From the ink to the pigment
In some cases the dissolution of the pigment may be as simple as boiling a tea (as in the case of mignonette), but it may also involve a number of complicated processing steps, as is the case with madder root. The pigments obtained in this fashion are extracts, completely dissolved out of their physical constituents. Their state is now that of an ink or a tincture. An ink allows you to colour, but cannot possibly constitute the basis for transparent layering effects.
When what you have is just a dissolved colour substance without a physical base, you cannot form superimposed colour layers. All in all, the plant chemist has had to disturb the equilibrium of the colourant in dissolving it.
And now, to obtain the pigment base, it is a matter of restoring it to the central zone of the physical-chemical spectrum. This recovery of centrality in the chemical process can be directly experienced by the senses. We begin with a transparent tincture, one that often still possesses a relatively unclear, fluorescent colour quality. With progressive neutralisation, accompanied by strong colour lighting and intensification, this gives rise at first to a slight milky turbidity.
As the colour quality continues to intensify an extremely fine colour pigment appears, still quite cloudy to begin with, and gradually settles on the base of the vessel. Through the action of the neutralisation agent aluminium salt, a mineral substance - clay - forms in fine crystals. The colour pigments are tied into this crystalline medium, and are now just as close to the light in their effect as they were in their former incarnation in the plant.