Six different pieces of gothic wooden panels or sculptures and baroque canvas paintings were chosen to identify the content, mineralogy and crystallinity of clay minerals present in pigments of original grounds and painting layers. Electron microscopy and microanalysis, X-ray diffraction and electron diffraction have been used to investigate minute heterogeneous material fragments. Reference commercial clay-rich pigments have also been measured. The greyish basal layer of some chalk grounds of gothic paintings and sculptures is more siliceous than expected with only a low content of clays. Baroque bole grounds are rich in clays, they contain predominantly kaolinite, mostly well-ordered; illite is common, smectite was enriched especially in the ground of a Baroque painting by Ch.A. Coypel.