Microstructures of cobalt blue enamel of first Imari porcelain made in the early 17th century (the first Edo era)

The microstructures and impurities of cobalt blue enamel used in the first Imari porcelain made in the early 17(th) century have been investigated. The fine structures are observed using a transmission electron microscope. The elements in the enamel are analyzed by EDX. A fine acicular crystal and a nanometer-sized circular precipitate are observed in the blue enamel. The acicular crystal contains 76 mol% Si, 16 mol% Al, 4.8 mol% Ca and small amounts of Na and K. The sizes of the crystals are 0.2-0.5 mu m width and 0.5-5 mu m length. The circular precipitates are 20-80 nm in diameter, contain Co species, and the structure is amorphous. The main constituents of the circular precipitate are 66 mol% Si, 9-10 mol% Al and Ca, and 1-4 mol% Na, Mg, and P. Furthermore, small amounts of Mn and Fe are detected. The concentration of Co in the circular precipitate is 0.7-1.4 mol% (average: about 1 mol%). The average concentration of the amorphous matrix is 97 mol% Si, and small amounts of Al, Ca, Na, and other trace elements are detected. From the analysis of the fine structures, the acicular crystals are found to precipitate primarily, and then the amorphous circular precipitates are separated from the amorphous matrix. In the transparent enamel used in Imari porcelain, fine crystals are distributed in an amorphous matrix. Their size is 20-80 nm in width and 50-600 nm in length. The composition of the crystal is approximately Si3Al2O7.5 and that of the amorphous matrix is Si1.0Al0.025O1.5.