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The combination of SEM, microchemical, XRD, micromorphological and other phosphate analyses for interpreting archaeological sites in Wallonia and Brussels (Belgium)

Publié le 20 décembre 2012
par Kai Fechner

Extrait du Bulletin of the archaeological micromorphology working group 1 (1997) : Round table III : phosphates (publication anciennement disponible sur le site internet : http://www.gre.ac.uk/ at05/micro/soilmain, fermé depuis).

Round Table I (Micromorphology and Phosphate) Chair - Jari Mikkelsen (Earth Science Institute, Gent) Reporter for Bulletin - John Crowther (Dept. Of Geography, University of Wales, Lampeter) 1st presenter - Jöhan Linderholm “The Umeå chemical method” (Dept. of Archaeology, Umeå University) 2nd presenter - Richard Macphail “Some results from Butser Ancient Farm and archaeological sites using the Umeå chemical method” (Institute of Archaeology, University College London) 3rd presenter - John Crowther 4th presenter - Kai Fechner "The combination of SEM, microchemical, XRD, micromorphological and other phosphate analyses for interpreting archaeological sites in Wallonia and Brussels (Groupe interdisciplinaire d’étude du paléoenvironnement,Université libre de Bruxelles).

The combination of SEM, microchemical, XRD, micromorphological and other phosphate analyses for interpreting archaeological sites in Wallonia and Brussels (Belgium)

Kai Fechner and Fabienne Kleiner

(anciennement : Groupe interdisciplinaire d’étude du paléoenvironnement Université libre de Bruxelles)

One of the main aims of our freelance research is to contribute to archaeological interpretation of pits and other dug structures on the archaeological sites found in rescue and program excavations. This research is executed at Brussels Free University and subsidized by the Walloon and the Brussels Regional Governments. In Middle Belgium, the sites are mostly situated on decalcified loess, rarely on secondary calcareous or tertiary non-calcareous clay. Many sites show greenish precipitations in or along the dug structures, especially from Roman times onwards, rarely in Neolithic or protohistoric contexts. The archaeological and palaeoenvironmental contexts sometimes seem specific enough to use these precipitations for the archaeological interpretation, just as we do with other artifacts.

By looking systematically at each sample of phosphate precipitation with a combination of SEM, microchemical, micromorphological, total phosphate and, when relevant, other analyses (chemistry, texture), it has appeared we might differentiate between different source materials of the phosphates. In many cases the interpretation of the archaeological structure and nearby activities is partly given by revealing the original presence of this material. The exploitation of a data basis of about 30 samples is still in its first stages and a number of interpretation problems still have to be solved.

The first example is a large pit in the Medieval rural habitat site of Tournai/Esplechin, excavated by the archaeological team of the Walloon Government. This site is situated at the westernmost edge of the Belgian line of the High Speed Train, close to the French border and on calcareous clay. In the pit, vivianite is associated with a very thick, clayey laminated basal deposit. It’s presence is only shown by X-ray diffraction. Other forms of phosphate revealed by SEM, microchemical and micromorphological analysis could have been formed later as they contain some calcium (leached from the top fill) and are not crystallized. The extreme acidity and the presence of big stones of the basal fill might be completing arguments for an interpretation as a flax retting pit. The top fill is calcareous and doesn’t contain any of the former characteristics.

A second example is a 13th century structure situated in the center of Brussels, in the site of the Marché aux Herbes. This is a rescue excavation carried out by Brussels Free University for the Brussels Government. A large concave pit is lined with a ca. 5 cm thick layer of homogeneous organic material. Strictly associated to this pit’s base we find spectacular accumulations of vivianite and pyrite (confirmed by different analysis). Siderite and iron precipitations are also present laterally outside this structure, the relative distribution of gypsum is not well determined. On the other hand, above the organic layer, small pieces of bone are found by SEM together with calcium-phosphates. The presence of numerous bark fragments and the strong pollution by vivianite and sulfates associated to the pit might be in favor of an interpretation as a tanning pit, but this remains hypothetical as a further study of the results is needed. The higher, calcium phosphate-rich levels are interpreted as butchery and shoe-making places by archaeozoological and archaeological data. The spatial and temporal variability of these precipitations clearly give us the evolution from a non-polluted soil to successive, well-dated stages of pollution of the wetland of the center of medieval Brussels.

At last, a large pit and the nearby ditch have been excavated by the team of the Archeosite of Aubechies for the Walloon Government in the site of Blicquy. This is a Roman rural sanctuary that includes a theater. Here it is a calcium-rich phosphate that is shown to be present by microchemical analyses and SEM in the lower fill of both structures. In thin-sections, this yellowish precipitation doesn’t differ from other types of non- or badly crystallized phosphates found in other sites (e.g. Quevy, Jollain-Merlin, Bruyelle). However, pieces of very weathered bones are also present regularly. All of these traces are found in a clayey laminated deposit. Both structures had almost vertical, probably wooden walls and the ditch seems to be linked to the border of the nearby theater. From the phosphate determination we conclude that an activity linked to some processing of bones might have produced this specific kind of rubbish that was then evacuated into the ditch and the pit. The idea that they served to drain cess away from the theater becomes very unlikely.

Bones in an alkaline soil and phosphate accumulations below a grave in an acid soil have been analyzed by the same means as reference materials (respectively Ottembourg and Rebaix). After analyzing a large number of samples with the same analytical techniques we find a clear subdivision between two main origins of archaeological phosphate in Middle Belgium :
- bones and their weathering forms ;
- organic materials and their weathering forms. The first is indicated by an association with weathered bone remains, the second by the presence of pure vivianite and, probably, other iron phosphates. The problem remains for isolated discoveries of phosphates of mixed and non-typical composition, in particular those with varying calcium content. For these forms we have to gibe an even greater attention to the study of Anne Gebhardt and Roger Langohr that concerned the site of Werken, close to our study area. On this site it can be seen particularly well how phosphates are affected by post-depositional evolution. A deeper insight into the literature on chemistry of the phosphate compounds will also be done to understand the sequence of formation and the micro-environment of the different phosphates revealed by our analyses. The full interpretation of the XRD should give a name to some of the mysterious iron and calcium phosphates.