Chiaroscuro and tenebroso
A gloss of popular discussions on the difference between Chiaroscuro and tenebrism in art reveals a broad non-consensus. Let's begin with a taxonomy of the terms, an ordering by genus and species. Chiaroscuro is genus. Tenebrism is species. That admits assertions such as, “every example of tenebrism in painting must also be an example of Chiaroscuro in painting.” And the complementary analogy, “not every example of Chiaroscuro in painting must, of necessity, also be an example of tenebrism.”
Implicit in any designation of a painting as “tenebrist” is the (unstated) premise that it is but one property of Chiaroscuro. Before defining what tenebrism is, it may be worthwhile to give an example of what it is not. For brevity, I won't give examples of specific works of art in particular. Instead, think of the primary meaning of Chiaroscuro as not—in itself—artistic.
Before nuanced art—when art was little more than charcoal smudges and rust stains—sentient humans gazed in awe at the drama of the weather, and at billowing, voluminous, late-Summer cumulus clouds, during the “Monsoon Season,” when thunderheads boiled to the Stratosphere. I imagine shepherds, such as those of Virgil's bucolic poems, with nothing to do but daydream, gazing in awe upon the spectacle
Those dreamers saw—or imagined that they saw—the figures of the gods and goddesses of pagan mythology in the clouds. The turbulence of cloud formation gave to these illusory figures animation, appearing and disappearing, in scenes of action and repose. And, most importantly for our purposes, they were not simple outlines of figures, but sensuously-modeled natural sculptural forms using a range of grays, from stark white, to the opposite—tenebrous—end of the scale.
The play of shadows viewed in clouds inspired the artistic representation of the figures envisioned by means of a simple mixture of charcoal dust, added to a binder of wet Quicklime (Calcium hydroxide), which combined to produce a scale of subtle differences of gray, and which mixture was then applied to white-washed walls al fresco. This gray modulation was the first appearance of Chiaroscuro in art.
Tonal painting was not originally dark. Too much charcoal mixed with a quicklime binder will not adhere for any length of time. The painting quickly falls-off and blows away. On a scale of gray from 1-10, in which 10 is black, and 1 is white, our charcoal-quicklime paint is limited for practical purposes to not more than 5. In practice this is faithful to the imaginary figures perceived in clouds, which never appear darker than 5 on a gray scale of one-to-ten.
Apollo, pagan sun god, ruled the day. Painting was originally conceived in, and for, the daylight. The gods of Mount Olympus were, therefore, the usual subjects of the 1-to-5 gray scale of blended tonal painting. Night subjects were typically depicted in starkly flat outline, as seen at night, against a background of firelight, for instance. Classical Greek black and red figure vases are famous examples of the outline style. A more nuanced scale of tonal gradation for rendering night subjects (tenebroso) would have to wait for the invention of oil painting.