Analysis of earthy pigments in grounds of Baroque paintings.

Sixteen samples of orange-red and yellow Fe-oxide earthy pigments mainly from bole grounds of Baroque paintings were studied by elemental and phase analysis and voltammetry. Fe, K, and Ti content were found to be suitable for further classification of those earthy pigments. According to the chemical and phase composition the yellow grounds were natural yellow ochres formed by intense chemical weathering in a moderate climate. Very similar phase and elemental composition of part of the orange-red boles indicated their similar geological origin or even their formation by calcination of yellow ochres. Part of the orange-red boles differed significantly from the yellow boles, especially in their increased content of Ti, indicating their relation to end products of intense weathering, e.g. laterites formed in a tropical climate. Analogous materials with a correspondingly large Ti content are not currently commercially available and their geological origin (provenance) is not clear. In several orange-red boles the intentional addition of rather coarse-grained haematite to natural ochre by the painters was assumed on the basis of Fe oxide content and crystallinity.