We use both organic and inorganic color pigments. The inorganic pigments are soil pigments and consist of metal compounds extracted from the soil and produced by crushing and processing stone material or by synthetic means. “Synthetic” means that the pigment has been altered in some way by human processes. Humans have been making color pigments for thousands of years, and the Egyptians, for example, made lead white color synthetically.
Examples of inorganic pigments include ocher, umber and terra di Siena. These pigments contain iron oxide and other metal oxides. Depending on a color’s oxide proportions to its other substances, it will have a shade from red to yellow to brown. Soil pigments are often found as both unburned and burnt; heating converts the yellow iron hydroxide content into red iron oxide. Inorganic color pigments are more stable and user-friendly in paint manufacturing.
Organic pigments consist of carbon compounds made from plant and animal materials or are extracted from substances such as petroleum. The organic material that provides color is processed so that the dye precipitates and is transferred to an inorganic substance, for example chalk, which can then be used as a pigment. Some examples of organic pigments are indigo (blue, from an East Asian shrub) and carmine (red, from the scale insect species cochineal).
The pigments we use come from different parts of the world, but above all, there are natural pigment soils in southern Europe. These pictures are from the pigment manufacturer Le Moulin à Couleurs (The Color Mill) in Ecordal in northeastern France.
Until the end of the 19th century, color pigments were often sold in the form of pieces or rods that the painter had to comminute, or break up, the pigment with a stone. Then the paint was pulverized on a flat stone slab, a grindstone, with the help of a muller, a cone-shaped stone with a flat bottom surface. Today, pigment is not bought in pieces; instead, it is delivered as ground dry pigment in powder form, and the manufacturer must grind the pigments further in a three-roll mill to distribute the grains evenly in linseed oil.