Los justos y los pecadores: una realidad dual

7.7.1. Comfort Triangles in conservation: museums and archives.

The storage of documents and works of art require very different conditions than those needed for human comfort (King, 1992). The preservation of delicate papers, valuable documents, historic paintings and other cultural artefacts require strict controls of the variation of temperature and humidity. Teygeler (2001), in an extensive annotated bibliography, emphasises the special problems of conservation, particularly in warm climates, and the disadvantages of artificial climate control, compared with natural conditioning.

Although temperature variations are less critical than the swings in humidity and the impact of light and radiation, the control of temperature variation is vital for reducing thermal movement and control of moisture content (King, 1992).

Hunter (1979) indicates the conditions of temperature and relative humidity recommended for paintings, archives, general conditions in museums and the comfort zones for visitors to museums in summer and winter, for application in the United Kingdom. As Table 8.1 shows, visitors prefer higher temperatures than the optimum for painting and other objects. These narrow limits of humidity and temperature are difficult to achieve without permanent artificial conditioning. King (1992) has questioned this approach, as intermittent air conditioning (due to power failures and economising at week-ends or at night) can be more harmful than natural conditioning in buildings designed to achieve gradual temperature changes over time.

For paintings, Fahey (2005) of the Henry Ford Museum, recommends a wider range of temperatures, between 18° and 21° C with 40 - 45% relative humidity in winter and 21°

to 24° C with 45 - 55 % relative humidity in summer.

Table 7.7. Temperature and humidity ranges for museums.

Artefacts and persons Minimum temperature

Source: Hunter (1979), Figure 33.39, page 291.

The Scottish Museum Council (Bouwmeester, 1995) recommends the following temperature and humidity limits in museums, with more realistic limits that, in many cases, allow natural conditioning, combined with partial heating in winter:

Temperature: Objects in museums 10 - 20°C.

Objects on display 16 - 20°C.

Objects in deposits 12 - 16°C (with climate control during transfer).

Humidity: Maximum humidity levels of 70 % (to prevent mould growth).

Minimum humidity levels of 40 % (to avoid excessive drying).

With temperatures below 10° C, humidity control becomes more difficult, especially when cool objects with low absolute humidity are transferred to or through spaces with warmer conditions. This can produce a severe condensation risk as well as the impact of sharp and damaging changes of humidity. Bouwmeester (1995) also comments on the potential risk of temperature fluctuations.

With these recommendations for the hygro-thermal design of museum spaces, specific Comfort Triangles can be developed for different gallery, museum and archive areas.

Figure 7.11. indicates the Comfort Triangles for different type of spaces and activities, demonstrating the limited combinations that can be implemented to achieve optimum conditions for visitors at museums and art galleries as well as for elements on display.

Table 7.8. Recommended conditions in museums and art galleries.

Application Minimum

The application of the Comfort Triangles for objects on display is more limited in height and width of the base, than the triangles for visitors, as well as having lower averages. The area of triangles for paintings have a small surface that coincides with that for visitors, while for papers and artefacts is even more reduced, with an average temperature of 19° C and a swing of only 2 degrees.

For deposits and stores, the recommended conditions do not coincide at all with those required for human comfort, for low levels of activity.

The requirements for museums are so strict that they can only be achieved by the use of full air conditioning, with both temperature and humidity control. However, these systems can cause severe problems when there are power cuts, mechanical failures or when switched off for maintenance. Condensation risk has to be considered when warmer humid air from outdoors comes in contact with cool objects on display, as well as a danger of mould growth in ducts.

For this reason, various authorities cited by Teygeler (2001) prefer natural conditioning to achieve a more relaxed control with slow temperature variations between more flexible limits.

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Average Temp

Swing °

Comfort Extended Paintings

Paintings hanging Objects

Papers

Objects, storage

Figure 7.11. Comfort Triangles for both visitors and conservation of objects on display in museums, galleries and archives.

In this context, the approach of the Comfort Triangles allows the visualisation of thermal swing control recommended by different authors (King 1992, Hunter 1987, Bouwmeester, 1995), considering that a seasonal variation in the average temperature and the daily swing within fixed limits.

The Comfort Triangles chart, as shown in Figure 7.12., illustrates average temperatures recommended for paintings with a range form 16° to 24°, with a maximum swing of 3 degrees, while the average temperature for visitors comfort ranges from 18º to 28º C, with a maximum swing of 8 degrees, as established in the previous sections of this chapter.

The zone of conditions apt for both objects and people, shown in grey, is more limited, from 18° to 24° C, with a maximum daily swing of only 3 degrees.

This definition of comfort limits, allows the use of certain bioclimatic strategies to control temperatures naturally or to reduce the need for artificial conditioning to a minimum.

This reduces the risk of damage as the building itself acts as a temperature moderator, rather the reliance of mechanical equipment and continuous energy use.

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Average temperature

Swing°

Comfort

Extended Comfort Paintings

Figure 7.12. Zones for thermal comfort of visitors and conservation of paintings.