Crystalline Deposits in New Display Cases at the Rijksmuseum: Characterisation and Origin

An unusual phenomenon occurred in new display cases at the Rijksmuseum four months after their installation in April 2013. White deposits were visible on glass windows, silicone door gaskets, black structural adhesive seals, and on works of art. The works of art most affected by these deposits were bronze sculptures, wooden and waxed objects, tempera, and oil paintings. In this study the deposits were chemically characterised using (pyrolysis-)gas chromatography-mass spectrometry ((Py-)GC-MS), µ-Raman spectroscopy (µ-Raman), ion chromatography (IC), and scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM-EDX). Five chemically different crystalline deposits were found. They were identified as organic salts of the base 2,2,6,6-tetramethyl-4-piperidinol (TMP-ol), a secondary amine, and five different carboxylic acids. It was found that TMP-ol, which is part of the UV-light stabiliser Tinuvin-770, emitted from the structural adhesive Terostat-9220. Terostat-9220 was used in large quantities in the display cases to adhere glass windows to metal parts. The carboxylic acids derived from both construction materials used to build the cases and from conservation materials present on the exhibited works of art. The carboxylic acids involved were 2,4-dichlorobenzoic acid, formic acid, methacrylic acid, palmitic acid, and an unknown carboxylic acid, respectively emitted from peroxide-cured silicone gaskets, panels of medium-density fibreboard (MDF), UV-adhesive, beeswax containing products, and from an unknown acidic conservation product or binding medium. The identification of the crystalline deposits was supported by their syntheses in the laboratory. Since 2013, similar deposits have been observed in a number of museum collections worldwide. A treatment for preventing further growth of the deposits was developed and applied in the Rijksmuseum showcases.

2013年4月安装于荷兰国立博物馆（Rijksmuseum）的新型展柜，在启用四个月后出现异常现象：玻璃橱窗、硅酮门密封胶、黑色结构密封胶及艺术品表面均可见白色沉积物。受该沉积物影响最严重的艺术品包括青铜雕塑、木质及上蜡物件、蛋彩画与油画。本研究采用（热解）气相色谱-质谱联用技术（(Py-)GC-MS）、显微拉曼光谱（µ-Raman）、离子色谱（IC）以及扫描电子显微镜联用能量色散X射线能谱（SEM-EDX）对沉积物开展化学表征。研究共检出五种化学成分相异的结晶沉积物，经鉴定均为仲胺2,2,6,6-四甲基-4-哌啶醇（TMP-ol）与五种不同羧酸形成的有机盐。经检测，作为紫外线光稳定剂Tinuvin-770组分的TMP-ol源自结构胶Terostat-9220，该胶曾大量用于展柜中玻璃与金属部件的粘接。羧酸来源分为展柜建造材料与展品修复材料两类，具体包括：过氧化物固化硅酮密封胶释放的2,4-二氯苯甲酸、中密度纤维板（medium-density fibreboard, MDF）释放的甲酸、UV胶释放的甲基丙烯酸、含蜂蜡产品释放的棕榈酸，以及某未知酸性修复材料或粘结剂释放的未知羧酸。研究团队通过实验室合成验证了上述结晶沉积物的成分。自2013年起，全球多家博物馆的馆藏展柜中均陆续出现类似沉积物。目前，针对该沉积物的抑制处理方案已被开发并应用于荷兰国立博物馆的展柜中。