Charcoal is a black, inert solid produced by the incomplete combustion of wood. Due to its high stability, it is widely preserved in archaeological sites and natural sediments. Traditional charcoal studies mainly rely on anatomical structure analysis for species identification, enabling the reconstruction of ancient vegetation environments and human fire-use behaviors. In recent years, environmental information embedded in charcoal has emerged as a new research focus. For instance, the chemical structure and reflectance of charcoal can be used to reconstruct past fire temperatures (intensity), while charcoal isotopic signals provide insights into ancient climate conditions, forest management, and fertilization practices. However, the fundamental processes and mechanisms underlying "carbonization" and "burial" in ancient charcoal studies remain inadequately understood. This knowledge gap may lead to severe misinterpretations of charcoal physicochemical and isotopic indicators.
To address this issue, the environmental archaeology team at Lanzhou University selected Quercusand Pinusas representative wood types and conducted a series of controlled muffle furnaceexperiments. These experiments examined changes in the physicochemical properties of charcoal under different heating temperatures and durations, quantitatively exploring the mechanisms of the carbonization process. In addition, to simulate the formation of combustion charcoal in natural environments and archaeological sites, the team conducted outdoor burning experiments. These burning experiments were designed to replicate the size and structure ofhearths and fire pitsfound in archaeological sites, simulating the production of charcoal during ancient domestic fire use. Furthermore, archaeological charcoal specimens of the same identified species were analyzed for comparison to investigate the effects of burial on charcoal properties. Charcoal samples obtained from different scenarios are shown in Figure 1.
Figure 1 Charcoal produced from muffle furnace-controlled experiments, outdoor combustion, and archaeological burial.
The study employed multiplephysicochemical analysis techniques, includingelemental analysis, reflectance measurements, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), solid-state ¹³C nuclear magnetic resonance (NMR), and thermogravimetric analysis (TGA).Data analysis methods includedprincipal component analysis (PCA), cluster analysis, discriminant analysis, and deep learning techniques.Additionally, thermodynamic and chemical kinetic theories were integrated to explain the changes in various physicochemical indicators during the wood carbonization process. The key findings are as follows:
(1) The carbonization threshold of wood occurs at300–350°C, resulting in complete blackening and increased fragility. This transition involves the conversion of cellulose-dominated wood into charcoal dominated by aromatic structures. During this process,tracheid wall thickness and lumen area decrease by approximately 20%, mass loss reaches 50%, carbon content increases to around 60%, while oxygen content decreases to about 35%. Reflectance rises from 0 to 0.5.The coupling relationships among different indicators are shown inFigure 2.
Figure 2 Changes in various physicochemical indicators during the carbonization process and their coupling relationships.
(2) Combustion is a complex thermodynamic process with continuously changing temperature over time and space, differing fundamentally from the controlled temperature conditions in amuffle furnace. However, the chemical structures of charcoal produced by both combustion and furnace heating are fully comparable. Therefore, muffle furnace-controlled experiments can serve as areference benchmarkfor interpreting combustion conditions in natural and archaeological settings. Spectroscopic data analysis reveals that approximately90% of combustion-derived charcoal exhibits high carbonization levels.The equivalent temperature range for this level of carbonization, when mapped onto muffle furnace conditions, is 400–500°C for Quercus charcoal and 550–600°C for Pinus charcoal.Notably, these temperatures representequivalent conditions from controlled experiments rather than the actual combustion temperatures.
(3) Archaeological charcoal, after thousands of years of burial, exhibitsoxidation characteristics, with the presence of distinctcarboxyl functional groupsin its chemical structure. This transformation significantly interferes with temperature reconstructions based on ancient charcoal formation. However,deep learning techniques—due to their powerful ability to extract complex information—can partially mitigate signal distortion caused by burial processes. The final results indicate thatcharcoal preserved in domestic fire-use contexts exhibits temperature conditions consistent with those observed in outdoor combustion simulations, with approximately 90% falling within high carbonization levels.
Figure 3 presents: (A–C)FTIR spectral clustering and principal component analysis resultsunder gradient-controlled temperature experiments. Clustering analysis classifies the carbonization process intoseven categories, further consolidated intofive carbonization levels: incomplete carbonization, low carbonization, medium carbonization, high carbonization, and ultra-high carbonization.(D)Deep learning (1D convolutional neural network) model predictions for combustion and archaeological charcoal.(E)Trends in wood/charcoal chemical structure variations with heating temperature and the impact of burial processes on charcoal structure.
In summary, this study successfullypredicted the equivalent formation temperature of charcoal generated in ancient domestic fire-use contexts.The results were further compared with temperature estimates fromreflectance and infrared spectral indicesin other studies. While slight variations exist due to differing experimental conditions in muffle furnace settings, the overall temperature ranges remainhighly consistent, aligning with thermodynamic models of combustion. During burning, the temperature decreases progressively from the outermost fuel layer to the inner core. Wood portions exposed to direct flames are more prone toash formation, while high-temperature charcoal exhibitspoor mechanical properties, making it more susceptible to fragmentation during burial. In contrast,low-temperature charcoalwithin the inner fuel core, which remains only partially carbonized, is more prone todegradation. Therefore,large charcoal fragments (>4 mm) recovered from flotation in archaeological sites likely represent the most stable and well-preserved portions, leading to relatively uniform formation temperature estimates.Furthermore, this study explored the complex interactions of multiple factors—includingtemperature, duration, and wood species—on the carbonization process.
This research providesimportant referencesfor sampling and interpreting charcoal indicators in archaeological and paleoenvironmental studies. It underscores thepotential applicationsof charcoal in both archaeology and paleoenvironmental reconstructions. The related findings have been recently published inJournal of Archaeological Research: ReportandFundamental Research.Doctoral studentGang Lifrom theSchool of Earth and Environmental Sciences, Lanzhou University, is the first author, withProfessor Guanghui Dongas the corresponding author.
Article links:
https://doi.org/10.1016/j.jasrep.2024.104963
https://doi.org/10.1016/j.fmre.2022.05.014
