Microstructures of Oil Color “Yellow Ocher”

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  • Kitada Masahiro
    Center for Archaeological Operations, Nara National Institute for Cultural Properties
  • Hayasaka Kohji
    Institute for Material Research, Tohoku University

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  • 油絵具・イエローオーカーに使われている顔料の微細構造
  • アブラエノグ ・ イエローオーカー ニ ツカワレテ イル ガンリョウ ノ ビサイ コウゾウ

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<p>The microstructure of oil color named yellow ocher has been investigated. The purpose is to obtain basic data of material properties, and for conservation and restoration of an oil painting. The four specimens used are yellow ocher made in the United Kingdom (specimen A), the French republic (specimen B) and Japan (specimens C and D) on the market. After the color specimens were dried for a year, the specimens are analyzed using an X-ray diffractometer, scanning electron microscope, energy dispersive X-ray spectroscope, and transmission electron microscope. The thin film used for transmission electron microscope observation is thinned in a focused ion beam apparatus. By means of X-ray diffractometry, goethite (αFeOOH), quartz (SiO2), kaolinite, and muscovite are detected from specimen A, goethite is detected only from specimen B, and goethite and calcite (CaCO3) are detected from specimen C and D. Colored mineral is goethite and the other minerals detected are colorless minerals. The other trace elements, for example, Mg, Al, and S are detected. It is thought that a part of O detected is introduced into the oil by oxidative polymerization. Acicular grains and aggregation of very fine grains are observed in specimen A by means of transmission electron microscopy. The length and width of the acicular grain are 0.2-0.8 μm and 50-200 nm, respectively, and the grain size is 5-10 nm for aggregated grain. The electron diffraction patterns of these grains indicate goethite. Therefore, color development grain is goethite.</p>

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