Air tightness and RH control in museum showcases: Concepts and testing procedures

The control of the Relative Humidity (RH) is a key factor for assuring a proper conservation of artworks. In the majority of museums/archives the collections are kept at a very stable and controlled RH by means of traditional air conditioning systems (HVAC systems). Nevertheless, the demanding requirements as far as the acceptable variation of RH is concerned (frequently less than 5 %), pose a lot of challenges and, frequently, it is hardly possible to achieve the desired goals by just using air conditioning systems. As a results, even highly sophisticated HVAC systems are, in many cases, not able to guarantee a satisfactorily constant and controlled hygric environment and, when they succeed, they give rise to significant energy consumptions (which can represent a non sustainable cost for the museum structures). Furthermore, the museum/archive exhibition halls often contain and display the so-called “mixed” collections, that is, a number of objects constituted by various materials are kept together in the same environment. These objects may require to be preserved at different RH levels (e.g. metals at low RH values, organic material at RH values around 50 %) and it becomes impossible to satisfy such condition by using full air conditioning systems (HVAC systems can only provide a uniform RH level throughout the whole room volume). A possible solution to these problems is represented by the adoption of museum (conservative) showcases. A showcase allows to realize a “box in box” configuration (being the first box the room of the building and the second the showcase). With this approach a first and rough climatic control is exerted by a usual HVAC system (which does not need to be particularly sophisticated nor powerful) while the “finer and local tuning” of the RH is achieved by means a passive control inside the showcase (using e.g. “buffer materials”). However, such approach reveals to be effective only if the showcases have a sufficiently good air/gas tightness and a suitable capacity in smoothening the indoor air temperature and relative humidity. In this paper, the concepts that allow the control of the RH by means of conservative showcases will be presented and critically discussed. The behaviour of the showcases will be analysed relating their air/gas tightness with the capability of controlling the indoor environmental conditions. The combined effect of gas diffusion and convection on the showcase microenvironment, with particular emphasis on the RH control, will be discussed and testing procedures (e.g. pressurization tests, tracer gas measurements and temperature/relative humidity response tests) will be described. Specifically, the results of a number of laboratory measurements (done on experimental/real showcases) will be shown and an analysis of the influence of the air tightness and gas permeability on the passive control of the RH inside the showcase will be developed. Furthermore, application of the proposed testing methods on real case studies, in actual museum conditions, will be illustrated and the results of medium to long term monitoring of the temperature and RH inside showcases before and after their retrofit, will be critically examined. Finally, laboratory and field measurement results will be critically analysed in order to abstract general recommendations about: - how to test the showcase air tightness, - how to assess the suitable air tightness of a showcase in relation to the desired level of control of the RH,