Preparation of red prehistoric pigments by heating

Goethite (alpha-FeOOH) and hematite (alpha-Fe2O3) were the first pigments o f art history, probably because of their intense colour, respectively yello w and red, and because of their natural abundance. Together with manganese oxides and charcoal, chosen for their black colour, they constituted the pr ehistoric painter's palette. Beautiful examples of their art can still be a dmired in decorated caves. A well-known property of goethite is that, when heated at 250 degrees C or 300 degrees C, it dehydrates and transforms into hematite, with of course the corresponding colour change. This fact was me ntioned as early as Antiquity by Roman painters, who used it as a "recipe" to obtain red pigment and different hues, from yellow orange to deep purple . Ochre factories at the beginning of the XXth century employed this method to make red pigment from goethite. No written document is available to kno w if Prehistorics discovered this property and used it. Archaeological fact s, as for instance iron oxides associated to fireplaces, tend to demonstrat e that they mastered this technique, but so far no reliable evidence has be en provided. The purpose of this work was therefore to search for character istic features enabling to distinguish between natural hematite and heated goethite, so that these archaeological assumptions could be confirmed. The methods we chose are powder X-ray diffraction (XRD) and transmission electr on microscopy (TEM). They are complementary, since XRD provides structural global information, while TEM provides morphological information at a nanom etric scale. To begin with, experiments were carried out on synthetic goeth ite as a reference. Then archaeological hematite samples excavated in a Pyr enean prehistoric site, the "grotte-abri du Moulin" in Troubat (where Magda lenian and Azilian inhabitants successively dwelt), were examined. All thes e data enabled us to set up a "strategy" that can be applied to any hematit e sample in order to determine its natural or artificial origin. The method has been adapted as well to tiny quantities and allows the examination of parietal paintings samples.