Characterization of Oil Spots on Black-Glazed Teabowl Made in Song Dynasty by Time-of-Flight Secondary Ion Mass Spectrometry

The ancient porcelain is the treasure of Chinese civilization. In this paper, multiple surface analysis tools were applied to characterize the attractive silvery bright spots on the glaze of black-glazed tea bowl fragment. The raindrop-shaped silver flecks covering the surface were symbols of North China oil spots. High-resolution time of flight-secondary ion mass spectrometry (TOF-SIMS) data confirmed the main component of the oil spot was iron oxide. Positive ions were Fe⁺ (m/z 55.93), FeO⁺ (m/z 71.93), Fe₂O⁺ (m/z 127.86), and negative ions were FeO^- (m/z 71.93), FeO^-₂ (m/z 87.92). A series of peaks were detected by micro-Raman spectroscopy, such as 230, 294, 412, 502, 611, 1 337 cm-1. This suggested existence of hematite (α-Fe₂O₃) in the spot. Secondary ion imaging of TOF-SIMS indicated the oil spots (~120 μm) was formed by self-organization and dispersion of hundreds hexagonal hematite crystal (~2-10 μm). Hematite was spatially complementary to alkaline calcareous glaze that contains silicon, aluminum, calcium, and sodium. TOF-SIMS depth profiling of the spot suggested a more than 5 μm continuous hematite layer on the glaze of black-glazed tea bowl. The main mineral of α-Fe₂O₃ crystals was hematite, which was ochre usually. Specular hematite, a subspecies of hematite with silver or black color, was believed to play critical role in the formation of the oil spot. With the optical microscope observations, we believed that the silvery material on the surface of the oil spots should be a thin layer of specular hematite and the lower ochre layer was hematite. In fact, the oil spot appeared silvery-white and reflective when viewed from 15° to 165°, while it appeared ochre when the angles of gaze was not greater than 15° from the glazed surface of the specimen. The extraordinary visual property of color changing with different angle of gaze formed the charm of Song dynasty oil spot tea bowls. With these results, a hypothesis for the formation of the North China oil spots was enlightened. The molten phase of Fe-rich soy-black glaze gradually formed micron-sized hematite particulates at sealed kiln of high temperature (1 250 ℃). When the kiln was unsealed, the hot porcelain was exposed to cold air immediately. A thin layer of specular hematite was generated on top of hematite particulates.