Preventive conservation methods are based upon the principle that deterioration and damage to works of art can be controlled or slowed down by managing the environmental conditions under which collections are housed and safeguarded. Therefore, it is possible to prolong the lifespan of objects by improving the indoor air quality (IAQ). For that reason, collection caretakers have great interest in improving the environmental conditions by performing the following tasks:
- Routine monitoring: Determine the IAQ as a function of time and detect an increase in one of the damaging parameters well before art objects have the time to respond to it;
- Diagnostic monitoring: Identify the largest risks related to the environmental parameters or investigate a specific degradation problem;
- Performance monitoring: Evaluate the performance of mitigation measures (ex. apply air purifiers) and optimize the taken actions in order to enhance the IAQ.
The problem with environmental control in museums is that they are usually limited to temperature and relative humidity. In some cases, this is supplemented with light and UV measurements. It is known that many other airborne substances such as particulate matter and reactive gases (ex., H2S, O3 or organic acids) play a crucial role in the deterioration processes of historical materials. However, in many cases these parameters are not monitored. Additional problems that hamper the evaluation of mitigation measures are:
- Complex relation between environmental parameters and transformation rates: It is the intention of preventive conservation to slow down the transformation rates of historical materials (see set C in Fig. 1) by mitigating the sources responsible for transformations (in set A of Fig. 1). The relations between the 3 sets as illustrated in Fig. 1 are to a large extent unknown. However, by measuring the transformation rates of a series of materials (set C) it should be possible to evaluate how harmful the environmental parameters are for a mixed collection (set B), allowing the identification of the most harmful sources (set A);
- How to measure IAQ: The IAQ is not only determined by temperature and relative humidity but by a much larger variation of quantities. Although it is not realistic to monitor all possible quantities simultaneously, the selected quantities should at least cover all the levels mentioned in Fig. 2;
- IAQ for mixed collections: The same environmental conditions might be suitable for some materials in a mixed collection, while it can be harmful for other materials in the same collection. Therefore, the IAQ is not only determined by environmental parameters but also by the materials and objects present in the mixed collection and by the properties of the building itself. Inspection of the collection and building is needed to evaluate their impact on the IAQ;
- Human decisions vs. analytical results: For all measured parameters a threshold value is needed that defines whether a magnitude is harmful or not. Defining threshold values is not always an analytical result but is sometimes a human decision. A methodology needs to be developed in how threshold values have to be defined.
Fig. 1: Example illustrating the complexity of the relations between 3 sets. A) set containing the sources responsible for transformation; B) Set containing the environmental parameters affecting the transformation of materials; and C) set containing all the transformation rates. It is the intention to describe the overall impact of set B on set C (i.e., the mixed collection) with the IAQ-index.
Complex relation between environmental parameters describing the IAQ (set A) and transformation rates of materials exposed to that air (set B) is illustrated by arrows.
Fig. 2: Overview of all the different parameters that can be monitored, classified in 4 different levels.