IPANEMA, Synchrotron SOLEIL, Saint Aubin, BP48 F-91192 Gif-sur-Yvette Cedex, France.
Anal Chem. 2011 Jul 1;83(13):5145-52. doi: 10.1021/ac200184f. Epub 2011 Jun 6.
Smalt was commonly used as a pigment by artists between the 16th and 18th centuries. It is a powdered blue potash glass colored by cobalt ions and often degrades causing dramatic changes in the appearance of paintings. The aim of the work presented in this paper was to investigate the changes in the structure and environment around the cobalt ion on deterioration, to further our understanding of the basis of the loss of color. Particles of well-preserved and altered smalt in microsamples from paintings in the National Gallery, London, and the Louvre, Paris, were analyzed using synchrotron micro-X-ray absorption spectroscopy at the Co K-edge. X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) measurements showed that in intense blue particles the cobalt is predominantly present as Co(2+) in tetrahedral coordination, whereas in colorless altered smalt the Co(2+) coordination number in the glass structure is increased and there is a shift from tetrahedral toward octahedral coordination. The extent of this shift correlates clearly with the alkali content, indicating that it is caused by leaching of potassium cations, which act as charge compensators and stabilize the tetrahedral coordination of the cobalt ions that is responsible for the blue color.
青金石在 16 至 18 世纪期间被艺术家们广泛用作颜料。它是一种由钴离子着色的粉末状蓝色钾玻璃，经常会退化，导致画作外观发生显著变化。本文研究工作的目的是调查钴离子在劣化过程中结构和周围环境的变化，以进一步了解颜色损失的基础。使用同步加速器微 X 射线吸收光谱技术在 Co K 边对来自伦敦国家美术馆和巴黎卢浮宫绘画的微样本中的保存完好和变质的青金石颗粒进行了分析。X 射线吸收近边光谱（XANES）和扩展 X 射线吸收精细结构（EXAFS）测量表明，在强烈蓝色的颗粒中，钴主要以四面体配位的 Co(2+)形式存在，而在无色变质的青金石中，玻璃结构中 Co(2+)的配位数增加，并且从四面体向八面体配位发生转变。这种转变的程度与碱含量明显相关，表明它是由钾阳离子的浸出引起的，钾阳离子作为电荷补偿剂稳定了负责蓝色的钴离子的四面体配位。
