| Abstract: | SUMMARIES. The study of several commercial paints and dry pigments, as well as paint samples removed from works of art, allowed the identification of magnesium cobalt arsenate, a pigment that has not previously been reported in the literature. This article focuses on the composition, characterization and occurrences of this pigment as well as other violet cobalt compounds used as artists' pigments, namely cobalt phosphate, cobalt phosphate octahydrate, ammonium cobalt phosphate hydrate and lithium cobalt phosphate. Cobalt arsenate, a pigment frequently mentioned in the literature on artists ' materials, has not yet been unambiguously identified. The introduction of these compounds as pigments is briefly discussed.
CONCLUSION. The study of several commercial paints and dry pigments, as well as paint samples removed from works of art, allowed the identification of magnesium cobalt arsenate. a pigment that has not previously been reported in the literature. Cobalt arsenate. a pigment frequently mentioned in the literature on artists' materials, has not yet been unambiguously identified. This study also showed that there are more cobalt violet pigments than the arsenate and phosphate forms mentioned in the pigment literature. Cobalt phosphate octahydrate, ammonium cobalt phosphate hydrate and lithium cobalt phosphate were identified. The last two are both available on the market today, and ammonium cobalt phosphate hydrate has been identified in twentieth-century paintings, proving that artists have indeed used it. Caution should be exercised in identifying the various cobalt violet pigments. Cobalt arsenate and magnesium cobalt arsenate cannot be distinguished by energy dispersive spectrometry alone. Similarly, this technique cannot be used to distinguish between anhydrous cobalt phosphate, the octahydrate, ammonium cobalt phosphate hydrate, and lithium cobalt phosphate. As illustrated by the present study. X-ray diffraction. FTIR spectroscopy and polarized light microscopy are all useful techniques for characterizing the various forms of cobalt violet. |
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