Sulfur K-edge Micro- and Full-field XANES Identifies Marker for Preparation Method of Ultramarine Pigment from Lapis Lazuli in Historical Paints

Ultramarine blue pigment from lapis lazuli is one of the most valued natural artist’s pigments used throughout history for its brilliant hue. Historical recipes describe various pigment preparation methods, however, little is understood about why such treatments were performed or how to distinguish them a posteriori on historical paintings. One such historically relevant treatment is heating of the lapis lazuli rock prior to extracting lazurite. X-ray absorption near-edge structure spectroscopy (XANES) at the sulfur K-edge in microbeam and full-field modes (analyzed with non-negative matrix factorization, NMF) is employed to monitor the changes in the sulfur species within lazurite following treatments. Sulfur signatures in lazurite show dependence on the heat-treatment of lapis lazuli rock from which it is derived. In particular, peaks at 2469.0 and 2471.2 eV, attributed to contributions from the trisulfur radical (S3•⁃), which is responsible for the blue color of lazurite, increase in relative intensity with heat of treatment ≥ 600 °C, paralleled by an intensified blue hue. The peak at 2472.5 eV (presumed as S8), on the other hand, decreases in relative intensity. The sulfur signature following heat-treatment is not only unique but also retained following accelerated aging of each pigment mixed as a paint with linseed oil. Further, XANES spectra using an attenuated focused microbeam were gathered on lazurite particles from five historical paint samples. In each, profiles matching that of pigment derived from heat-treated rock were identified, providing a marker for artists’ pigments that had been extracted from heat-treated lapis lazuli.

以青金石（lapis lazuli）为原料制得的群青颜料，凭借其鲜亮色泽，是历史上最具价值的天然美术颜料之一。尽管历史文献记载了多种该颜料的制备工艺，但学界对这类处理工艺的实施缘由，以及如何在古代绘画中后验鉴别这些工艺的差异仍知之甚少。其中一种具有历史研究价值的处理工艺，便是在提取天蓝石（lazurite）前对青金石原石进行加热处理。本研究采用微束与全场模式下的硫K边X射线吸收近边结构光谱（X-ray absorption near-edge structure spectroscopy, XANES），结合非负矩阵分解（non-negative matrix factorization, NMF）分析，以监测经各类处理后天蓝石内部硫物种的变化。天蓝石中的硫特征信号与其来源青金石原石的热处理工艺存在显著相关性：具体而言，归属为三硫自由基（trisulfur radical, S3•⁃）——该自由基正是天蓝石蓝色色泽的成因——的2469.0 eV与2471.2 eV处特征峰，其相对强度随热处理温度≥600℃而升高，同时伴随颜料蓝色泽的加深；而2472.5 eV处的特征峰（推测对应单质硫S₈），其相对强度则随之降低。经热处理后的硫特征信号不仅具有独特性，且在将该颜料与亚麻籽油混合制成涂料并经受加速老化后仍可稳定保留。此外，本研究还利用衰减聚焦微束技术，采集了5件古代涂料样品中天蓝石颗粒的XANES光谱，在每一件样品中均识别出与经热处理青金石原石制得的颜料相匹配的谱图特征，这为鉴别源自热处理青金石的美术颜料提供了可靠的识别标记。