Identification of adulterants in artistic earth pigments using a multi-technique approach

The composition of natural earths from Kremer and others bought in different art shops in Lima was studied to determine their authenticity. They were analyzed by Raman Microscopy, X-Ray Diffraction, X-Ray Fluorescence, and Light reflectance. The results show that Kremer pigments have a natural origin, as they are rich in minerals and clay. By contrast, the pigments bought in Lima are synthetic and contain adulterants. One manufacturer used Cadmium-based pigments for adulteration in Umbers and Siennas, probably to obtain a color similar to the original. A second manufacturer used talc as a filler for adulteration in Sienna and Ochre. The third brand is probably a case of re-packaging construction pigments as art pigments. Manufacturers should label these pigments as Mars instead of Earth pigments, and their actual composition should be known prior to their use.