Yellow, brown and red iron oxide -- pigments of natural or synthetic origin cannot be determined from the small samples which can be taken from paintings with the same accuracy as is possible with infrared-absorption spectrography, using micro samples. The kinds of clay minerals (kaolinite, montmorillionite, nacrite, illite, etc.) which are constituents of earth minerals can be determined, with the aim to classify ochres from different origins used in painting. 24 spectra of iron oxide pigments are illustrated and discussed. -- AATA