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

Abstract

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 Xray 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.

Identifying VOCs in exhibition cases and efflorescence on museum objects exhibited at Smithsonian’s National Museum of the American Indian-New York

“…The detection of TMP-ol in the modified silicone (MS) polymer Terostat by SPME-GC-MS and DART-MS, is in agreement with previous studies from the Museum of Fine Arts (Boston, USA) and the Rijksmuseum (Amsterdam, The Netherlands) carried out in similar display cases [1][2][3]. These studies also linked the efflorescence with the presence of Tinuvin ® 770, used as an additive in the structural adhesive (Terostat) applied during the manufacture of their display cases.…”

“…The preparation method of Tinuvin ® 770 describes the amount of substance ratio of sebacic derivative and TMP-ol as 1: (2-2.2), confirming the excess of TMP-ol in the initial mixture [5]. This excess of TMP-ol gives rise to free TMP-ol in the final composition of Terostat [3]). In the Infinity of Nations exhibition casework, the free TMPol has migrated from the structural adhesive (Terostat MS 937) to the case environment.…”

“…With the organic component of the efflorescence characterized, efforts were focused on determining the source of TMP-ol and the origin of chlorinated compounds that can form the chlorinated salt, reported in previous studies [1,3] and detected in this study by SEM-EDS analysis.…”

“…Other museums have described similar issues, observing efflorescence in exhibition cases from the same manufacturer installed between 2009 and 2014 [1][2][3]. While other museums have noted efflorescence on case sealing gaskets, glass door interiors, and occasionally on certain types of objects, at NMAI-NY the efflorescence is not observed on gasket materials but appears on a variety of component materials of collections items in these exhibit cases: ceramic, stone, wood, plant fibers and seeds, feathers, fur, textiles, and tanned hide.…”

“…The first study focused on this issue, carried out by the Museum of Fine Arts, Boston (USA) and the University of Turin (Italy), suggested that the formation of this nonvolatile substance may have been caused by a reaction between the volatile 2,2,6,6-tetramethyl-4-piperidinol (TMP-ol) and an acidic compound, containing chlorine or sodium, also present in the case environment [1,2]. In a recent publication by the Rijksmuseum, five types of crystalline deposits are described as a result of an acidbasic reaction between TMP-ol and different carboxylic acids (2,4-dichlorobenzoic acid, formic acid, methacrylic acid, palmitic acid and an unknown carboxylic acid) 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 [3].…”