Karakteristika og identifikation af okker - samt dets anvendelse som pigment i ældre skandinavisk helle- og kalkmaleri

Abstract

This thesis focuses on the material referred to as ochre. The aim of the project is to present the composition and formation of ochres and the analytical methods that can be used to identify them. Contextual examples of how it has been used as a pigment are introduced. One of the main purposes of this study is to form the foundation for a later master thesis. The subject was mostly studied through literature surveys. A case study of six presumed ochre samples from Sweden, Australia and a pigment cabinet at the Department of Conservation in Gothenburg was carried out using stereo and polarisation light microscopy, Scanning electron microscopy with energydispersive X-ray spectroscopy (SEM-EDX) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Ochres are often mixtures of iron containing minerals such as hematite and/or goethite and clay minerals. Hematite and goethite are distinguishable in, among others, colour, crystal form and crystallization. By using X-ray diffraction analysis, the crystallization can be determined. The formation of ochre is a result of the decomposition of rocks through chemical weathering. Hematite and goethite are favoured by specific environmental conditions. Dry and tropical climate supports formation of the former, humid and temperate climate of the latter. Characterization of the six case study samples is based on the literature, the examination and the analysis results. The two Swedish samples seem to contain both goethite and ferrihydrite and the four other samples presumably contain goethite, hematite or a mixture of goethite and hematite. Scandinavian Stone Age rock art motives are usually animals or humans painted in red with animal fat as the binding media. Goethite is yellow in colour but is the one favoured by the Scandinavian climate. Hematite is red and can be obtained by heating goethite higher than 260°C. A processing of the original earth pigment before application on the rock surface would therefore be likely during the Scandinavian Stone Age. When it comes to the Scandinavian frescos/seccos, ochre has been used as the main pigment in fresco painted parts due to its stability in alkaline environments. The binding medium can be calcium oxide and the pigment is applied to both wet and dry surfaces.