences in early taphonomy of the bones.     

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.     

Physical and Geochemical Characteristics of Soils from Angel Mounds State Historic Site,Indiana, including a Case Study of Sherds from Unit A

Abstract

Using physical and chemical characteristics of soil to evaluate archaeological artifacts and their preservation potential along with associated features is well established. The soil characteristics and geochemistry of the Angel Mounds site (12Vg1) indicate the soil geochemistry is very homogenous and composed of loams and silt loams with a range of pH values from 4.69 to 6.17, mean value of 5.40 ± 0.47. The slightly acidic nature of the soil favors the preservation of pollen and soil diatoms, with minor corrosion potential for the bone, shell, and sherd artifacts. A comparison of the geochemistry of the soils from Unit A and site-wide sampling to sherds from Unit A suggests that elevated phosphorus (P) in the sherds is either the result of the addition of a P-rich temper material or from the sourcing of nonlocal clay-rich sediment, and not from use-derived residue or from the soils themselves.     

Diagenesis and survival of osteocalcin in archaeological bone

It has been demonstrated that the protein osteocalcin can survive in bone in the archaeological record, and postulated that it has the potential to survive over geological time periods. The precise mechanism for this longevity of survival is not yet fully understood, and has not been extensively studied in comparison to other diagenetic aspects of archaeological bone. We report a comparison between osteocalcin survival and the state of preservation of more than 60 bones from 14 archaeological sites. The amount of osteocalcin, assayed immunologically, was compared with diagenetic parameters that measure: the amount of ‘collagen’ in the bone, the mineral changes, the porosity, and the histological preservation of the material. The findings indicate that microbial taphonomy and mineral alteration of bone have a profoundly damaging effect on the preservation of osteocalcin.     

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.     

Sclerochronological analysis of archaeological mollusc assemblages: methods,applications and future prospects

Accreting skeletal tissues found in bone, teeth, otoliths and molluscan shell act as sensitive recorders of local environmental and climatic conditions. Owing to their robust nature, ubiquity and abundance in the archaeological record as well as the potential for high-resolution data acquisition, the accreting skeletal tissues of archaeological molluscs are increasingly employed as palaeoenvironmental proxies. Researchers have chiefly utilised such proxies to extend instrumental records of environmental conditions through palaeoenvironmental reconstruction and explore the impact of environmental and climatic change on human populations. However, the use of environmental proxies from the archaeological record can be hampered by a number of methodological challenges including inadequate sampling strategies, appropriate calibration, the use of inappropriate proxies and the broad extrapolation of localised results. This paper reviews the use of molluscan shell from archaeological contexts as palaeoenvironmental proxies. We focus on the application of sclerochronology—a suite of high-resolution physical and geochemical data recovery methods widely used in conjunction with molluscan shell. This paper presents an overview of the potential of these techniques in approaching more nuanced understandings of human-environment interactions and how they can be more successfully incorporated into archaeological research.     

Determining the preservation rating of submerged archaeology in the post-glacial southern North Sea: a first-order geomorphological approach

Abstract

During most of the last glaciation, the southern North Sea floor was exposed and accessible to humans. Archaeological finds are concentrated around Brown Bank, Dogger Bank and the Norfolk Banks, but the Palaeolithic and Mesolithic archaeological potential of these areas is poorly known. Management of submerged archaeological heritage requires knowledge about location (potential) and context, ascribing significance (value) and determining risk. Thus, the preservation potential of primary and secondary archaeological material around these three areas is considered a) in the context of the post-glacial evolution of the southern North Sea and b) regarding natural and anthropogenic processes. A detailed review is followed by original research material describing an approach to assessing preservation potential. In general, low-energy deposits associated with former intertidal, floodplain or lacustrine environments are likely to preserve primary archaeological material, including organic remains, whilst (high-energy) riverine environments are more likely to preserve inorganic secondary archaeological material. The main possible anthropogenic impacts on submerged archaeology result from beam trawling, which may disturb deposits at the seafloor, and aggregate dredging, which may remove secondary deposits. Trawling and aggregate dredging are increasingly contributing to knowledge, through reporting finds through established protocols, and through assisting in funding relevant to multi-disciplinary research.     

Quantification and Visualization of In Situ Degradation at the World Heritage Site Bryggen in Bergen,Norway

Abstract

Environmental monitoring at the World Heritage Site of Bryggen in Bergen, Norway, has shown damaging settling rates caused by degradation of underlying archaeological deposits. Measurements of piezometric head, oxygen, and soil moisture content, as well as chemical analyses of water and soil samples are key elements of the environmental monitoring.

Groundwater monitoring and geochemical analyses reveal a complex and dynamic flow through the natural and anthropogenic stratigraphy. The preservation conditions within the organic archaeological deposits are strongly correlated with oxygen and soil moisture content, that are controlled by the groundwater flow conditions at the site. To quantify decay rates, it is thus essential to understand the wider hydrogeological context of the site. This paper presents recent advancements in quantifying decay rates in the saturated zone at Bryggen. The paper also shows that 3D geo-archaeological modelling can contribute to preservation management by visually combining results of geological, archaeological, geochemical, and hydrological investigations. This opens up for improved multidisciplinary understanding of preservation potential, thereby contributing to an improved protection of archaeological deposits in situ.     

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.     

Negotiations with the Animate Forest: Hunting Shrines in the Guatemalan Highlands

Ethnoarchaeological research at highland Maya hunting shrines documents the material remains of interactions between two types of animate beings: humans and the forest. When either active agent enters the others’ domain there are accompanying ceremonial activities to assuage the inherent danger, often leaving physical traces in the material record. These traces, if found in the archaeological record, might reveal similar ancient interactions. Using the material correlates of modern hunting rituals, we explore the utility of ethnoarchaeological research in identifying negotiations with non-human agents associated with the animate forest – an active agent in many societies.     

Preservation of fungi in archaeological charcoal

During the analysis of wood charcoal remains from archaeological sites, it is common to find different microorganisms and different forms of degradation present in the plant tissue. However, one may encounter difficulties when attempting to identify these microorganisms and determine when their attack occurred. This paper focuses on preservation aspects related to the microorganisms in wood and demonstrates the structural changes that take place in different types of decayed wood after it was converted into charcoal. The study seeks to determine whether the microbial attack found in archaeological woods took place before the burning of the wood or after. Burning experiments were conducted using wood that had been decayed by various types of fungi including white-rot, brown-rot, and soft-rot. The laboratory burnt wood samples showed decay patterns that were comparable to those observed in archaeological charcoal samples, indicating that signs of fungal infestation and features of decay can be preserved after burning with micromorphological details of mycelium and cell wall attack evident. This indication may provide important information related to the gathering of deadwood as fuelwood. In addition, examples of decayed wood preserved in archaeological charcoal assemblages are described. Their relationship to the archaeological context and environmental conditions may suggest different interpretative models concerning wood management strategies applied by past societies.     

The ethnoarchaeology of firewood management in the Fang villages of Equatorial Guinea, central Africa: Implications for the interpretation of wood fuel remains from archaeological sites

In archaeological literature, the study of trees and wood remains is a topic of relatively marginal interest, especially compared to texts on crops and human–animal relations. However, charcoal is the most frequent botanical remain found in archaeological sites. Charcoal analysis can therefore play a major role in the development of studies in both landscape and palaeoethnobotanical reconstruction. The majority of the archaeological charcoal assemblages reflect the exploitation of wood as an energy source (fuel). The archaeological study of firewood selection has been predominantly developed from “eco-utilitarian” or “subsistence economy” perspectives, but has not yet considered fuel collection and use as one of the most enduring categories of human–environment interactions, nor has archaeology looked into its potential as a source of empirical information on past perceptions of, and interactions with, ancient landscapes. The aim of this paper is to expand previous archaeological work on the interpretation of charcoal macro-remains through the study of firewood collection as a historically constituted, socially mediated and archaeologically observable landscape practice. In order to achieve this, we present an ethnoarchaeological case study from the Fang society of Equatorial Guinea (central Africa) aimed at gaining a better understanding of the complex interactions between cultural, ecological and economic variables in firewood collection strategies.     

In situ preservation of archaeological bone: a histological study within a multidisciplinary approach

To make preservation in situ a serious option for the management of archaeological sites, research has to be done on the factors affecting conservation of different archaeological materials, including bone. A European project has been started which deals with bone degradation in a multidisciplinary way. The goals of the project are to develop techniques to describe the preservation of archaeological bone, to make a classification of soil environments according to their preservation potential and to detect what factors in the environment of the bone affect its conservation. One technique used in this project to determine the state of preservation of archaeological bone is histology. The relevance of this technique for archaeological heritage management research is discussed.