Natural
These are pigments not made from the tissues or chemical residues of animals or plants. They can be further divided into those which are natural and those which are man-made. The natural are a simple matter to discuss. All our predecessors did was to dig them out of the ground or from rock faces. In most instances these were dry ground with a pestle and mortar until they made a fine powder. In the case of Lapis Lazuli (the original version of Ultramarine Blue) excessive grinding simply causes the blue colour to disappear! There are many books which tell romantic stories of the mystique which, sometimes inappropriately, surrounds these pigments. Vermilion, for instance, in its natural form was called Cinnabar, a granular terra-cotta-like mineral which was gathered by shooting arrows at seams exposed in cliff faces.
We live in a modern world where the word ”natural” is normally associated with superior quality. Yes, you could take a natural earth as did the colour makers of the Renaissance and laboriously grind it, but the result, unromantically, may well be unstable physically, and contain such impurities as to make it colorifically impermanent. To be honest, as an artist’s colourman, I find the natural earth pigments a nightmare to use in making paint, and if possible I avoid them! But for Terre Verte and Raw Sienna there are no manufactured substitutes. Both have been micronised to give them consistency. I find that Terre Verte is very stable, in that adverse changes in consistency can be avoided after a few days. But Raw Sienna is a different matter: not only does it contain “free radicals” (other elements) which are wild cards awaiting something to react with, but it has a tendency sometimes to flocculate, which means that particles, once evenly dispersed, might later regroup like miniature magnets and form lumps. You have been warned!
Manufactured
Unsurprisingly, paint makers, over thousands of years, have attempted to regularize supplies of stable pigments by initiating chemical processes (until quite recently, without fully understanding them) which create more reliable versions of such “found” materials. A good example of this is Lead Carbonate, which is a naturally occurring chemical, but which has been made for centuries by the “stack” process, and is of course the constituent pigment of the uniquely handling Lead Whites. Most of the earth colours are in fact mixtures of Iron Oxide and Manganese or Magnesium salts, and modern chemistry allows these to be manufactured to the highest standards of consistency and purity. As such chemical knowledge developed, other metal compounds were found to be useful. Between 1780 and 1900 chemists progressively discovered that Zinc, Chrome, Cobalt, Manganese, Titanium and above all Cadmium based compounds provided the basis for some of the most brilliant and durable colours you will find on your palette today. I find that man-made inorganic pigments are a dream to make paint from - and to paint with. The only disappointment was the Lead Chromates (not to be confused with Chromium Oxide) which have been abandoned due to their tendency to darken and to react with other colours. The rest have been unqualified successes.