Charred bone: Physical and chemical changes during laboratory simulated heating under reducing conditions and its relevance for the study of fire use in archaeology
Introduction
Fire has played a key role in the development of humankind and fundamentally changed our relationship with the world (Goudsblom, 1992). The chronological distribution of heated remains in the archaeological record of Europe suggests that fire (the chemical process through which heat is generated) has been an integral part of the human technological repertoire from the later Middle Pleistocene (~ 350 ka) onwards (Roebroeks and Villa, 2011), an interpretation that fits well with recently reported data on fire use from the Levant (Shimelmitz et al., 2014). Since fire is used in various domestic and non-domestic settings, data on its use are crucial for our understanding of human subsistence strategies, fuel management, various pyrotechnologies, mortuary practices, and even landscape management (Scherjon et al., 2015, Théry-Parisot, 2002). In order to reconstruct the heating conditions archaeological fire remains were exposed to, and in turn gain insight into the timing and nature of the specific human behaviour that produced them (e.g. the function of fireplaces), the effect of heat on the physical and chemical properties of the materials entering the archaeological record needs to be understood.
For a fire to be ignited, and remain ablaze, the three components of the fire triangle need to be present: heat from an external heat source, oxygen, and a fuel (Emmons and Atreya, 1982). It is the carbon-rich organic part of the fuel that carries the energy that can be transformed into heat. When organic matter is exposed to heat, thermal energy will be absorbed, mainly through radiation, causing the temperature of the material to increase. When temperatures approach around 300 °C, chemical reactions start to take place that gradually change the original organic constituents of the material (i.e. charring) (Braadbaart et al., 2007). This is a reaction that requires heat, but no oxygen, resulting in the formation of aromatic compounds (i.e. char) and the release of volatile gasses (Braadbaart et al., 2007, Rein, 2009). This first charring is a necessary chemical step towards combustion. In the presence of air, when temperatures remain sufficiently high (> 300 °C), the char and volatiles oxidise (i.e. combustion), producing more thermal energy and flames. When the oxidation is completed, all organic material will have been removed, leaving only ash (i.e. the inorganic components of the fuel) (e.g. Braadbaart et al., 2012, Rein, 2009). In the absence of oxygen (i.e. charring), with increasing temperatures (> 400 °C), the molecular structure changes, resulting in the formation of polyaromatic, planar sheets and increased ordering of the char (e.g. Braadbaart and Poole, 2008). It is important to note that two types of heating can be distinguished: combustion (or burning) and charring, both of which require the formation of char. Consequently, charring is not restricted to reducing conditions. In addition, it should be noted that the physical and chemical properties of heated materials do not just depend on temperature and the presence or absence of oxygen, but also on other heating conditions, such as heating rate (°C/min) and exposure time (Braadbaart et al., 2007, Rein, 2009).
While a lot of work has been done on the effect of heat on organic materials, particularly on wood (e.g. Ascough et al., 2008, Braadbaart and Poole, 2008, Cohen-Ofri et al., 2006, Scott, 2010), the understanding of another common bioorganic fire residue, heated bone, is far more fragmentary. The presence of a high amount of inorganic compounds in bone makes it distinct from wood and other organic plant and/or animal tissues. While wood is composed of only about 2 wt% inorganic compounds (Braadbaart and Poole, 2008), bone on average contains 70 wt% inorganic material (White and Folkens, 2005). Bone is a composite material in which equal volumes of the organic and inorganic constituents are intimately intergrown (for details see Section 2) (Pasteris et al., 2014). Because of these specific properties, bone is affected differently by heat, than organic compounds, for example in terms of the ease with which air - and thus oxygen - disperses into the material. Combined with the prevalence of heated bone in the archaeological record, this highlights the importance of research focussing on heated bone.
Initially, studies in this direction concentrated on the macroscopically visible manifestations of heating (e.g. Kalsbeek and Richter, 2006, Shipman et al., 1984, Stiner et al., 1995). Later, focus was shifted towards the physical and chemical changes underlying the visible manifestations by applying a broad range of analytical techniques including Thermogravimetric analysis (TGA) (e.g. Ellingham et al., 2015b, Etok et al., 2007, Haberko et al., 2006, Jankovic et al., 2009, Lozano et al., 2003, Mkukuma et al., 2004), x-ray fluorescence (XRF) (e.g. Kalsbeek and Richter, 2006, Thompson et al., 2011), x-ray diffraction (XRD) (e.g. Enzo et al., 2007, Piga et al., 2008, Rogers and Daniels, 2002), Fourier transform infrared spectroscopy (FTIR) (e.g. Figueiredo et al., 2010, Lebon et al., 2008, Lebon et al., 2010, Mkukuma et al., 2004, Thompson et al., 2009, Thompson et al., 2013), Raman spectroscopy (e.g. Pasteris et al., 2004), and transmission electron microscopy (TEM) (e.g. Koon et al., 2003, Koon et al., 2010). While all of these studies discuss the thermal alteration of bone, they all concern combustion (i.e. heating in the presence of air), with the exception of Mkukuma et al. (2004), and generally focus on just one or two analytical methods, a limited temperature range or the effect of heat on one specific bone property (e.g. crystallinity). This means we only have a partial understanding of the range of heated bone potentially available in the archaeological record, both in terms of heating conditions (e.g. charring vs. combustion; temperature) and of the interaction of the organic and inorganic properties targeted by the different analytical techniques.
In order to gain a more comprehensive understanding of the effect of heating (i.e. the chemical process of fire) on the physical and chemical properties of bone, we performed a series of controlled laboratory-based heating experiments, in reducing conditions (i.e. charring), covering a broad temperature range (20–900 °C), and applied a variety of different analytical techniques. This allows us to gain insight in the effect of heat on both the organic and inorganic compounds in bone and their interaction, as well as assess the usefulness of the different techniques for the reconstruction of heating conditions in archaeological contexts. The combination of techniques applied in this study was chosen in order to infer changes in physical (colour, mass loss, TGA, reflectance analysis), elemental (XRF, CHN), molecular (FTIR, Raman, DTMS), and structural (XRD) properties. Furthermore, this specific combination of techniques allows us to address changes in bone organic and inorganic content separately, as well as combined. By taking the process of charring as a starting point, we were able to exert more control over the experimental conditions and provide valuable initial baseline data for further research. From an archaeological perspective, improving our understanding of charred bone is important since the presence of charred organic materials, including bone, in the archaeological record implies that not all char is oxidised during heating, even though the presence of air may be expected in an open fire. It should be noted that charring can also occur within an open fire (Albini, 1993). The data generated by this study will help archaeologists identify the full range of heated bone and reconstruct the heating conditions (e.g. temperature; charring vs. combustion) bones from the archaeological record were exposed to. Shedding more light on fire function and specific human behaviour (e.g. Braadbaart and Poole, 2008). Furthermore, this study provides the first comparative standard for charred bone that archaeologists can use when reconstructing the taphonomic history of bones heated in the past. Essentially, gaining insight into the process of charring is the necessary first step towards understanding combustion and the effect of diagenesis on heated bone.
Section snippets
What is bone?
Bone tissue is a composite material that consists of three major parts: a large inorganic fraction (about 70 wt%), a much smaller organic fraction (about 20 wt%), and water (about 10 wt%). The relative proportions of these constituents depend on the type of bone (cortical or trabecular) and may vary as a result of development and pathology (Kuhn et al., 2008, Pasteris, 2014, Weiner, 2010, 102, 105; White and Folkens, 2005, 33). The organic fraction mainly consists of an assortment of proteins, of
Sample preparation and heating experiments
Samples were taken from the cortical part of the femur of a mature (9 years old) female bovine (Bos taurus). Bovine bone was chosen as an analogy for the large herbivore bones dominating most Pleistocene assemblages. Flesh and fat were mechanically removed, after which the bone was further cleaned with water at a temperature of 40 °C and air-dried. Cleaned bone was cut into longitudinal samples measuring approximately 5 × 5 × 35 mm. All samples were cut from the same bone.
All samples were individually
Colour
Bone colour (Fig. 1, Table 1) changed from white through yellow-white and yellow to reddish brown at 300 °C, after this initial change the bones darkened to black with increasing temperature at 600 °C and higher (cf. Munsell, 1954).
Mass loss
Mass loss during heating occurs in two stages (Fig. 2, Table 1): below 370 °C the increase in mass loss follows an approximately linear trend from 10.53 wt% at 200 °C to 24.93 wt% at 370 °C (± 6 wt%/100 °C), above 400 °C the increase in mass loss slows down to an approximately
Discussion
The results of the different analytical techniques are combined to address the aims of this paper: determining the influence of heating under reducing conditions on the organic and inorganic content of bone, and determining the applicability of the various analytical techniques in addressing archaeological questions related to the identification of heat-induced changes in charred bone and reconstructing past heating conditions. Gaining insight into charred bone and the methods available to
Conclusion
The use of fire has played a defining role within the development of humankind. Heated bone, being more taphonomically durable than charcoal due to the presence of a large mineral component, forms an important source of information for gaining a comprehensive understanding of the history and specifics of fire use through time. It is therefore essential to be able to unambiguously identify heated bone in the archaeological record, to trace back when and where fire was used, and to reliably
Acknowledgements
We would like to thank the following people from the Dutch Cultural Heritage Agency (RCE) for their help with the measurements: Suzan de Groot for assistance with the FTIR and Raman, Elisa Selviasiuk for performing the Raman, and Henk van Keulen for his assistance with the pyMS. Thanks also go out to Wil Roebroeks and Marie Soressi (Faculty of Archaeology, Leiden University) for helpful discussions on the subject and feedback on earlier versions of the paper, and to Paul Kozowyk for
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