HUMIDIFICATION
The challenges of museums and libraries
T
he materials used in works of art are primarily: canvas, wood, paper and parchment; nonetheless, the conservation of these requires limits that are much stricter than those relating to the material alone. An alteration in the dimensions of the canvas of a painting may cause the paint to peel; the same eff ect with wood may produce cracks in the painted panels, or in the case of antique furniture detachment of the joints and separation of the decorations.
Furthermore, there may be deformations to manuscripts, with a loss of elasticity and consequent fragility.
As a consequence, accurate temperature- humidity control is essential. Room temperature is usually established according to the needs of visitors, and is set between 18°C and 20°C. The relative humidity for the correct conservation of the works displayed should lie in the limited range between 45% and 50%. Conservation is compatible with lower humidity levels than those indicated as long as
38 December 2016
the temperature is also lower. For example, at a temperature of 10°C a relative humidity of 30% is acceptable, while the use of heating, together with the introduction of cold outside air with a low moisture content, leads to very low relative humidity values that are poorly tolerated by the works displayed.
Obviously, certain limits must also not be exceeded at the opposite end, as high humidity levels may lead to local condensation and the formation of mould, as well as unacceptable dimensional alterations.
Similar problems to those described above also arise in libraries, particularly where precious documents are kept.
The ideal conditions are a temperature of 18°C ± 1°C and a relative humidity of 40% ± 5% all year round in rooms where books are stored. In reading rooms, the acceptable range of values is wider, with a humidity of 50% ± 10% and a temperature that is more suitable for the comfort of users, both due to the shorter reference time and the lesser importance of
works that are more frequently read compared to manuscripts or parchments of historical-artistic interest.
It may seem puzzling why there are such strict requirements for the conservation of works that have lasted in more or less good condition for centuries, without special attention to ambient conditions.
There are three main diff erences from the past that have led to fl uctuations in relative humidity levels in museums and the like: heating in winter, attendance by visitors and lighting. This document will focus on the fi rst two. The concept of relative humidity can, in quite simple terms, be defi ned as the quantity of water contained in air relative to the quantity the same air could potentially hold. When air reaches 100% relative humidity, it is saturated, and adding additional water vapour leads to condensation. One of the basic properties of air is that its capacity to hold water vapour increases with temperature. Consequently, in winter, even if the outside air is very humid, once this is introduced
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