Scandinavian Rock Art Pigments and Their Preparation - A Pilot Study on the Use of SCiO in Heritage Science

Abstract: This Master’s thesis focuses on firing, aging and provenance of ochres and ochreous soils. It investigates which preparation methods are most significant as separation factors when near infrared (NIR) spectra of prepared pigments are statistically processed. It is performed from a rock art and heritage science perspective. One of the aims of this project is to serve as a pilot study on the application of SCiO®- a commercial, pocketsize NIR spectrometer - for pigment examinations. An experimental research outline sheds light on the studied objects: six soil samples from Denmark and Sweden and three artistic pigments were fired at 300°C, 600°C and 900°C for different periods of time. The pigments were applied on a rock surface with water and blood as binding media. One sample was prepared for natural outdoor exposure. All samples were analysed with SCiO. Principal component analyses (PCA) and cross-validation models based on SCiO-spectra were used to find patterns of statistical separation. Some samples wer complimentarily analysed with scanning electron microscopy-energydispersive X-ray spectroscopy. Literature studies were inspirational for the experimental setup and enable interpretation of the results. The Swedish Tumlehed rock painting is included as a case study where the results from the experiment are related back to the rock painting. SCiO’s spectral range is limited (700-1100nm) but some information can still be derived from the spectra of prepared pigments. PCA-plots and models show that firing temperature can separate samples from the same location. Samples can, to some extent, be ascribed to main provenance groups (‘Denmark’, ‘Sweden’ or ‘other’) regardless of heating temperature; although, firing is necessary. Iron content differences between Swedish samples are plausible explanations for PCA separation. Age (or exposure) seems to make some separations, also en terms of the type of binding media. When making a PCA-plot with SCiO-scans from Tumlehed rock painting, a noticed separation is visible between water related figures and a deer figure. Suggestions for this separation are either that the pigments have been fired at different temperatures (or same temperature for different time periods) or that the wavy patterns and a fish have been painted with pigment sourced from an aqueous environment (such as a stream) whereas the pigment for painting the deer is derived from ochreous soils. Samples set size, and the limitations of SCiO and the software that is supplied with the SCiO most definitely have negative impact on the results. SCiO is, without calibration, mostly suitable as a screening tool.