Charcoal discovered during archaeological excavations is not only used for carbon-14 dating. Their study also makes it possible to identify wood species, reconstruct ancient environments, and better understand the practices of past societies.

This discipline, called anthracology, is based on the observation of the anatomical structures of charred wood. Discover how archaeological charcoal is analyzed in the laboratory and what information it provides to archaeologists.

What is anthracology and its role in archaeology?

Anthracology is a branch of archaeobotany specializing in the study of charcoal from archaeological contexts. It relies on microscopic observation of the anatomical structures of charred wood to identify plant families, genera, and, when possible, species.

The charcoal found during preventive excavations can originate from hearths, combustion structures, burnt building elements, or natural fires. Depending on their state of preservation and combustion conditions, charcoal can retain enough anatomical structures to allow for botanical identification.

However, analysis is not limited to simple recognition: it reveals human practices (choice of species, harvesting methods, combustion techniques) and the environments exploited by ancient societies.

Why is charcoal considered a precious remnant?

In the context of preventive excavations, charcoal holds a special place. It is both a witness to local paleovegetation and a material that can be used for carbon-14 dating (up to approximately 60,000 years).

In practical terms, anthracology helps archaeologists answer several strategic questions:

• What environments surrounded the excavated site? Charcoal helps to restore ancient vegetation and exploited woody resources.
  
• Which coals to choose for dating? Their nature, state of preservation, and archaeological context help in choosing relevant samples.
  
• How to avoid bias? Identifying the wood and the part of the tree used can help spot risks related to the "old wood" effect.

Anthracology alone does not allow for the reconstruction of an entire ancient landscape. It usefully complements other archaeobotanical approaches, such as palynology, which studies pollen, or carpology, which focuses on seeds and fruits.

In preventive archaeology, it thus contributes to better characterizing ancient environments, understanding the exploitation of plant resources, and guiding the selection of samples for carbon-14 dating.

How does an anthracological analysis take place in a laboratory?

Anthracological analysis begins with the preparation of charcoal surfaces, refreshed with a razor blade to make the anatomical structures more legible.

The samples are then observed according to the three anatomical planes of the wood: transverse, tangential longitudinal, and radial longitudinal. Examination of these different faces generally allows for the identification of the botanical family or genus, and sometimes the species.

Observations are made using a stereomicroscope (Olympus® SZ61 binocular loupe) and a dark-field metallographic microscope (Olympus® BX53M), under "natural" light (calibrated white light), coupled with digital cameras.

What anthracological criteria are analyzed?

Botanical identification is only one part of anthracological study. Specialists also examine several anatomical characteristics and alterations that provide information about the part of the tree used, the state of the wood before combustion, and the conditions under which it burned.

Identify the origin and the part of the tree used

• The presence of bark and pith: by simultaneously observing these two elements on a sample, we deduce the calibration of the stem.

• Reaction wood (characteristic of small branches or leaning trunks). This criterion, combined with a strong curvature, indicates that we are dealing with a small branch.

• Calibration assessment: analyzing the curvature of growth rings and the angle of woody rays will identify the part of the tree from which the charcoal originated. Charcoal is classified into 4 categories:

To determine the state of the wood before burning it.

• The presence of tyloses: tyloses form in the vessels of certain hardwoods during heartwood formation. The presence of tyloses helps us to determine potential species, but also to indicate the location of the sample.

• The presence of fungal hyphae: filaments can be observed in the vessels of charcoal. They indicate the development of fungi in the wood before its combustion, particularly during the degradation phases of a dead or dying tree. Their observation thus provides information on the state of the wood before its use.

• Degradation by boring insects or worms: similarly, the presence of galleries in coals is proof of an attack by boring insects or worms. It is possible to find the carbonized organisms in these galleries. These clues are proof that the wood was dead and rotten before its combustion. In some cases, the sapwood of a living subject can be attacked by such organisms.

Analyze the combustion conditions

• The presence of radial shrinkage cracks and vitrification: water-saturated wood will exhibit a large number of shrinkage cracks. Vitrification is a complex phenomenon that occurs during combustion. The aspects of vitrification depend on the nature of the wood (species, size, moisture content) and the combustion conditions (temperature and oxygenation of the firebox). We will distinguish 4 aspects of vitrification corresponding to 4 levels of carbonization:

Study of wood growth and its use

• The width of the growth rings and growth rate. A narrow width indicates that the wood grew slowly (unfavorable growth conditions or an old subject). Conversely, wider rings indicate greater growth, linked for example to favorable environmental conditions or a younger tree. Therefore, the growth rate, i.e. the regularity of the ring widths, indicates whether the subject grew uniformly or if there were events that temporarily hindered growth (climatic conditions, fungal attack, tree injuries, etc.).

• Woodworking traces: surface grooves or scratches can attest to work done on the wood using tools.

Archaeobotany and radiocarbon dating: why combine these methods?

Charcoal can be used to date an archaeological event through the measurement of carbon 14. However, the age of the wood analyzed does not always correspond to that of the event studied.

This is particularly the case with the "old wood" effect: charcoal from the heart of an ancient tree can be significantly older than the archaeological structure to which it is associated. Dating may then provide an age prior to the event being investigated.

Anthracological study helps to characterize fragments and select the most relevant samples, taking into account their nature and context. It does not eliminate all risks of error, but contributes to a better interpretation of the results.

Combining anthracology and radiocarbon dating thus allows for the cross-referencing of botanical and chronological information, in order to place the remains within their archaeological context.

Why are these observations important?

The observations made on the coals are compiled in tables and graphs, then presented in an analysis report.

Anthracological study thus makes it possible to better understand the plant resources exploited, the characteristics of the wood used, and certain combustion conditions. By cross-referencing this information with the archaeological context and, where relevant, with carbon-14 dating, specialists have complementary data for interpreting the remains.

Joey Montagut

Archaeobotanist, Joey graduated from the École Supérieure du Bois (Nantes – 2002). He specialized in the identification of archaeological and contemporary woods (Xylology) and charcoals (Anthracology).