Working with natural pigments may offer a renaissance in our understanding of color language. This article discusses what the unique properties of natural mineral pigments can offer to contemporary painters.
Old Alchemists will be my witnesses, who have never either by chance or by experiment succeeded in creating the smallest element, which can be created by nature.
—Leonardo da Vinci, 1508 
Thanks to developments in chemistry over the past 300 years, painters today have hundreds of pure and permanent colors from which to choose. Although fewer pigments were available to painters of Medieval Europe, they had minerals, earth, plants, bones, shells, and insects, and they knew how to transform these into pigments. The question arises whether the extensive range of modern synthetic pigments provides artists of today with anything superior to the natural and artificial pigments used by Medieval and Renaissance masters.
Although they spent countless hours grinding pigments with muller against a slab, what Medieval and Renaissance painters learned about these materials became invaluable later when they created the masterpieces of their time. The direct experience gained with these materials while grinding away we have primarily lost—the visual effect of colors is dependent upon the physical characteristics of the pigment. Each pigment has different characteristics that reflect and absorb light differently.
During the past few centuries, three categories were developed to describe a color in white light—hue, saturation, and tone. Hue is the attribute of colors that permits them to be classified as red, yellow, green, blue, etc. Saturation refers to the purity of colors or freedom from dilution with white. Tone is the amount of black present in the hue. These attributes describe the visual appearance of color, which is the dominant wavelength of light observed in transmitted or reflected light. To truly grasp the language of color, an artist must understand the chroma  pigments they use in painting.
Color Is Also Dependent Upon Particle Size and Shape
A pigment's chroma depends upon its particle size and shape and the absorption characteristics of its chemical constituents. The size of a pigment particle varies considerably from less than 10 micrometers (or microns μ), comparable to milled flour, to over 100 micrometers, which corresponds to fine sand. To make a comparison, most artificial pigments, such as cobalt blue and ultramarine, have a particle size of less than 1 micrometer. In contrast, larger particles of azurite, which these synthetics have displaced mainly on the artists’ palette, vary considerably between 1 to 120 micrometers.
Heating cobalt chloride and aluminum chloride together makes cobalt blue. This chemical reaction produces particles of unusual fineness and uniformity. On the other hand, the natural pigment azurite is prepared by crushing samples of the mineral obtained from copper ore deposits. When the mineral azurite is crushed, aggregates of copper carbonate crystals are shattered into tiny grains. This results in individual particles of irregular shapes and sizes. Not only do the particles of azurite vary in size and shape, but also their composition. The mineral may contain inclusions, which are small amounts of other minerals, such as malachite, in the case of the mineral azurite. As a result, azurite will reflect and transmit light in other areas of the spectrum, such as red, green, and yellow. Compared to cobalt blue, which attains a purity rarely found in nature, azurite appears more chromatically intense because it reflects a broader part of the spectrum than its synthetic counterpart.
The Importance of Luminous Grounds
Another factor concerning a pigment's chroma is the grounds' luminosity. Light passing through layers of paint to the ground is refracted or bent by particles of pigment and paint binder. The amount of refraction depends upon the crystal structure and transparency of the pigment particles. It is also influenced by the paint binder's chemical composition surrounding the pigment particles. The larger and more irregular the particles, the more light can pass through the paint film to the white ground and be reflected from it (compare the schematics in the illustration below).
How particle size and shape affect a pigment's apparent color is demonstrated in this schematic, showing a comparison of synthetic cobalt blue and the natural mineral azurite.
To illustrate the effect particle size, structure and composition have on color; let’s use a typical example from the dye industry. If velvet, silk, cotton, and linen are dyed identically, they are described as having the same color. However, light falling on the different textures imparts each with a different appearance. The same phenomenon is involved in the differences between synthetic and natural pigments. Although modern pigments have a variety of particle sizes and shapes, these have far simpler shapes and are smaller and more homogeneous than natural pigments.
Pigments Formulated for Mass Production
As products of the chemical industry, synthetic pigments are defined quantitatively. Their advantages mainly serve the dye and paint industries, in which producing paints for artists plays a minor role. Modern pigments are formulated to improve color nuances, brightness, and stability in paint without concern for their chromatic intensity. To achieve maximum desirability in paints today, pigments are made more homogenous in shape, size, and composition. For example, particle sizes are reduced to the smallest possible to increase the covering power of a pigment. The smaller the particles, the more the color nuances of the pigment are absorbed into its primary hue, as in inks with no texture. Particles more consistent in shape and size also tend not to settle quickly and separate from their binder once inside a paint bottle or tube. This increases the shelf life and, thereby, marketability of paint but reduces its effectiveness as a color for artists’ use. As Anita Albus wrote in Art of Arts, “The result is not perfection, but sterility.” 
Synthetic pigments in dry powder form available from most artists' supply retailers are purchased from the same sources used by paint manufacturers. It is ridiculous for artists to use the same pigments used by industry to commercially mass-produced products. Isn’t it strange that painters are one of the few artistic groups to succumb to economic pressures and use such ready-made pigments? Even in the applied arts such as cooking, it is customary for the artists to produce their juices, even though commercially available extracts have been around for over a hundred years.
Whereas modern pigments offer certain advantages to today’s painters, many things to learn from their predecessors can enhance artists’ comprehension of color language. Natural pigments that have proven stable over time may well provide the basis for a new renaissance in the visual arts.