In this paper, fluctuations in relative humidity have been studied, as humidity is particularly important for dimensional changes which may compromise the condition of works of art [3, 9, 10].Many museum objects, artifacts and works of art are composed of complex hygroscopic materials (for example, painted and unpainted wood, ivory, varnishes and glues) that respond dimensionally to variations in relative humidity and temperature in order to maintain equilibrium with the surrounding environment [11-13.] Dimensional changes may occur with fluctuations in humidity; material expansion or swelling may take place with increased values of RH (and moisture content of the material) and contraction or shrinking may be provoked with decreased humidity.
Swelling and shrinking of the wooden support of a panel painting, frame or sculpture can lead to the development of cracks and irreversible buckling or warping or the paint or wooden layers. Further, such dimensional changes may critically influence the strength of adhesion of the constituent layers in a painting or polychrome work of art, and may provoke subsequent loss of mechanical integrity [10-14]. As painted artworks are multilayered structures composed of heterogeneous materials with varying porosity, hygroscopicity and flexibility, with dimensional changes which follow a change in humidity, a multitude of non-uniform internally generated stresses and strains may be generated [15-18].The amount of stress or load which a material can safely undergo before fracture is governed by Hooke’s law, which forms the basis of the theory of elasticity.
In brief it states that for certain ranges of stress, the strain produced is proportional to the stress applied and disappears on its removal (elastic region); whereas in the limit of proportionality the linearity ceases (plastic region), the material reaches the elastic limit (yield point), accompanied with permanent change (strain) leading to fracture even without further loading. The elastic limit has been used to define the onset of damage in works of art and related materials [17-21]. In this context, if RH fluctuations can cause shrinking or swelling stresses that exceed the elastic limit, damage is provoked and fracture is expected in case of further loading. However, even following irreversible and permanent changes (damage), effects of dimensional changes may initially be invisible [22, 23] until visible and irreversible damage occursThe cumulative effects of dimensional displacement may present visible phenomenon which are well-known to conservators; for example, these can range from changes in craqueleur patterns on the surface of a varnished painting to paint losses, detachments among layers Carfilzomib and cracks in a wooden support [24, 25].