Natural iron oxide pigments or iron oxide earth pigments, collectively referred to as iron oxides, comprise both oxides and oxide hydroxides of iron. Hematite (α-Fe2O3) is the most common iron oxide in red earth pigments, and the iron oxide hydroxide goethite (α-FeOOH) is the most frequently found iron compound in yellow earth pigments. Umbers are brown earth pigments containing both oxides of iron and manganese.
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Names for natural iron oxide pigments are complex and inconsistent. Yellow pigments based on iron oxide hydroxides are referred to by many names, including goethite, limonite, raw sienna, Mars yellow, yellow earth, yellow ocher, yellow iron oxide, yellow hydrated oxide, and iron hydroxide pigment. Similarly, natural red pigments based on anhydrous iron oxide are referred to as hematite, red earth, red ocher, burnt sienna, Mars red, and red iron oxide.
The problem is compounded further when manufacturers label synthetic iron oxide pigments with names typically associated only with earth pigments, such as a synthetic yellow iron oxide labeled yellow ocher.
Iron oxide pigment is an appropriate name for natural and synthetic materials that may contain mixtures of iron oxides and iron oxide hydroxides. The names earth and ocher are suitable when it is clear that the pigment comes from a natural source. Earth is a more general term since ocher refers to a specific type of iron oxide deposit containing kaolinite (clay) and quartz. Hematite is used to differentiate the crushing of mineral hematite, which contains a high proportion of iron oxide in a finely divided form and is usually associated with clay or other minerals. Umber refers to a chemically distinct class of brown earth pigment containing both manganese and iron oxides.
The Colour Index (, 1:16&#;17, 120&#;122, 64&#;65) lists natural red iron oxide pigments as CI Pigment Red 102 (nos. , , and ). Synthetic red iron oxide pigments are listed as CI Pigment Red 101 (nos. , , and ). The natural and synthetic yellow iron oxide pigments are listed as CI Pigment Yellow 43 and 42 (no. ), respectively. CI Pigment Brown 6 and 7 (nos. , , and ) designate synthetic and natural brown iron oxide pigments, including umber.
Generally, we can summarize the various iron oxide pigment names and their Colour Index designations as follows:
Earth Pigment Types
Colour Index
Common Names
Yellow Earth Pigments Pigment Yellow 43 Limonite, goethite, jarosite, yellow ocher, yellow earth, raw sienna, yellow iron oxide Red Earth Pigment Pigment Red 102 Hematite, red ocher, red earth, burnt sienna, red iron oxide Brown Earth Pigments Pigment Brown 7 Pyrolusite, brown ocher, umber, burnt umber
To determine whether a pigment is from a natural source, such as an earth iron oxide pigment, look for the Colour Index designation, such as Pigment Yellow 43 (PY43), Pigment Red 102 (PR102), or Pigment Brown 7 (PBr7).
The labels on paint tubes indicate whether they contain natural or synthetic pigments. The Colour Index Generic Name (CIGN) Pigment Yellow (PY 23) indicates natural raw sienna, while Pigment Red 102 (PR 102) indicates natural burnt sienna. Aspiring artists should be aware of these distinctions and choose their materials accordingly to achieve the desired effects in their artwork. The Colour Index for natural sienna can also be written as Pigment Brown 7 (PBr 7), as noted in Table 1 Suitable Pigment List of the ASTM D Standard Specification for Artists&#; Oil, Resin-Oil, and Alkyd Paints.
Color
Mineral
Chemical Formula
Structure
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Crystal System
Yellow Goethite FeOOH Ramsdellite / hcp oxygen Orthorhombic Akagneite Hollandite/bcc oxygen Tetragonal Lepidocrocite Boehmite / ccp oxygen Orthorhombic &#; Cdl2 Hexagonal Red Hematite Fe2O3 Corundum Hexagonal Maghemite Defect spinel Cubic Black Magnetite Fe3O4 Inverse spinel Cubic
Iron oxide occurs naturally in various forms and colors. Indeed, iron is the second most abundant element on Earth, representing over 5% of the Earth&#;s crust. These natural products tend to consist of coarse particles and contain impurities, making them appear less chromatic. The chemistry of iron is a very complex field where, under different natural geological conditions, several different crystal forms of the same compound, differing due to the arrangements of molecules within the unit cells of the crystalline lattice of each crystal form, are formed (see table above).
The different crystal forms and accessory minerals associated with each deposit of iron oxide earth create a nearly unlimited variety of color, texture, and rheological behavior when dispersed in paint. This variety gives artists ore choices of expression and is one of the main advantages of natural earth iron oxide pigments in art.
Earth has been used as a pigment since prehistoric times. Earth pigments include an enormous range of colours, from warm and strong reds, to cool and gentle greens. In this article, I look at the most commonly used earth pigments, their synthetic alternatives, and how to identify them. I&#;ve used watercolour to try out the pigments, but the information is relevant for all media.
Earth pigments are colours which occur naturally in the earth. Their colour is determined by the various metal oxides (usually iron and manganese oxide), clays, and minerals that are present, something that is often unique to the geology of a particular location. Earth pigments are extremely lightfast and permanent, making them ideal for artist colours.
Synthetic iron oxide pigments, also known as Mars pigments, were first created in the 18th Century, but they began to gain popularity as alternatives to natural earth pigments in the early 20th Century. They are produced by precipitating salts of iron with alum and an alkali, such as lime or potash. Synthetic iron oxides include yellow, red, black, and violet pigments, and the exact formulation and type of iron salt used determines the colour of the pigment.
While they are chemically similar, synthetic iron oxides tend to be highly saturated and &#;cleaner&#; in colour than natural iron oxides, which can contain naturally occurring impurities. Generally speaking, synthetic iron oxides have a smaller pigment particle size than natural earth-based pigments. This makes them usually higher in tinting strength and, in watercolour, more staining and harder to lift. But because of the huge variation in the properties of both natural and synthetic iron oxides, it&#;s difficult to make definitive comparisons between the two. But even so, it&#;s helpful to identify the pigments that are used in artists paints today.
Natural Yellow Ochre (PY43) contains hydrated iron oxide, which gives the earth its golden yellow colour. Natural Red Ochre pigments (PR102) are derived from earths that contain high amounts of hematite, a blood-red mineral. Red and Yellow earths can be found all over the world, and Red and Yellow Ochre can be found in the earliest prehistoric cave paintings.
Many paints labelled Red and Yellow Ochre are made using synthetic iron oxides, instead of naturally occurring iron oxides. These synthetic alternatives can be identified by the pigment index numbers PR101 (Red Ochre), and PY42 (Yellow Ochre). Below is a selection of synthetic red iron oxide pigments (PR101), and natural red iron oxide pigments (PR102). It&#;s clear that synthetic iron oxides can imitate the many colour variations of natural earths.
Pigment index number PBr7 broadly refers to pigments containing natural brown iron oxides. These include Sienna and Umber, two of the most commonly used earth colours in an artist palette.
Sienna is named after the city of Siena, Italy. It is a yellow-brown earth pigment that was originally sourced from the Italian province of Tuscany. The colour became extremely popular with artists of the Rennaissance in the 14th Century. Similarly, Umber is named after the Italian region of Umbria where the pigment was originally mined. The presence of manganese in the earth makes Umber darker and more green than Sienna. &#;Burnt&#; variations of earth colours can be produced by heating the raw pigment. This process, called calcination, partially converts the brown iron oxides into hematite, a red iron oxide.
In some Burnt Sienna paints, PBr7 is replaced by synthetic pigment PR101. Raw Umber is sometimes made with PBr6, a pigment made by oxidising synthetic black iron oxide.
Also known as Terre Verte, natural green earths contain silica clay and green minerals. Like the other earth pigments, they have been used since prehistoric times. Most Terre Verte and green earth pigments are very delicate with a low tinting strength that makes them unassertive in mixtures, but they are characterful pigments in their own right and can be used to neutralise red and orange pigments, or in underpaintings for portraits (a technique known as verdaccio, which can help create life-like flesh tones).
The pigment index number for green earth is PG23. Many paints labelled Terre Verte or Green Earth contain no natural green earths, but rather a mixture of green and brown pigments.
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