Loose pigments are used only in pastels...

Loose pigments are used only in pastels, which have to be protected by glass, since the color grains are captured by the roughened base of the flesh side of parchment or by the layer of size on paper, but remain as sensitive to touch as the colored dust on the wing of a butterfly. The butterfly’s brilliant colors are created by a system built up in layers, in which iridescent effects are produced by the interplay between the pigment and the various physical structures of the scales. Painters used to imitate this process from nature. Although there were one or two pigments they could obtain ready-made from the apothecary or from a monastery, they were also familiar with the material from which their paintings were made in its raw form. In front of their very eyes they had the metals, minerals, earths, plants, woods, bones, lice, shells, and snails needed, and they knew the processes by which to transform these into pigments. They knew which materials could be sublimated, calcined, smelted, eluted, ground, pounded, precipitated, boiled, dried, and distilled, and in their apprenticeships they had all spent endless hours grinding pigments with the muller against the slab. Compared with today, only a few colors were available—but the complex systems for using them in the fifteenth century make today’s techniques look like child’s play. We have forgotten what every butterfly “knows”: that the visual effect of colors is created by the interplay of tone and body. The color tone is one thing, and the shape and structure of the color are another. Each pigment has a different body, which refracts, reflects, and absorbs light in a different way. The blue tone of artificial ultramarine, produced industrially since 1830, is apparently indistinguishable from that of natural ultramarine, laboriously obtained from the semiprecious stone lapis lazuli. A glance through the microscope reveals the difference, which emerges so clearly in the painting that it is visible even to the untrained eye. The synthetic pigment, with its small, homogeneous round crystals, produces a uniformly consistent blue surface, while genuine ultramarine—the most expensive of all the pigments, with its large, irregular crystals of varying transparency and with naturally embedded particles of calcite, pyrite, mica, and quartz—appears like a glittering firmament. Above all, the transparent splinters of the calcite crystals embedded in the lapis lazuli sparkle like stars within the deep blue.
-- The Art of Arts (2001), Anita Albus (trans. by Michael Robertson)