Names for Iron Oxide Earth Pigments

Names for Iron Oxide Earth Pigments

Natural iron oxide pigments, also known as iron oxide earth pigments, encompass both oxides and hydroxides of iron, commonly categorized as iron oxides. The predominant iron oxide found in red earth pigments is hematite (α-Fe2O3), while yellow earth pigments typically comprise the iron oxide hydroxide goethite (α-FeOOH). Umbers are categorized as brown earth pigments and contain a mixture of both iron and manganese oxides.

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The nomenclature for natural iron oxide pigments can be quite intricate and often varies. Yellow pigments derived from iron oxide hydroxides go by a variety of names such as goethite, limonite, raw sienna, Mars yellow, yellow earth, yellow ocher, and yellow iron oxide, alongside yellow hydrated oxide and iron hydroxide pigment. Similarly, natural red pigments composed of anhydrous iron oxide are referred to as hematite, red earth, red ocher, burnt sienna, Mars red, and red iron oxide.

This naming complexity increases as synthetic iron oxide pigments are sometimes labeled with titles commonly associated only with their natural counterparts. For example, a synthetic yellow iron oxide might be labeled as yellow ocher, adding to the potential for confusion.

The term "iron oxide pigment" serves as an inclusive description for both natural and synthetic materials that may comprise combinations of iron oxides and hydroxides. The designations “earth” and “ocher” are fitting only when the pigment is confirmed as a natural product. The term “earth” presents a broader classification, while “ocher” is more specific, generally referring to a specific deposit of iron oxide that also contains kaolinite (clay) and quartz. Hematite refers to the process of crushing mineral hematite, which is largely composed of finely divided iron oxide associated with clay and other minerals. The term umber denotes a specific subgroup of brown earth pigments that feature both manganese and iron oxides.

The Colour Index classifies natural red iron oxide pigments as CI Pigment Red 102. Conversely, synthetic red iron oxide pigments are assigned the designation CI Pigment Red 101. The natural yellow iron oxide pigments fall under the labels CI Pigment Yellow 43, while their synthetic counterparts are designated as CI Pigment Yellow 42. Furthermore, synthetic and natural brown iron oxide pigments, including umber, are identified as CI Pigment Brown 6 and 7.

We can summarize the various names of iron oxide pigments and their corresponding Colour Index classifications 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

For artists and consumers seeking to determine the source of a pigment, referencing the Colour Index designation is crucial. Designations such as Pigment Yellow 43 (PY43), Pigment Red 102 (PR102), and Pigment Brown 7 (PBr7) can indicate whether a pigment is naturally occurring.

Labels on paint containers also reveal the nature of the pigments inside; for instance, the Colour Index Generic Name (CIGN) Pigment Yellow (PY 23) corresponds to natural raw sienna, whereas Pigment Red 102 (PR 102) signifies natural burnt sienna. Artists should remain vigilant regarding these distinctions for achieving desired results in their artwork. It's worth noting that the references for natural sienna can also be identified as Pigment Brown 7 (PBr 7) in regulatory documents like the Suitable Pigment List of the ASTM D Standard Specification for Artists' Oil, Resin-Oil, and Alkyd Paints.

Iron oxide appears in various natural forms and colors. As the second most prevalent element in the Earth's crust, iron represents over 5% of it. Natural iron oxide pigments are typically composed of coarse particulate matter and may contain impurities, leading to less intense chromatic properties. The chemistry of iron is intricate, with several crystal forms arising under varying geological conditions, resulting in different molecular arrangements within each crystal structure.

This diversity in crystal structures and accompanying minerals associated with each deposit of iron oxide earth contributes to an immense variety of colors, textures, and rheological properties when mixed into paints. This extensive range enhances artists' creative options, underscoring the benefits of utilizing natural earth iron oxide pigments in artistic applications.