Master thesis
The following text is giving a summary of my master thesis about Mineralogy and geochemistry of earthy pigments ad their identification in paintings, which was defended in 2005. Whole work is available here. Please contact me whenever You have any questions or in a need of the translation. Proper citation of the work is: Bláhová, A., 2005. Master thesis: Mineralogy of Fe-rich pigments from czech region and its identification in paintings (in czech: Mineralogie železitých hlinek z českých lokalit a jejich identifikace v malbě). Charlese University in Prague, Faculty of Sciences, Institute of Geochemistry, Mineralogy, and natural resources, Prague, 67 pp.
Part of the work will be published in special issue of Applied Clay Science.
SUMMARY
Natural mineral pigments are a marginal topic of contemporary geological sciences. The pigments are interesting rather for restorers and suppliers of artist’s pigments, who focus mostly on technical parameters of the pigment behaviour in binders etc. There are only several geological reports on Czech pigments, and a what is completely missing is a general survey of their mineralogical composition and their classification to groups of quality corresponding to European historical pigments. The lack of information on the Czech pigments has already been noticed by Seifert (1991) and Konta (1995). The classification of mineral ferric pigments in terms of mineralogical-chemical composition and the clues to their provenance are the main aims of this diploma thesis.
In this work, 55 samples of ferric pigments were studied. The samples were collected in the accessible Czech localities of mineral pigments with past mining documented in literature. The set of samples was compared to 40 reference materials from the currently commercially available production, which mostly represents the historically documented European localities. Several traditional (X-ray powder diffraction, chemical analysis, atomic absorption spectroscopy) as well as less common techniques (voltammetry of microparticles, diffuse reflectance electron spectroscopy, measurement of magnetic susceptibility) were used for a complex study of the samples. The most careful attention was paid to the nature of the colour agents in the samples, even if they were present in the concentration below the detection limit of the X-ay diffraction analysis (low concentration of ferric hydroxyoxides, magnetite, and manganese oxides). The main colouring agents in the samples studied are goethite, jarosite (yellow hues), hematite (red hues), poorly crystalline goethite, ferrihydrite (ochreous to brownish hues), and celadonite, glauconite, and smectite (green hues). The resulting colour is – beside the main colouring agents – affected by particle size and also by further mineral admixtures (magnetite, Mn-oxides). In the case of brownish and dark brownish hues of natural mineral pigments, denoted assienas and umbras, an admixture of manganese oxides is reported to be responsible for their dark hues, however, in the samples studied, the brownish hue was more frequently caused by poorly crystalline ferric hydroxyoxides. There were a lot of other, non-coloured phases identified in the pigments, most frequently clay minerals (kaolinite, smectite), quartz, calcite, dolomite, gypsum, feldspars, and micas.
The study of the green earths was a continuation of previous work on Kadaň green earth, and novel results were obtained. In newly received samples, originating from the period of mining in the early 20th century, a presence of celadonite was confirmed, beside the interstratified celadonite-smectite. The best available localities of ferric pigments were found in Hořenec (first-class kaolinite-hematite pigment rich in anatase, with the composition close to the bole grounds of Baroque paintings), and Dobříč, Senec, Zliv, and Horní Lukavice, which are closely resembling French ochre (containing goethite, kaolinite, mica, and quartz). Some typical historical materials, namely umbras and calcium sulphate type of ochres have not been available in the Czech Republic. The resulting knowledge was used to identification of pigment materials in 4 microsamples of different baroque bole grounds. At least in one case the material used was not of the Czech origin but it was rather of the Italian provenance, and two samples could possibly represent typical Czech ochres.
