Ochre (rarely spelled ocher and often referred to as yellow ochre) is one of a variety of forms of iron oxide which are described as earth-based pigments. These pigments, used by ancient and modern artists, are made of iron oxyhydroxide, which is to say they are natural minerals and compounds composed of varying proportions of iron (Fe3 or Fe2), oxygen (O) and hydrogen (H).
Other natural forms of earth pigments related to ochre include sienna, which is similar to yellow ochre but warmer in color and more translucent; and umber, which has goethite as its primary component and incorporates various levels of manganese. Red oxides or red ochres are hematite-rich forms of yellow ochres, commonly formed from aerobic natural weathering of iron-bearing minerals.
Prehistoric and Historic Uses
Natural iron-rich oxides provided red-yellow-brown paints and dyes for a wide range of prehistoric uses, including but in no way limited to rock art paintings, pottery, wall paintings and cave art, and human tattoos. Ochre is the earliest known pigment used by humans to paint our world--perhaps as long ago as 300,000 years. Other documented or implied uses are as medicines, as a preservative agent for animal hide preparation, and as a loading agent for adhesives (called mastics).
Ochre is often associated with human burials: for example, the Upper Paleolithic cave site of Arene Candide has an early use of ochre at a burial of a young man 23,500 years ago. The site of Paviland Cave in the UK, dated to about the same time, had a burial so soaked in red ochre he was (somewhat mistakenly) called the "Red Lady".
Natural Earth Pigments
Before the 18th and 19th century, most pigments used by artists were of natural origin, made up of mixtures of organic dyes, resins, waxes, and minerals. Natural earth pigments like ochres consist of three parts: the principle color-producing component (hydrous or anhydrous iron oxide), the secondary or modifying color component (manganese oxides within umbers or carbonaceous material within brown or black pigments) and the base or carrier of the color (almost always clay, the weathered product of silicate rocks).
Ochre is thought generally to be red, but in fact is a naturally-occurring yellow mineral pigment, consisting of clay, siliceous materials and the hydrated form of iron oxide known as limonite. Limonite is a general term referring to all forms of hydrated iron oxide, including goethite, which is the fundamental component of the ochre earths.
Getting Red from Yellow
Ochre contains a minimum of 12% iron oxyhydroxide, but the amount can range up to 30% or more, giving rise to the wide range of colors from light yellow to red and brown. The intensity of color depends on the degree of oxidation and hydration of the iron oxides, and the color becomes browner depending on the percentage of manganese dioxide, and redder based on the percentage of hematite.
Since ochre is sensitive to oxidation and hydration, the yellow can be turned red by heating goethite (FeOOH) bearing pigments in yellow earth and converting some of it to hematite. Exposing yellow goethite to temperatures above 300 degrees Celcius will gradually dehydrate the mineral, converting it first to orange-yellow and then red as hematite is produced. Evidence of heat-treatment of ochre dates at least as early as the Middle Stone Age deposits in Blombos cave, South Africa.
How Old Is Ochre Use?
Ochre is very common on archaeological sites worldwide. Certainly, Upper Paleolithic cave art in Europe and Australia contain the generous use of the mineral: but ochre use is much older. The earliest possible use of ochre discovered so far is from a Homo erectus site about 285,000 years old. At the site called GnJh-03 in the Kapthurin formation of Kenya, a total of five kilograms (11 pounds) of ochre in more than 70 pieces was discovered.
By 250,000-200,000 years ago, Neanderthals were using ochre, at the Maastricht Belvédère site in The Netherlands (Roebroeks) and the Benzu rock shelter in Spain.
Ochre and Human Evolution
Ochre was part of the first art of the Middle Stone Age (MSA) phase in Africa called Howiesons Poort. The early modern human assemblages of 100,000-year-old MSA sites including Blombos Cave and Klein Kliphuis in South Africa have been found to include examples of engraved ochre, slabs of ochre with carved patterns deliberately cut into the surface.
Spanish paleontologist Carlos Duarte (2014) has even suggested that using red ochre as a pigment in tattoos (and otherwise ingested) may have had a role in human evolution, as it would have been a source of iron directly to the human brain, perhaps making us smarter. The presence of ochre mixed with milk proteins on an artifact from a 49,000-year-old MSA level at Sibudu cave in South Africa is suggested to have been used to make the ochre liquid, probably by killing a lactating bovid (Villa 2015).
Identifying the Sources
The yellow-red-brown ochre pigments used in paintings and dyes are often a mixture of mineral elements, both in their natural state and as a result of deliberate mixing by the artist. Much of recent research on ochre and its natural earth relatives has been focused on identifying the specific elements of a pigment used in a particular paint or dye. Determining what a pigment is made up of allows the archaeologist to find out the source where the paint was mined or collected, which could provide information about long-distance trade. Mineral analysis helps in conservation and restoration practices; and in modern art studies, assists in the technical examination for authentication, identification of a specific artist, or the objective description of an artist's techniques.
Such analyses have been difficult in the past because older techniques required the destruction of some of the paint fragments. More recently, studies that use microscopic amounts of paint or even completely non-invasive studies such as various types of spectrometry, digital microscopy, x-ray fluorescence, spectral reflectance, and x-ray diffraction have been used successfully to split out the minerals used, and determine the type and treatment of the pigment.
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- Cloutis E, MacKay A, Norman L, and Goltz D. 2016. Identification of historic artists' pigments using spectral reflectance and X-ray diffraction properties I. Iron oxide and oxy-hydroxide-rich pigments. Journal of Near Infrared Spectroscopy 24(1):27-45.
- Dayet L, Le Bourdonnec FX, Daniel F, Porraz G, and Texier PJ. 2015. Ochre Provenance and Procurement Strategies During The Middle Stone Age at Diepkloof Rock Shelter, South Africa. Archaeometry:n/a-n/a.
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- Moyo S, Mphuthi D, Cukrowska E, Henshilwood CS, van Niekerk K, and Chimuka L. 2016. Blombos Cave: Middle Stone Age ochre differentiation through FTIR, ICP OES, ED XRF and XRD. Quaternary International 404, Part B:20-29.
- Rifkin RF. 2012. Processing ochre in the Middle Stone Age: Testing the inference of prehistoric behaviours from actualistically derived experimental data. Journal of Anthropological Archaeology 31(2):174-195.
- Roebroeks W, Sier MJ, Kellberg Nielsen T, De Loecker D, Pares JM, Arps CES, and Mucher HJ. 2012. Use of red ochre by early Neandertals. Proceedings of the National Academy of Sciences 109(6):1889-1894.
- Villa P, Pollarolo L, Degano I, Birolo L, Pasero M, Biagioni C, Douka K, Vinciguerra R, Lucejko JJ, and Wadley L. 2015. A Milk and Ochre Paint Mixture Used 49,000 Years Ago at Sibudu, South Africa. PLoS ONE 10(6):e0131273.