CHAPTER 2: The Degradation of Works of Art Materials

This chapter deals with the general physicochemical properties of works of art materials, strictly correlated to the degradation and alteration processes. The principal chemical and physico-structural properties of both organic and inorganic materials constituting wall paintings and easel paintings are discussed. The mechanisms leading to the degradation of these works of art are analysed, classifying them into three broad categories: natural weathering; pollution – indoor and outdoor; and general chemical, physical and mechanical stresses. The natural weathering of artworks materials is then discussed, focusing on the role of temperature, humidity and light, with particular attention to the phenomenon of salt crystallization. The environmental conditions for the best conservation of indoor and outdoor works of art are highlighted. The remaining chemical effects driving decay processes are analysed by referring to condensation of water, with consequent wetting and capillary suction. In this frame the effects of deliquescent and hygroscopic salts are taken into account, together with the topic of salt efflorescence. Finally, the physical alteration associated with the combined action of light and oxygen from air is treated in relationship to two classes of compounds particularly sensitive to these phenomena, i.e. drying oils and natural varnishes.