Microstructures and Decomposition of Cu-Containing Green Pigment Used in the Late Edo Era

The paper painted with green pigment containing Cu discolours and its mechanical properties also deteriorate. The microstructure and decomposition of the green pigment have been investigated. Specimens are obtained from a hand-coloured wood-block print from the late Edo era. A transmission electron microscope and EDX spectrometer are used to analyse the green pigment and changes in the quality of the pigment. The green pigment is polycrystalline copper acetoarsenite Cu(C2H3O2)2•3Cu(AsO2)2, and contains Cu-As-O compound, the composition of which is different from that of copper acetoarsenite. Small crystals grow radially and form a spherical and its crystal. Cu2O, As-S compounds and the Cu-As-O compound produced in the decomposition process of copper acetoarsenite are detected around the discoloured cellulose fibers. Of these, it is considered that the coloured Cu2O and As-S compounds stain the fiber.