Ochre

Ochre is a diverse family of natural clay earth pigments.

These pigments are composed of ferric oxide mixed with varying amounts of clay and sand. Because the chemical mixtures vary, the colors range from pale yellow to deep orange or brown. The term "ochre" is also used to describe the specific light brownish-yellow color produced by these pigments. Understanding ochre is important because it connects the chemistry of the Earth to the history of human culture and art.

The color of ochre is determined by specific mineral compositions. The major ingredient in most ochres is iron(III) oxide-hydroxide, which is known as limonite. This mineral provides the characteristic yellow color found in yellow ochre, also called gold ochre.

When the iron oxide is dehydrated, it becomes hematite. This mineral gives red ochre its distinct reddish tint. Other minerals can change the hue further. For example, sienna contains limonite plus less than 5% manganese oxide. Umber pigments contain a much higher proportion of manganese, between 5% and 20%, resulting in a dark brown color.

There are several distinct types of ochre within this mineral family. Yellow ochre is primarily composed of hydrated iron(III) oxyhydroxide. Red ochre relies on the mineral hematite for its color. Brown ochre is made of goethite, which is a partly hydrated iron oxide. There is also a rare variant called purple ochre. Chemically, purple ochre is identical to red ochre. However, it appears as a different hue because of light diffraction. This happens because purple ochre has a greater average particle size than red ochre.

Archaeological evidence shows that humans have used ochre for hundreds of thousands of years. In Africa, evidence of red ochre processing dates back to around 300,000 years ago. This period coincides with the emergence of Homo sapiens. In Australia, use of the pigment is dated to 50,000 years ago. New research has also found evidence of its use in Asia dating to 40,000 years ago.

These findings suggest that ochre played a role in the cognitive and cultural evolution of early humans.

The significance of ochre is seen in how prehistoric people selected and moved it. They did not just pick up any clay; they chose specific pieces based on color, texture, and hardness. In Southern Africa, deep red-iron ochres were transported over very long distances. This occurred even when local deposits were available. Such movement implies that specific pigments held unique cultural or ritual importance. It also suggests that early long-distance exchange networks existed during the Middle Stone Age.

Ochre has been used in many famous historical contexts. In the Lascaux cave in France, an image of a horse was colored with yellow ochre approximately 17,300 years ago.

In Ancient Egypt, yellow ochre was used extensively in tomb paintings. Egyptians associated the color yellow with gold, which they believed was eternal and indestructible. In these paintings, women were often depicted with yellow ochre or gold faces. Red ochre was also used as a rouge or lip gloss for women.

In Ancient Greece, red ochre was called miltos. In Athens, public slaves used ropes dipped in miltos to sweep the Agora. This was a way to mark citizens who loitered, as the paint would stain their clothes.

Beyond art, ochre has served many practical and medicinal purposes. In Ancient Egypt, ochre clays were used medicinally, as noted in the Ebers Papyrus from about 1550 BC. Some modern tribal peoples use ochre to treat animal skins or as an insect repellent. It can also be used to staunch bleeding or provide protection from the sun. In England, a version called ruddle is used by farmers to mark sheep. This shows how a single mineral family can serve as art, medicine, and a tool for animal management.