Making archaeological charcoal visible

Scanning electronic image of an archaeological charcoal in the transverse fracture [Attribution: Alexa Höhn; Copyright: Alexa Höhn].

Making archaeological charcoal visible

Wood has been an important raw material since prehistoric times: digging sticks, spears and tool handles, shelters and roof constructions, spoons, bowls and furniture, but also sculptures and gaming figures, are just a few examples for objects that have been made of wood – and still are. And, wood is important as fuel – for cooking and heating. Within archaeological sites, wood remains provide information about the specific use of this resource, but it is precisely the remains that have found their way into settlements on a daily basis, as firewood, that provide valuable information about the environment and how it has been changed by climate and human impact. By tree-ring analysis and through radiocarbon dating, wood remains also contribute to the chronological classification of finds and sites.

As organic material, wood is decomposed by bacteria and fungi. Therefore, it only preserves – even through millennia – if these microorganisms cannot access it or if its structure is changed in a way that makes it unattractive for them. Wood charcoal is formed in anoxic burning environments, where pieces of wood do not burn to ashes, but instead their chemical composition is altered. Through the loss of its organic compounds, charcoals are unattractive as food for most microorganisms. At the same time, however, the microscopic structure of the wood is largely preserved. Therefore, it is possible even after millennia have passed to determine which trees and shrubs the charcoals originally came from. Because of this, charcoal is an important archive of information about the past for archaeologists. Within the priority programme “Entangled Africa”, research of the project Cultivated Landscapes is mainly dedicated to this find material.
Charcoal samples in the laboratory [Attribution: Alexa Höhn; Copyright: Alexa Höhn].
Achaeological charcoal sample [Attribution: Alexa Höhn; Copyright: Alexa Höhn].
Katalog der Holzkohletypen in der iDAI.objects
Catalog of charcoal types in the iDAI.objects [Attribution: Alexa Höhn; Copyright: DAI].
Now a first batch of photos and data of the project are available in open-access and make the (identification) work of charcoal research in tropical Africa visible in the truest sense of the word. In the iDAI.objects (Arachne) online available catalogue of the project, the first scanning electron microscope photos of archaeological charcoals from West African sites are currently aggregated. The focus is on the Mege site in Nigeria, with documentation of additional charcoal types and from other sites explored within the project to follow gradually. Their publication not only facilitates cooperation with other charcoal researchers, but the pictures of this unspectacular material are also very aesthetically pleasing.

Scanning electron microscopy makes it possible to take accurately focused photos of charcoal in a way that is hardly possible with a light microscope: The surfaces of the charcoal fragments are uneven, thus clear images rarely succeed due to the shallow depth of field. Even “focus stacking”, the digital compilation of photos from different levels of focus, does not always produce satisfactory results with charcoal fragments. Due to its great depth of field and higher resolution, the scanning electron microscope (SEM) allows the best documentation of archaeological charcoal fragments. These SEM images are part of most charcoal analytical publications, but space in printed publications is limited, even if online files are supported as supplements in journals. Thus, the selection is limited to the most beautiful and important images, while atypical or poorly preserved fragments are rarely illustrated.

With the catalogue of the project now started in iDAI.objects, it is possible to document the variability of charcoal types. In this way, colleagues all over the world can be involved in discussions about the definition and delimitation of different types of charcoal. The accompanying description of the characteristic features of each charcoal type in the catalogue explains how a clear delimitation on species, genus or family level is possible – and where difficulties and possibilities for confusion might exist. In this way, the material may be used as a reference in the laboratory and serves to ease the start of working with (West-)African materials for beginners. Moreover, with the integration of the catalogue in the future Entangled Africa Data Explorer, this basic work of charcoal research in tropical Africa will become easier to find and gain visibility.

However, the visibility of the analytical results based on these determinations, i.e. the counted proportions of charcoal fragments in the respective samples, also needs to be improved. A first step is the current input of anthracological results into the “Sahel-Sudan-Archaeobotany Database” in ArboDat 2018© (Lower Saxony Institute for Historical Coastal Research, Wilhelmshaven). The cross-cutting analysis of data from different sites in West Africa is simplified many times over by the availability of the results within a single database.
View of the datasets in the "Sahel-Sudan-Archaeobotany Database" [Attribution: Alexa Höhn; Copyright: ArboDat 2018© (Lower Saxony Institute for Historical Coastal Research, Wilhelmshaven)].
Datenpunkte im nordhemisphärischen Afrika in Neotoma. Screenshot am 21. Juni 2021 [Attribution: Alexa Höhn; Copyright: Neotoma].
Still, these data are not yet publicly accessible. This is where the integration of our own data into another online repository comes in: Neotoma, a worldwide palaeoecological database. Especially in the drier areas of West Africa, it is not easy to find pollen-bearing deposits, usually used for environmental reconstructions and inferences about former land use. Charcoal from archaeological excavations can provide information on woody vegetation and supplement pollen data. Mege and other sites with available archaeological charcoal data are not yet a point on the Neotoma Explorer map, but it should not be long now …
Only in cooperation with various projects within the SPP was it possible to make the archaeological charcoals and their potential visible. Huge thanks go to the teams of the Learning through connecting, DeGree and the coordination projects of “Entangled Africa” for their support!

Alexa Höhn

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