Study by X-ray microanalysis of 155 coloured grounds from French paintings of the seventeenth and eighteenth centuries allowed us to establish the nature of their constituents. The colour was introduced by three varieties of pigment, brown earths, ochres and iron oxides which were used in combination with calcium carbonate, lead white and minium. This confirms the recipes mentioned in the old treatises. The presence of barium sulphate, unexpected in this period, is connected with the nature of the coloured pigments and the place of execution of the works.