Using FTIR-ATR, analytical colour and mercury for unravelling the cremation ritual of Tyresta Viking Age burial mound (South-Central Sweden)

The funerary rituals in Viking Age Scandinavia are known by their complexity and diversity including inhumation, boat burials, and cremation. Cremations have been extensively studied macroscopically, but the application of geochemical techniques, although highly informative to investigate cremation in more detail, had not been widely explored yet. In the Viking Age the inclusion of both animals and humans on the funeral pyres makes its research challenging. In the present study, we demonstrate the usefulness of molecular analysis (FTIR-ATR), direct mercury analysis (DMA) and analytical colour (in the CIELab space) for the characterization of Viking Age cremated remains of a human individual buried with dogs, a horse, a cat and other animals – as well as evaluate naked-eye methods for assessing the temperature of firing. We demonstrate that the spectroscopical signal is highly correlated with analytical colour parameters, and with mercury concentration (that still present even in bones exposed at high temperatures). Mercury concentration was higher in human bones than animals, suggesting an ante-mortem pollution. Human bones were probably heated at temperatures lower than 1100°C but higher than 900°C, while animals were cremated at lower temperature. We also identified a secondary cremation in a layer previously interpreted as a Badger burrow. A new MIR-index, _T PV, is proposed, and we suggest that the cyanamide band is related to oxygen availability during burning. Regarding naked-eye colour method, we support the splitting into two categories the white colour degree regarding the porosity of the bone, to better approach higher temperatures.