Selected Evaluation Methods

The literature review showed different methods to evaluate the preservation conditions for a work of art in a historic building. Roughly, these recommendations can be divided into two groups (Figure 2.10):

 those who focus on general prescriptions for the indoor climate

 those who focus on the behaviour of a particular art object

Figure 2.10:Approach studied in this work to estimate damage risk.

Methods belonging to the first group look whether the simulated temperature and relative humidity fall between allowed boundaries. If they do not, it is concluded that there is a risk of damage. These methods are rather easy to use because only temperature and relative humidity of the indoor environment have to be known to evaluate the preservation conditions. For defining the allowed boundaries, different guidelines to evaluate the indoor climate (see paragraph 2.1.3).

When generalizing the guidelines found in literature, it seems as if every single organisation strives to find its ow guidelines or recommendations, although these are all quite similar. Table 2.4 summarises the recommendations from major organisations. Concerning the relative humidity, the guidelines include an average value with allowable fluctuations or an allowable range. These values are mostly centred in the mid-range of 40-60%RH. In this dissertation, the ASHRAE method to evaluate the preservation conditions is used, and more specifically class B.

The reason is that this method is widely used and incorporates short-term and seasonal fluctuations providing flexibility for historical buildings. Because, a criticism on these methods is that they are too rigorous, compared to the observed damage of the object and therefore leading to too stringent climate conditions, the method that focuses on object behaviour is also used.

PREDICTING DAMAGE RISK:GUIDELINES &DAMAGE FUNCTIONS 35

Table 2.4: Comparison of the boundary values for the indoor air temperature and relative humidity given by the ASHRAE, the Smithsonian institute and the Bizot group. For ASHRAE class B was selected as this is

what can be achieved in a historical building (based on [68,138]).

Parameter ASHRAE (class B) Smithsonian Institute Bizot Group

T [°C]

Setpoint 50 or annual average 45

Minimum 37 (30) 40

Maximum 75 53 (60) 60

Daily fluctuation ±10 ±10

Seasonal fluctuation ±10

2.2.1 ASHRAE Method

ASHRAE does not provide a clear description of how short-term and seasonal fluctuations should be used. Several authors use different methods [139–141], that can lead to different results. In this work, the method to evaluate the indoor climate as described by Martens [141,142] is used.

This method is a so-called reverse method because ASHRAE classes are normally used as design parameters and not as an evaluation tool. The principle of the ASHRAE method is to define a bandwidth which the short-term (daily) fluctuations may not exceed. This bandwidth is derived from the average course over the year, including long-term (seasonal) variations.

To define the bandwidth, first the seasonal variation is calculated. To calculate the allowed seasonal variation the yearly average or control set point can be used. Since the work of art has been in the historic building for decades, the target values will be based on the yearly average which is more representative than the control set-point. The long-time variation defined in the ASHREA-class (Table 2.1) is added or subtracted to the yearly average to set the boundaries for the allowable seasonal fluctuation (Figure 2.11a).

Next, the moving average used to express the average course, is calculated for the temperature and relative humidity fluctuations of the space (eq.(2.2)). By using the moving average, short-term fluctuations are smoothed out and long-short-term variations are highlighted. The period for calculating the moving average covers three months and is centred¹². Furthermore, the restriction that the temperature should be between 15°C and 25°C [143] is not taken into account. This is because this guideline is beneficial for human comfort, but not for the preservation conditions since for a work of art a lower temperature results in a lower chemical degradation. Because the course of the moving average for an entire year is needed, the sampling period needs to be

12 This guarantees that variations in the average temperature and relative humidity are not being shifted in time.

extended by one and a half month before and after. When the moving average exceeds the boundaries set by the seasonal yearly average, the moving average is truncated by these boundaries (Figure 2.11b).

x̅_moving=xi−1.5 month+ ⋯ + xi+ ⋯ + xi+1.5 month

n (2.2)

Then the allowable short-time variation defined by the chosen ASHRAE class is added to or subtracted from the average course to set the boundaries for the allowable short-term fluctuations (Figure 2.11c).

Figure 2.11: Reversed ASHRAE method used in characterising the preservation conditions.

To evaluate if the measured or simulated indoor temperature or relative humidity satisfies the conditions of the desired ASHRAE class, it should be checked whether the daily minimum and maximum lie between the boundaries (Figure 2.11d). Results are expressed as the percentage of time that target values for temperature, relative humidity or humidity were met during the the combination of temperature and relative humidity were met during the monitoring campaign or simulation period. These percentages give an indication of how far the conditions for the indoor climate deviate from the conditions required to achieve a certain preservation class. It is important to remark that once the conditions for a certain class are not continuously fulfilled , there is risk of damage, no matter whether the preservation conditions are within the specified class for 99% of the time or for only 10% of the time.

PREDICTING DAMAGE RISK:GUIDELINES &DAMAGE FUNCTIONS 37

2.2.2 Object Oriented Method

The methods from the second group also take object–related characteristics into account in addition to temperature and relative humidity. An example is the three-types of degradation principle, including damage functions for chemical, biological and mechanical deterioration.

Martens [141] used such a method to assess the preservation conditions of an indoor environment by linking temperature, relative humidity and an object type to the three-types of object degradation (Figure 2.12). Four types of objects were defined: paper, panel painting, wooden sculpture and furniture. The damage risk assessment for each type of degradation is based on methods described in literature. For example, to assess the risk of mechanical damage for a panel painting, the study of Mecklenburg [144] and Bratasz [145] was used.

Figure 2.12: Approach developed by Martens [141].

In this work, a similar approach as the approach from Martens [141] is used, meaning that the climate assessment will be made for the three degradation types. In contrast with the work of Martens, the mechanical damage risk will not be estimated by using temperature and relative humidity directly, and linking them to previous defined boundaries. Instead temperature and relative humidity will serve as input for the estimation of the moisture content in the object. To do so, the moisture transport of the work of art will be modelled using a coupled BES-HAM model (Chapter 3).

This approach can itself be used for different kinds of objects. However, to estimate the mechanical damage risk, a proper evaluation of the moisture content of an object is necessary as the moisture content is directly related to shrink and expansion of materials. To calculate the changes in moisture content of the object, material characteristics related to the object are required [56]. Therefore in this work, one type of work of art object was chosen as case study, that is wooden panel paintings, from the 15^th -16^th century, which are part of the Early-Netherlandish paintings¹³. This type of panel paintings was selected for two reasons. On the one

13'Early Netherlandish paintings' refers to a style developed in the 15th and 16th century Northern Renaissance and the painters are known as the so-called ‘Primitifs flamands’. During this period, Flanders was the centre of the Northern Renaissance. Next to the many artworks that were made for the Burgundian court, paintings were also made to be hung in churches.

hand, wooden panels are an important category of organic works of art and are representative models of inhomogeneous organic constructions [146]. On the other hand, the Early-Netherlandish panel paintings are very important to preserve, including many famous pieces that are protected by the government. [15].

The three types of damage will be assed. The focus in this work lies on the mechanical degradation. To have a thorough comparison with the general method, chemical and biological degradation were also measured, either to a limited extent. However, in reality it is up to the user to decide what to take into account and what not.

 Chemical damage risk: ‘Lifetime multiplier’ [112] and Equivalent lifetime multiplier [147] to assess improvement of the relative lifetime.

 Biological damage risk: Only mould risk is assessed which is mainly related to a too high relative humidity. Other kinds of biological degradation like insects, are not taken into account. In this work, the hygro-thermal model of Sedlbauer is used to assess the mould risk [148]. This model is available through the freeware WUFI-Bio [149] and a theoretical background of this model is described in Annex 2C. The discussion whether which mould model should be used, is outside the scope of this work.

 Mechanical damage risk: Numerical simulation to predict the moisture response of the object (HAM). Although experimental studies of dimensional changes of works of art are also possible and interesting, they are time consuming and expensive and they are not always sufficient [150]. Furthermore, experiments with objects of cultural heritage are often not possible since the objects are unique, and possible damages cannot be accepted. In this study, based on the results of a numerical simulation, mechanical damage and fatigue damage will be assessed.

- Yield: Related to material characteristics. Object specific research necessary.

- Fatigue Damage: A similar approach as suggested by Bratasz [136] will be used. The method of ‘proofed fluctuation’ of Michalski [130] seems less usable for the purposes of this dissertation. This is because in case the proofed fluctuation causes undesirable damage, necessitating a restoration of the work of art, one would like to avoid this fluctuation at all costs. It is thus known that the proofed fluctuation was a too large fluctuation.

However, predicting an allowed threshold based on this value, becomes a difficult objective exercise. For this purpose, experiments and results are useful.

The results of both methods will be compared. In case a discrepancy is noticed in the results, the discussion will focus on finding out in which case a more individualized approach (from the second group of methods (§2.2.2 ) is required and in which case a more general approach (according to the first group of methods §2.2.1) is sufficient.