Abstract An important problem encountered during the preservation of paintings and other artworks is the fading of the original colors due to exposure of the colorants to light. This fact is clearly evidenced in some of Vincent Van Gogh's paintings in which an organic red, eosin or geranium lake, is present. The identification of eosin and the characterization of its degradation products in paintings represents a challenge because of (i) the generally low concentration of the pigment remaining after an aging period of ca 100 years, (ii) the scarcity of the paint micro samples available for analysis and the difficulty of obtaining additional ones and (iii) the complexity of the degradation behavior of eosin when it is mixed with organic or inorganic pigments, binding media or varnish. This study presents an accelerated aging experiment of eosin paint models in order to understand better the discoloration process; more specifically the influence of different metals with which eosin forms complexes and of the presence of admixture pigments such as lead white and cobalt blue on the lightfastness of eosin is evaluated. Paint model samples were prepared using eosin, lead white, and cobalt blue in different mixing ratios and were characterized with several techniques before and after aging. The possible formation of intermediate molecular forms during the aging experiment and the influence of pigment ratios on the discoloration process were monitored at periodic intervals using a combination of UV–Visible and attenuated total Reflectance-Fourier transform infrared (ATR-FTIR) spectroscopies. Raman spectroscopy, scanning electron microscopy coupled to energy-dispersive X-ray analysis (SEM-EDX) and optical microscopy (OM) analyses were performed to gain information about the discoloration processes taking place within the paint models. Eosin precipitated on lead, aluminum and potassium/aluminum salts was used. These three lakes showed similar discoloration rates under light exposure. In contrast, the presence and relative abundance of the admixture pigments lead white and cobalt blue had a significant influence on the (speed of the) eosin discoloration process. The presence of lead white and cobalt blue appears to stimulate the eosin degradation. However, the cobalt blue shows less influence in the discoloration process, showing a protective effect during the first stages of the aging. This may be qualitatively explained in terms of the ability of lead white to scatter light towards eosin molecules and the absorption characteristics of cobalt blue in the green range of the electromagnetic spectrum, shielding eosin from incoming light. The color changes observed in the paint reconstructions are similar to discoloration phenomena visible in some Van Gogh paintings and can offer an explanation of the gradual discoloration process that took place over the years. These insights will be helpful to estimate the original hues color used/intended by the artist.

中文翻译：

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对伊红湖退化的保护和刺激作用。第 1 部分：铅白和钴蓝

摘要 绘画和其他艺术品在保存过程中遇到的一个重要问题是由于着色剂暴露在光线下而导致原始颜色褪色。这一事实在文森特梵高的一些画作中得到了明显的证明，其中存在有机红色、曙红或天竺葵湖。伊红的鉴定及其在绘画中的降解产物的表征是一个挑战，因为（i）在大约 100 年的老化期后剩余的颜料浓度通常较低，（ii）可用于绘画的颜料微样品稀缺分析和获得额外的难度以及 (iii) 当伊红与有机或无机颜料、粘合介质或清漆混合时，其降解行为的复杂性。本研究展示了曙红涂料模型的加速老化实验，以更好地了解变色过程；更具体地说，评估了与曙红形成复合物的不同金属和混合颜料（例如铅白和钴蓝）的存在对曙红的耐光性的影响。油漆模型样品是使用曙红、铅白和钴蓝以不同的混合比例制备的，并在老化前后使用多种技术进行表征。使用紫外-可见光和衰减全反射-傅立叶变换红外 (ATR-FTIR) 光谱仪的组合定期监测老化实验过程中可能形成的中间分子形式以及颜料比例对变色过程的影响。拉曼光谱，扫描电子显微镜与能量色散 X 射线分析 (SEM-EDX) 和光学显微镜 (OM) 分析相结合，以获得有关油漆模型内发生的变色过程的信息。使用沉淀在铅、铝和钾/铝盐上的曙红。这三个湖泊在光照下显示出相似的变色率。相比之下，铅白和钴蓝混合颜料的存在和相对丰度对曙红变色过程（速度）有显着影响。铅白和钴蓝的存在似乎会刺激伊红降解。然而，钴蓝在变色过程中表现出的影响较小，在老化的第一阶段表现出保护作用。这可以从铅白将光散射到曙红分子的能力和钴蓝在电磁光谱绿色范围内的吸收特性方面进行定性解释，使曙红免受入射光的影响。在油漆重建中观察到的颜色变化类似于在一些梵高画作中可见的变色现象，可以解释多年来发生的逐渐变色过程。这些见解将有助于估计艺术家使用/打算使用的原始色调。在油漆重建中观察到的颜色变化类似于在一些梵高画作中可见的变色现象，可以解释多年来发生的逐渐变色过程。这些见解将有助于估计艺术家使用/打算使用的原始色调。在油漆重建中观察到的颜色变化类似于在一些梵高画作中可见的变色现象，可以解释多年来发生的逐渐变色过程。这些见解将有助于估计艺术家使用/打算使用的原始色调。