What Earth Pigment Adulteration Looks Like in Practice
Suppose you open a tube of raw umber and find that the color is stronger than expected — noticeably brighter, with higher tinting power than other raw umbers you have used. You might attribute this to batch variation or a different source deposit. You probably would not suspect that the tube contains cadmium compounds.
Earth pigment adulteration is not a historical curiosity. A 2022 study by Caceres-Rivero and colleagues documents exactly that scenario, analyzing products sold as natural earths in mainstream art shops and finding cadmium compounds, industrial fillers, and repackaged construction colorants. The authors purchased eight earth pigments from a well-documented supplier (Kremer Pigmente) and eleven “earth” pigments from art shops in Lima, Peru (Quiss, Spondylus, and Baycolor). Using Raman microscopy, powder X-ray diffraction (XRD), X-ray fluorescence (XRF), and visible reflectance colorimetry, they characterized the actual composition of each product. Their findings break into three distinct fraud patterns that any painter working with earth colors should understand.
What Genuine Earth Pigments Look Like — And Why Adulteration Is Hard to See
Before describing the fraud patterns, it is useful to know what authentic earth pigments look like when tested, because mineral complexity is itself an authenticity indicator.
The Kremer reference samples showed what genuine mined earths produce: complex mixtures of iron oxides together with the mineral assemblages typical of their geological origin. The authors identified quartz, calcite, dolomite, kaolinite, gypsum, and rutile in various combinations. XRF showed a characteristic soil elemental profile — potassium, calcium, titanium, manganese, iron, and trace elements, including rubidium, strontium, yttrium, zirconium, and niobium. The green earth sample contained celadonite, glauconite, and associated clayey minerals (Caceres-Rivero et al., 2022, p. 3).
The point is this: a genuine mineral earth is not a simple or “clean” material. Its complexity is evidence of natural geological origin. When a pigment sold as a natural earth shows an unusually simple or anomalous elemental signature, that simplicity is suspicious rather than reassuring.
Three Patterns of Earth Pigment Adulteration
1. Color-corrected earths: cadmium compounds added to umbers and siennas
The Quiss pigments showed a sharply different analytical profile from their Kremer counterparts. XRF spectra revealed abnormally high concentrations of cadmium, selenium, zinc, and sulfur — elements with no business in a soil-derived pigment — alongside the absence of the soil-typical trace elements (potassium, calcium, titanium, rubidium, strontium, yttrium, zirconium, niobium) that appeared consistently in the Kremer samples (Caceres-Rivero et al., 2022, p. 4). XRD then identified crystalline phases consistent with cadmium sulfoselenide and cadmium sulfide in Quiss Raw Umber — compounds associated with cadmium red and cadmium orange pigments.
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