Non-invasive and non-destructive Raman spectroscopic techniques have been used to detect the stratigraphic variation of tints in semi-translucent films, comprising stacked layers with varying low concentrations of carbon-black pigment dispersed in a painting varnish imitating historical artist materials, as in the ‘Sfumato’ technique often associated with Leonardo da Vinci's works of art. Understanding the structures behind this effect could help to conserve such paintings. Micro-Raman spectroscopy is one of the analytical techniques usually applied to painting materials that has proved suitable for tackling the challenging detection and characterization of carbon-based pigments in organic-rich paintings. Model transparent samples have been fabricated following a selected recipe from historical sources and analysed using Raman-scattering-based experimental techniques, including micro-spatially offset Raman spectroscopy (micro-SORS). Single glaze layers spanning a range of concentrations, and multilayer systems mimicking the resulting stratigraphy of the Sfumato glaze technique, have been studied. Raman-spectroscopy performance to detect the pigment was assessed on the single layers; the spectral behaviour was characterized according to the concentration of pigment. The micro-SORS technique was tested on the multilayer systems and appears to be able to distinguish between different stratigraphic samples, varying in concentration of the same carbon-based pigment, and the order of layers. This proof-of-concept observation is promising. It calls for further studies to be undertaken to obtain comprehensive results about an increased number of model painting materials, especially for carbon-based materials mixed with other pigments.

中文翻译：

---

使用微空间偏移拉曼光谱研究碳基颜料着色的层状釉料

非侵入性和非破坏性拉曼光谱技术已用于检测半透明薄膜中色调的地层变化，该半透明薄膜由堆叠层组成，这些层具有不同的低浓度炭黑颜料，分散在模仿历史艺术家材料的绘画清漆中，如“晕染”技术经常与达芬奇的艺术作品联系在一起。了解这种效果背后的结构有助于保护这些画作。显微拉曼光谱是通常应用于绘画材料的分析技术之一，已被证明适合解决富含有机物绘画中碳基颜料的挑战性检测和表征。模型透明样品是按照历史来源中选定的配方制造的，并使用基于拉曼散射的实验技术（包括微空间偏移拉曼光谱（micro-SORS））进行分析。人们已经研究了跨越一系列浓度的单釉层，以及模仿晕涂釉技术所产生的地层学的多层系统。在单层上评估检测颜料的拉曼光谱性能；光谱行为根据颜料的浓度进行表征。微 SORS 技术在多层系统上进行了测试，似乎能够区分不同的地层样品、相同碳基颜料的浓度和层顺序的变化。这种概念验证观察是有希望的。需要进行进一步的研究，以获得更多模型绘画材料的综合结果，特别是与其他颜料混合的碳基材料。