Understanding Ocher: From Earth to Artist's Palette

Ocher is perhaps the most widespread pigment in an artist’s palette and has been known since prehistoric times. Despite its familiarity, defining ocher precisely can be challenging. Is it a type of earth, a mineral, an iron ore, a pigment, or simply a color? And when referring to its color, do we mean yellow, red, brown, or orange? Clearly, the term "ocher" holds multiple meanings. Let's explore them step by step. 

A yellow ocher paint swatch

A red ocher paint swatch

Geological Perspective

While commonly associated with iron minerals, ocher has a broader geological definition. It refers to the colorful crusts formed on metalliferous ores as they decompose. Typically, ochers appear as fine-grained oxidation layers or crusts but can also occur as soils or even form entire rocks. Chemically, ochers are mixtures of metal oxides (products of oxidation) and hydroxides (products of interaction with water) with diverse compositions and appearances. Beyond iron ochers, other notable types include:

• Lead ocher (plumbic ocher): Mainly composed of lead oxides such as massicot and litharge, displaying vibrant colors from yellow to orange and red.
• Antimony ocher: Consists of oxides and hydroxides of antimony (stibiconite, cervantite), forming subdued shades from pale yellow to brown.
• Molybdic ocher: Contains molybdite or ferromolybdite, exhibiting colors from pale yellow to brown and occasionally greyish blue.
• Tin ocher (varlamoffite): Consists of tin oxides/hydroxides mixed with iron oxides/hydroxides, typically yellow.
• Tungstic ocher (tungstite, ferritungstite): Hydrated tungsten oxide producing bright yellow colors ranging from greenish-yellow to gold.
• Bismuth ocher: Includes oxides and carbonates like bismite or bismutite, varying in color from greenish to yellow and brown.
• Uranic ocher (gummite): Made of uranium oxides-hydroxides, typically colored yellow, orange, or brown.

However, oxidation isn't the only process leading to colorful crust formation on ores. Ocher-like formations with different chemical compositions include:

• Cadmium ocher: Composed of cadmium sulfides (greenockite, hawleyite), ranging from bright yellow to red.
• Nickel ocher or Nickel bloom (Annabergite): A nickel arsenate typically forming green crusts on nickel minerals.

Thus, from a geological viewpoint, "ocher" is not a single species as it encompasses various secondary minerals. Nonetheless, in everyday language, "ocher" usually refers to iron minerals oxidation products.

Massicot (plumbic ochre) from Monte Cristo mine, Goodsprings District, Clark County, Nevada, USA. Photo by Robert M. Lavinsky from Wikimedia Commons distributed under the CC BY-NC-SA 2.0 license.

Iron Ocher: The Commonly Known Ocher

When referred to without qualifiers, "ocher" typically means iron-rich earths or rocks formed from oxidized iron ores. These naturally occurring substances exhibit colors ranging from greenish-yellow to deep maroon, depending on their specific composition. The primary minerals forming iron ocher are:

• Goethite: The major constituent of yellow ochers; crystalline iron(III) oxide-hydroxide (FeO(OH)), responsible for bright yellow hues.
• Hematite: This principal mineral of red ochers consists of crystalline red iron(III) oxide (Fe₂O₃).

The term "limonite" is often cited as a main ingredient of yellow ocher, but it's not a single mineral. Instead, it is a field term that describes mixtures of yellow-brown iron oxides and hydroxides, primarily non-crystalline goethite, with lepidocrocite, hematite, maghemite, and natural impurities.

A piece of limonite containing iron oxide hydroxide. Quite a fragile aggregate easily leaving marks on paper.

Alongside these primary minerals, ocher naturally includes various impurities, such as sand and clay. Typical ocher composition includes:

• Iron oxides/hydroxides: 5-40% (occasionally up to 75%)
• Clay: 20-25%
• Calcium minerals (e.g., gypsum): ~2-3%
• Quartz (silica/sand): up to 40%
• Water: 7-8%
• Trace amounts of mica and feldspar

A Member of the Earth Pigments Family

Ochers naturally occur as earth or lumps, making them abundant and easily accessible. Their availability made ocher one of humanity's earliest artistic pigments. Indeed, red from the red ocher was the first color used by the cavemen.

Ocher belongs to the "earth pigments" family, emphasizing their natural origins. Related earth pigments like siennas and umbers are essentially variations of ocher, differentiated mainly by manganese oxide content. Typically, ochers lack manganese oxide, siennas include up to 5% of it, while umbers contain around 5-20%. This classification is generally useful, though not strictly fixed.

From Earth to Pigment

Transforming raw ocher into usable pigment involves several purification processes, including grinding, washing, levigation, and drying, to eliminate impurities and achieve desired particle sizes.

Levigation specifically aids in removing unwanted minerals. The raw ocher is mixed with water, allowing heavier particles to settle and lighter pigment particles to remain suspended. After several hours this water-pigment mixture is decanted, and the process repeats until achieving the desired purity.

Color can also be modified through additional treatments. For example, roasting yellow ocher transforms it into red ocher.

Factors Influencing Ocher’s Color

The color of ocher pigment depends on several factors:

• Mineral composition: Goethite contributes yellow hues, while hematite provides red tones.
• Particle size: Fine goethite particles yield a greenish-yellow color, whereas coarser grains produce brownish-yellow. Hematite colors range from light red to dark violet depending on particle size.
• Mineral admixtures: Minerals like dark brown manganese oxides create darker, brownish shades.

Pigment Properties and Quality Indicators

To highlight certain properties of ocher, the standardized abbreviations can be used next to the pigment's or paint's name:

• J (Jaune): Yellow
• L (Lave): Levigated
• C (Citron): Lemon yellow
• T (Très): Very
• S (Sur fin): Superfine (or Superior)
• E (Extra): Extra quality
• F (Foncé): Dark
• O (Or): Gold
• R (Rouge): Red

Natural vs. Synthetic Ochers

Next to natural ochers, there are also synthetic varieties. Synthetic yellow ocher is made from iron (III) oxide-hydroxide, and red ocher from iron (III) oxide. In the world of art pigments and paints, the natural yellow ocher is identified as pigment yellow 43 (PY 43), while its synthetic counterpart is labeled PY 42. Red Ocher, depending on its source, is referred to as PR101 for synthetic red iron oxide or PR102 for natural red ocher.

There is minimal difference between paints made from natural and synthetic ocher. Synthetic production methods yield a highly pure pigment free from the natural impurities found in earth-derived ochers. In contrast, the presence of clay in natural ochers can cause watercolor paints to feel sticky or oily on the brush and make them harder to rinse. Natural ochers watercolor paints also tend to separate in wash mixtures, requiring frequent stirring during use.

Ocher as a Color Name

"Ocher" originates from the Ancient Greek "ōkhrós," meaning pale or yellowish, describing the natural earth color. In contemporary usage, "ocher" typically refers to a muted yellow spectrum, ranging from greenish dark yellow to golden and brownish tones. However, there is no universally standardized definition for "ocher color." Next to the yellow ocher referred simply as "ocher", there is also red, orange, brown and even green ones.