Solutrean and Magdalenian ferruginous rocks heat-treatment: accidental and/or deliberate action?
Résumé
Heating of prehistoric coloring materials can induce radical changes in color indicative of structural
matter transformation. For instance, the structure of the yellow iron oxide-rich mineral, goethite,
changes into the red iron oxide-rich mineral, hematite, when it is heated to around 250-300°C. For a
long time, heating has been thought to be the reason for the high frequencies of red rocks used in camp
sites and the red pigments in rock art paintings. However, records of heat-treatment of coloring materials
are usually not well documented; the contextual information is not clear enough to confirm intentional
heating. Two Solutrean camp sites (the flint workshop Les Maîtreaux and the hunting site Combe
Sauniere I) and one middle Magdalenian cave with rock art (Grotte Blanchard, La Garenne) allow us to
study the heating process of ferruginous rocks. All three sites, which have been excavated relatively
recently, have well-defined archaeological records and strong associations between the ferruginous rocks
and other artifacts. With the use of X-ray diffraction and electron m-diffraction for identifying structural
modification and SEM-FEG and TEM-FEG for detecting dehydration nano-pores, we have strong evidence
for intentional heat-treatment of yellow goethite-rich materials in two archaeological contexts and one
site for unintentional heating, where rocks were only partially transformed. Intentional heating to obtain
red hematite from primary goethite would have required ingenious methods of temperature control in
fireplace settings and purpose-built ground ovens.
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