Istituto di Chimica Inorganica e delle Superfici, Consiglio Nazionale delle Ricerche, Padova, Italy.
Anal Bioanal Chem. 2012 Feb;402(6):2195-208. doi: 10.1007/s00216-011-5645-4. Epub 2011 Dec 29.
In this paper, we propose an analytical methodology for attributing provenance to natural lapis lazuli pigments employed in works of art, and for distinguishing whether they are of natural or synthetic origin. A multitechnique characterization of lazurite and accessory phases in lapis lazuli stones from Afghan, Siberian and Chilean quarries, on the pigments obtained by their purification, and on synthetic ultramarine pigments was performed. According to the results obtained, infrared spectroscopy is not a suitable technique for distinguishing the provenance of lapis lazuli, but a particular absorbance band makes it relatively easy to determine whether it is of natural or synthetic origin. On the other hand, EDS elemental composition and XRD patterns show the presence of specific mineral phases associated with specific lapis lazuli sources, and can be used to distinguish the provenance of the stones as well as-albeit to a lesser extent-the corresponding purified blue pigments. In contrast, FEG-SEM observations clearly show different stone textures depending on their provenance, although these distinctive features do not persist in the corresponding pigments. PCA analyses of EDS data allow Afghan lapis lazuli stone to be distinguished from Chilean and Siberian ones, and can distinguish between the pigments resulting from their purification as well as synthetic blue ones. Although this methodology was developed using a limited number of samples, it was tested on lapis lazuli pigments collected from three paintings (from the fourteenth to sixteenth centuries) in order to perform a preliminary validation of the technique, and based on the results, the provenance of the blue pigments employed in those artworks is proposed. Finally, upon analytically monitoring the process of purifying lapis lazuli to obtain the corresponding pigments, it was found that ion-exchange reactions occur between the alkali modifiers of silicate/aluminosilicate phases and free carboxylic acids present in the doughy mixture of natural terpenes and ground stone, namely pastello. These reactions favor (i) the retention of silicate phases in the organic mixture and (ii) the selective extraction of lazurite due to the formation of Brønsted acidic sites [Al(OH)Si], which are responsible for its high hydrophilicity in comparison to the one of the other species present in the lapis lazuli stone.
本文提出了一种分析方法，用于确定艺术作品中使用的天然青金石颜料的产地，并区分其是天然来源还是合成来源。对来自阿富汗、西伯利亚和智利矿区的青金石矿石、通过其提纯获得的颜料以及合成群青颜料中的蓝方石和伴生相进行了多技术特征分析。根据所得结果，红外光谱不是区分青金石产地的合适技术，但特定的吸收带使其相对容易确定其是天然来源还是合成来源。另一方面，EDS 元素组成和 XRD 图谱显示存在与特定青金石来源相关的特定矿物相，可用于区分石头的产地，以及——尽管程度较小——相应的提纯蓝色颜料。相比之下，FEG-SEM 观察清楚地显示出不同产地的石头具有不同的纹理，尽管这些特征在相应的颜料中并不存在。EDS 数据的 PCA 分析可以将阿富汗青金石与智利和西伯利亚的青金石区分开来，并且可以区分它们的提纯颜料与合成蓝色颜料。尽管该方法是使用有限数量的样品开发的，但它已在从三件（十四至十六世纪）绘画中收集的青金石颜料上进行了测试，以对该技术进行初步验证，并基于结果提出了这些艺术作品中使用的蓝色颜料的产地。最后，在对青金石的提纯过程进行分析监测以获得相应的颜料时，发现硅酸盐/铝硅酸盐相的碱改性剂与天然萜烯和磨碎的石头的面团混合物中存在的游离羧酸之间发生离子交换反应，即蜡笔。这些反应有利于 (i) 硅酸盐相在有机混合物中的保留，以及 (ii) 蓝方石的选择性提取，这是由于形成了 Brønsted 酸性位 [Al(OH)Si]，与蓝金石矿石中存在的其他物质相比，它负责其高亲水性。
