New kriteria for classification and differentiation between clay and iron oxide pigment sof various origins

Pigments containing iron, although they form a very large group, have never been considered very suitable for datation of color layers and identification of the origin of the painting, due to their abundant occurrence in nature, good availability and widespread use in all historical and pre-historical periods of time. In this paper we have verified that mineralogical composition of clay minerals in earthy pigments is a suitable tool for more detailed specification of material provenance and, in the case of ground layers, also the provenance of the artwork as such. It was obviously convenient to preferably use raw materials from a close and thus also cheaper source. For the purposes of statistical comparison we evaluated elemental composition of earthy grounds on 70 paintings from the 16(th)-18(th) centuries from Czech collections. We performed detailed mineralogical analyses for a selected representative number of grounds on 35 samples in total. We discerned 6 types of material in total 2 types of boles, 3 types of other earths, and one type representing iron-rich red from oxidized zones of hydrothermal ore deposits. We were able to distinguish between earthy pigments of Central European origin (coming from Czech and Bavarian locations) and those coming from North-Italian sources. Thus we were able to assign anonymous paintings to an appropriate place of origin