The color of the sky and the atmosphere

Blue is difficult to conceptualize and is normally rarely found in nature. Without human intervention, there are few really blue plants on Earth, and blue-feathered birds live in small areas. The sky and the sea? The cavemen and the ancients were not easy to fool: the air of the sky and the water of the sea are not blue; they took samples with their hands, examined them and found no color. It is no coincidence that the symbolism associates the blue of the ether and the abyssal, the timeless, the unknown celestial universe, reserved for divinity.

According to recent research, prehistoric people did not perceive many colors. At first they could only distinguish black, white, red, later yellow and green, and blue was completely unknown to them. It is not known when mankind began to perceive and become aware of the color blue and where the first mentions appeared. In early antiquity there was no notion of blue: in Homer's Odyssey, the ocean is described as "a sea red as wine", and this was not a metaphor: it did not relate a battle scene and referred to the Mediterranean, which must be had about the same color as now.

The oldest known writings completely ignore the idea of ​​blue: the term does not appear in the old tablets of Ugarit (8th century BC), nor in the Bible (the word tehelet does not mean blue, as misinterpreted, but purple, pigment known in antiquity and which was extracted from the shells of shells in Israel and Lebanon) nor in the Qur'an.

The human eye perceives as blue any surface with a wavelength between 420 and 495 nanometers (the opinions of specialists differ, especially in terms of the lower threshold, which some will higher). Blue has the shortest wave of the primary colors and is perceived as the "coldest" color, as opposed to red, the "warmest" and the longest wavelength. The psycho-physiological effect of hot or cold is given by the fact that through the retina that perceives color our pulse is influenced: warm colors increase it a little, and cold ones lower it slightly, which physically creates the feeling of warming or cooling, of energizing. or relaxation.

In water, long-wave colors are absorbed and that is why white surfaces acquire shades of blue below a certain level, and the deeper we dive, the darker the water shades become.

The color of the sky and the atmosphere, blue gives depth to the image: it is the effect of the atmospheric (or aerial, as it is improperly called) perspective that makes the surfaces with more and more diffuse shades seem more distant, which tend to get closer to the color of the sky. background.

The first blue pigment was azurite, a natural mineral: basic copper carbonate. Its presence has been documented in ancient Egyptian and Japanese art, in Chinese murals dating back to the Ming and Sung dynasties, and was the most widely used blue during the Middle Ages and the European Renaissance. It began to be made artificially from the seventeenth century, and its use was slowed in the eighteenth century by the appearance of the spectacular Prussian blue.

The first modern artificially obtained blue pigment appeared by chance in Johann Jacob Diesbach's laboratory in Berlin in 1704, when the chemist was trying to obtain a new red pigment. He accidentally mixed potash with animal blood and instead of the blood taking on a more intense hue, a vibrant blue appeared. Prussian (or Berlin) blue began to be used as a pigment in 1724. Picasso used it exclusively in the works of the blue period, and Katsushika Hokusai created with him the Great Wave of Kanagawa and other works in the 36th series of views of Mount Fuji. .