The proper control of indoor microclimate conditions in museums represents a crucial situation for guaranteeing the correct preservation of the exhibited works of art [1], [2], especially when museums are hosted by historical buildings [3], [4], being cultural heritage items themselves. Such conditions are often achieved by means of Heating, Ventilating and Air-Conditioning (HVAC) systems that, essentially, operate on the thermodynamic properties of the indoor air of the museum’s rooms [5], [6]. Nowadays a wide debate is open among the operators about the features that new technological systems must have in order to be installed in museums, also involving the role of the so-called Best Available Technologies [7], [8]. However, several museums are still equipped with old and/or improperly sized HVAC systems [9]. This causes that these systems, apart from their significant energy use [10], are often subjected to interruptions for maintenance operations or, accidentally, for undesired failures. These eventualities introduce an undesired delay of the planned breaks, likely resulting in unsuitable microclimate conditions for the works of art. On the other hand, such drawbacks could determine unsuitable indoor microclimate conditions with possible discomfort for the museum visitors and workers.

It must be noted that these requirements can be controversial, since the environmental conditions required for preserving the works of art sometimes are not able to guarantee comfort of people at the same time [11], [12], sometimes they might also conflict with the requisites of energy efficiency [13], [14]. Also, different works of art generally require different environmental conditions for their proper preservation [15], [16]. Furthermore, a study reported in La Gennusa et al. [17] showed how, when buildings like museums and galleries are involved, the methods to be used for assessing the indoor air quality, should not be too invasive in order not to interfere with the exhibition scene.

An Italian standard [18], which defines proper physical indoor parameters for items exhibited in galleries and museums, asserts that curators are in charge of the final decision about the environmental conditions of each item, since the most suitable microclimatic conditions strongly depend on the environmental history of the item itself, and this latter is usually well known only to the curator. Curators and/or people responsible for the general running of museums are indeed the ones called to assess effective strategies in order of limiting an excessive interruption of the HVAC service. For this purpose, they must pay the highest attention to properly draw up the contracts that regulate the relationships with the companies in charge of the design, maintenance and repairing services of the museum’s HVAC system [19], [20]. In fact, among other things, the contractors “… pay the bill when they fail to respond to an emergency within the agreed-on time period …” [21].

According to the above-cited considerations, the evaluation of the quality of microclimate conditions of museums consists not only in verifying that the environmental parameters fall within the tolerance intervals for the preservation of works of art, but also in taking into account the probability of occurrence of the HVAC systems’ malfunctioning (time during which such parameters fall outside the optimal range).

The present work, after a short review of some available indexes of risk for the preservation of works of art, introduces a new simple indicator (the EI indicator) intended as a support for curators to properly evaluate the planned interruptions for maintenance operations of the HVAC systems and to establish the maximum time of acceptable discontinuity in service. This indicator can also be utilized for defining appropriate penalties to be paid by the companies in case of overruns of the interruption periods [22]-[24].

The feasibility of the proposed new indicator has been verified in the “Museo Regionale” of Palermo (Italy), two halls of which – previously subjected to measurements of indoor parameters – have been considered to provide an example of the EI indicator’s calculation.

Since the relevance of risks related to the HVAC systems’ discontinuity period is associated with their maintenance and management effectiveness, some literature indexes attempting to estimate the probable risk of damage in connection with assigned environmental conditions associated to pre-set reference values, are described in the following sections [25]. The aim of this preliminary analysis is to verify whether some of these existing indicators can be utilized for supporting curators in their relationships with HVAC systems’ providers.

The indexes reported in the previous section, although very useful for the specific purpose for which they have been implemented, do not completely meet the scope of the present work, which is singling out an operational parameter that curators could usefully apply when approaching the assessment of suitable contracts concerning the maintenance and management of HVAC systems in exhibition halls.

In fact, the TWPI index takes into account only the chemical degradation of organic materials, mainly referring to the changes in the air temperature values, and it does not suitably consider other types of damage. On the other hand, the indicator based on the kinetic theory depends on coefficients whose fixed values are not always well known. As for the PI(T) and PI(RH) indexes, due to their inherent definition, they consider the two environmental physical parameters singularly, in this way not considering the damage induced by the contemporary effect of air temperature and relative humidity whose synergic contribution is of paramount relevance in establishing the environmental related risk to works of art. In addition, despite the fact that the PI(T-RH) index would be theoretically eligible for the purpose of this work, it does not consider the distance between the actual environmental conditions and the suggested maximum and minimum limits values of air temperature and relative humidity set by the current regulations.

On the ground of the above reported considerations, in this section a new approach is proposed in order to allow curators to evaluate the reliability of the companies responsible for the maintenance and management of HVAC systems, based on the damage risk assessment related to the works of art exhibited in museums. This approach tries to take into account not only the percentage of time during which the physical parameters characterizing a given environment fall outside the suggested optimal range, but also the gap between such parameters and their relative safety ranges over a considered period of time. This will allow to single out the situations in which artefacts are more likely exposed to risk conditions in case of a HVAC system failure.

Noticeably, a relevant feature of such indicator should be its capability of properly taking into account the knowledge of the curators concerning the history of the considered works of art.

New indicators of damage risk: The Integrated Over Range and Integrated Under Range indexes

The proposed approach initially requires an analysis of the environmental conditions of the considered storage/exhibition halls when the climatization system is not in operation. In other words, a monitoring campaign of the two involved exhibition halls (hosting the Virgin Annunciate and the Triumph of Death, respectively) in free-floating conditions has been performed. This is important in order of establishing the ex-ante behaviour of the indoor physical parameters (air temperature and relative humidity, in this case).

The proposed approach is based on the evaluation of doses of air temperature (T) and/or Relative Humidity (RH) on the microclimatic environment surrounding the under study work of art rebuilt considering a response time constant of three days [33]. In this aim, two indexes for each microclimatic parameter are here introduced (Figure 1):

Figure 1.

Graphical representation of the IOR and IUR indicators

• The Integrated Over Range (IOR) index, which is the integration over time of positive differences between the monitored values and the upper limits of the optimal range:

  $IO{R}_T={\displaystyle \sum }_{i=1}^{Os}{{\displaystyle \int }}_{B_i}^{E_i}\Delta T\times dt$ (3)

  $IO{R}_{RH}={\displaystyle \sum }_{i=1}^{Os}{{\displaystyle \int }}_{B_i}^{E_i}\Delta RH\times dt$ (4)

• The Integrated Under Range (IUR) that is the integration over time of negative differences between the monitored values and the lower limits of the optimal range:

  $IU{R}_T={\displaystyle \sum }_{i=1}^{Us}{{\displaystyle \int }}_{B_i}^{E_i}\Delta T\times dt$ (5)

  $IU{R}_{RH}={\displaystyle \sum }_{i=1}^{Us}{{\displaystyle \int }}_{B_i}^{E_i}\Delta RH\times dt$ (6)

  where Os represents the number of exceedances above the considered optimal range, Us represents the number of exceedances below the considered optimal range, B_i represents the initial instant of time of the i-th exceedance and E_i represents the final instant of time of the i-th exceedance.

The considered response time constant takes into account how the work of art reacts to the modality of variation of the microclimatic parameters. In fact, a high distance from the optimal ranges of air temperature and relative humidity confined in a short period of time would imply different effects compared to a smaller distance from the optimal ranges but prolonged over a longer period of time.

Therefore, the microclimatic quality of the environment with respect to the safety of the exhibited works of art is simply defined by means of two indicators for each parameter, respectively, referred to air temperature and relative humidity. Hence, a total of four indicators are required to define the quality of the environmental conditions in relation to an eventual malfunctioning of the HVAC system.

As previously mentioned, the feasibility of the proposed approach has been verified in the “Museo Regionale” of Palermo (Italy), two halls of which have been considered to provide an example application of the EI indicator’s calculation. As already reported, the indoor air physical parameters of these two rooms have been preliminary measured in free-floating conditions.

Description of “Palazzo Abatellis” museum

The museum is hosted by Palazzo Abatellis (Figure 2), a palace located in the historic centre of the city, along Via Alloro, which is the main artery of the Kalsa ancient district that, at the end of the 15^th century, by virtue of its proximity to the port, recorded its maximum urban development.

Figure 2.

Palazzo Abatellis entrance (top left), internal atrium (top right) and exhibition halls’ layout (bottom left and right) (source: “Archivio fotografico di Palazzo Abatellis”, http://www.celeste-ots.it/musei_siciliani/museo10_1.htm)

The museum was built in the late 15^th century by Matteo Carnalivari as the residence of Francesco Abatellis, port master of the Kingdom of Sicily, and it is one of the most significant buildings of Gothic-Catalan architecture in western Sicily. The building was later used as a monastery and underwent many transformations over time. Severely damaged by the bombings of 1943, the building was lately restored by the Superintendence of Palermo. In 1953-54, with the museum set up by Carlo Scarpa, it became the headquarter of the National (now Regional) Gallery of Sicily. This Gallery reports the evolution of the figurative culture in Palermo, and in western Sicily, from the 12^th to the 18^th century. The Gallery’s collections come from acquisitions, donations and confiscation of assets of suppressed religious bodies. Before joining the current site, these goods were part of the Picture Gallery of the Royal University and then, starting from 1866, the collections of the National Museum of Palermo.

The works of art (Figure 3) considered in this case study are described in the following:

• The Triumph of Death (exhibition hall #2), a fresco considered as one of the most representative works of the late Gothic painting in Italy. The author of the work, which dates back to 1446, is unknown;

• The Virgin Annunciate (exhibition hall #10), a painting by the Italian Renaissance by Antonello da Messina, probably painted in Sicily in 1476, shows the Virgin interrupted at her reading by the Angel of the Annunciation.

According to the Italian standard UNI 10829 [18] and the Italian Ministerial Degree of 10 May 2001 [28], the recommended ranges of air temperature T and relative humidity RH for these works of art are: T comprised between 19 °C and 24 °C and RH comprised between 50% and 60% for the Virgin Annunciate, while T comprised between 10 °C and 24 °C and RH comprised between 55% and 65% for the Triumph of Death.

Figure 3.

The Triumph of Death on the left and the Virgin Annunciate on the right (source: “Archivio fotografico di Palazzo Abatellis”, http://www.regione.sicilia/beniculturali)

The monitoring of the two considered microclimatic parameters (air temperature and relative humidity), in free-floating conditions, inside the halls hosting the two works of art has been performed for a whole year (2011), as reported in Figure 4.

Figure 4.

Yearly trend of the monitored values of air temperature (T) and relative humidity (RH) in the exhibition halls hosting the two considered works of art

The results of the simulations have been analysed using a monthly statistical analysis. In order to compare the existing indexes with the new proposed indicator, the results obtained from the performed simulations were normalized by using a scale of values comprised between 0 and 1, where 1 represents a critical microclimatic condition for the work of art, and 0 corresponds to an acceptable condition. Specifically, the statistical analyses lead to some consideration reported in the following.

Concerning the PI(T-RH) index for the Virgin Annunciate as well as for the Triumph of Death, in both cases of the hypothesized service interruption period it gave on average higher values respect to the others indexes. Moreover, the PI (T-RH) presented a wider change as regards both the symmetry of the distribution and the dispersion of the values. A slight difference concerning the most critical periods was found between the two works of art, which turned out to be June-September and December-January for the Virgin Annunciate and July-September for the Triumph of Death, regardless of the duration of the HVAC interruption period.

With reference to the results of the KT index, the influence of the different interruption periods emerged in both the symmetry and the dispersion of values with regard to the Virgin Annunciate, for which the most critical months resulted to be April, May, October and November. Relatively to the Triumph of Death, instead, KT presented an almost identical trend for the two considered interruption periods, with higher values in the interval June-October.

In order to better emphasize the advantages introduced by the new proposed indicator EI, it was decided to report in the following Figure 5 a comparison between the monthly median values of the EI indicator and the TWPI index, as this latter, among the existing indexes, represents the one theoretically more suitable for the purpose of this work.

Looking at the TWPI results reported in Figure 5, it can be observed how the graphs regarding the Virgin Annunciate show a very similar trend for both interruption periods, hence the TWPI does not seem to be particularly influenced by the change in the period of interruption, the critical period goes from June to October. This index, however, does not give useful information for the Triumph of Death.

In Figure 5 the behaviour of the new EI indicator is also reported. As it is evident, relatively to the Virgin Annunciate, EI appears to be strongly influenced by the season change and by the variation of the interruption interval. In fact, going from 5 to 15 days of interruption of the HVAC system, an increase in the values of the indicator can be observed. The fact that a similar behaviour is not equally evident in the graphs relative to the Triumph of Death may be due to two main reasons:

• The recommended range of air temperature T for frescos (10-24 °C) is wider than that suggested for paintings on wood (19-24 °C);

• The different exhibiting choices for the two works of art, since the Virgin Annunciate is contained in a showcase located at the centre of exhibition hall #10, while the Triumph of Death is put against a wall in exhibition hall #2. Despite this, the EI indicator seems to give more useful information than the TWPI, which shows an almost flattened trend.

Figure 5.

Comparison between the EI indicator and TWPI index relative to the considered works of art, for the two hypothesized HVAC system interruption periods

The observed differences among the existing indexes and the new proposed indicator (and the hypothesized periods of interruption) are ascribable to their intrinsic definitions. In fact, PI(T-RH) does not consider the distance between the monitored values of air temperature and relative humidity and their relative suggested limits. The index KT, instead (although showing the best intrinsic structure, able to consider several effects induced by the microclimate on the specific materials constituting the examined work of art), depends on fixed (not present in the scientific literature) coefficients that are based on the different typologies of works of art. The TWPI, finally, is only suited for works of art composed of organic materials, besides, it shows a strong dependence on the air temperature and, conversely, a weak dependence on the relative humidity. On the contrary, the new indicator EI is built-up to effectively evaluate the distance of the monitored values of air temperature and relative humidity from the corresponding allowed relative ranges, and it also takes into account the response of each specific work of art by means of the considered “response time constant”.

This feature candidates the EI indicator as a suitable one for evaluating the shortest conceivable period of HVAC malfunctioning to which possible damages to the works of art are related. Therefore, EI could be adopted as a decision support for curators in regulating their contracts with the companies providing the service of maintenance and management for HVAC systems (i.e. evaluating the planned interruptions for maintenance operations and establishing the maximum time of acceptable disservice).

To sum up, the main advantages of the proposed new EI indicator are the following:

• The capability of considering not only the number and duration of times in which the environmental indoor parameters fall outside the recommended ranges, but also the actual “distance” from such ranges;

• The use of a “response time constant”, characteristic for each single work of art, which allows to take into account how the work of art reacts to the modality of variations of the microclimatic physical parameters;

• The possibility through proper weighting coefficients to consider the different impact that the environmental parameters have on the different works of art, in order to make the estimation of the damage as unbiased as possible.

HVAC systems are requested not only to ensure the comfort indoor thermal conditions to visitors, but also to guarantee optimal conditions of rooms where cultural goods are positioned, in line with the prescriptions for their exhibition. Hence, in case of a HVAC malfunctioning, it is important to quantify the possible damage induced to the exhibited or stored works of art. This would represent a useful information for museums’ curators in the aim of properly regulating the stipulation of contracts with companies in charge of the management and maintenance of the HVAC systems, by including economic considerations related to the system’s discontinuity period. The proposal presented here is a first attempt to establish a protocol to support curators in this attempt.

For this purpose, a tentative integrated, economic-wise, indicator (EI) was built up by means of two indexes that represent the whole time during which indoor air temperature and relative humidity fall respectively above (IOR) and below (IUR) the recommended ranges for the given work of art. The indicator is thought on the conviction that the duration of the systems failure periods is related to the possible damage occurring to the works of art. Concerning its mathematical expression, two weighting parameters, respectively, referring to air temperature and relative humidity, have been introduced, in order of considering their different impacts on the exhibited artwork.

In the paper, the new proposed indicator has been compared with the existing TWPI through an application to an old heritage museum building in Sicily. The comparison has shown the feasibility and the effectiveness of EI, while TWPI confirmed its almost invariance with respects to the period of interruption of the HVAC system.

In conclusion, further investigations must be carried out, especially regarding the values to attribute to the > and > coefficients, which the EI indicator depends on, also for better taking into account the influence of the season of the year in which the break occurs. Anyway, the role of curators and their personal knowledge of the history of the artwork in a given museum is of paramount importance for the final assessment of the EI index.