Novak, Morana; (2023) Evaluation of the environmental performance of archival boxes for cellulose acetate storage. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

At present, archival boxes are used as a preventive measure to control the microenvironments in museums and archives. However, their efficiency to protect valuable artefacts against environmental fluctuations is still not entirely understood. This is especially noted for cellulose acetate-based objects, whose optimal storage solutions are still under debate. It is generally agreed that enclosures used to store cellulose acetate objects need to be able to protect objects against moisture, which causes the hydrolytic degradation of this material. Consequently, as these objects degrade, different volatile organic compounds, including acetic acid, are produced and released. Storing cellulose acetate in airtight enclosures, such as metal and plastic ones, could lead to increased degradation of this material, as released acetic acid, which acts as a degradation catalyst, gets trapped inside the enclosure. Therefore, the scope of this project was to test the suitability of the selected archival boxes to mitigate moisture fluctuations and remove acetic acid. This study was done in several steps. First, an analytical methodology to assess how archival boxes buffer RH fluctuations was proposed and evaluated. This was done by testing several box and board properties, namely air exchange rate and moisture sorption and transmission. In addition, the effect of several box configurations, such as size, design, board type, surface modifications, as well as filling and stacking of boxes, on moisture buffering was tested. While non-modified boxes made of paperboard responded more rapidly to humidity changes, a surface modification of the paperboard reduced interaction between a box and its external environment, leading to more stable internal RH. Therefore, archival boxes are suitable for the management of humidity fluctuations in storage areas. Furthermore, a novel empirical model, based on the mass transfer of moisture, was validated using the RH monitoring data. After the validation, the model was used to predict the RH values inside an archival box for various storage conditions. In addition to humidity buffering, the ability of archival boxes to remove excessive acetic acid from box headspace was also tested. The sampling and ion chromatographic methods were developed and validated, in order to quantify acetic acid emissions from cellulose acetate objects, including 3D objects and photographic material. It was concluded that some plastic objects can emit high levels of acetic acid, and therefore should not be stored in an airtight enclosure. An empirical model was developed and validated, and the calculated acetic acid emissions were used as the model inputs. This was done in order to predict the equilibrium acetic acid concentrations emitted from individual objects for several storage scenarios (an archival box, a surface-modified archival box, a display case, and a storage room). This model indicated that archival boxes showed lower concentrations of acetic acid, compared to more airtight surface-modified boxes of the same volume. For display cases and storage rooms, the crucial factor that determines the removal of acetic acid is not their airtightness, but their larger volume, which dilutes a released acid. Consequently, a lower acetic acid concentration, compared to archival boxes, is going to be observed. This research is of use to the preventive conservators and collection managers, who work on the storage strategies in heritage institutions. The project’s findings could inform them on the suitability of boxes to effectively work as a microenvironmental control, buffer moisture fluctuations or prevent the accumulation of acetic acid, which is emitted from cellulose acetate. In addition, the industry and manufacturers can use these results to get an insight into how their storage products perform in indoor environments and find how their existing storage products could be improved to accommodate the storage needs of cellulose acetate objects. Furthermore, plastic conservators could get an idea of the range of acetic acid that individual cellulose acetate objects in collections can release, and its cumulative effect in storage areas where these objects are stored. This could inform the decision-making and improve preventive conservation practices and storage strategies for modern materials in museums and archival collections.

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