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.