Materials Science Research for the Conservation of Sculpture and Monuments
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Materials Science
Research for the Conservation of Sculpture and Monuments
George W. Scherer, Robert Flatt, and George Wheeler Introduction Mountains give the illusion of durability because their grand scale hides the continual weathering of their surfaces. When the same stone is used to create a sculpture, the loss of a few millimeters suffices to spoil, or even eliminate, the features of a face; the rate of loss may be surprising, but it is not necessarily unnatural. In this article, we review the causes of deterioration of sculpture and monuments made of stone and masonry, and examine various approaches to retarding or preventing the damage. Although our focus is on stone, we recognize that the same mechanisms of deterioration act in porous artifacts as diverse as pottery, porous pigments, frits, frescoes, and bone.1 Materials science has a central role to play in this field. The problem is particularly challenging because one has to treat an existing material, rather than create a more suitable one; moreover, one often has little or no control over the conditions of exposure. The goal of the conservation scientist is to diagnose the cause of the damage, then to choose and apply a treatment that will repair the damage and arrest the process of deterioration. There can be no generally applicable treatment for a given type of material because the damage reflects the microclimate to which the object is exposed, as well as its history (duration of exposure, previous repairs, etc.). Unfortunately, the main causes of deterioration are only understood schematically. The details of the mechanisms that control
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the type of damage and the rate at which damage occurs are not clear. To provide accurate diagnosis and effective treatment, we must learn the fundamental mechanisms of deterioration. Table I outlines some of the phenomena that are of importance in stone* conservation and suggests some avenues for materials research. Although the list is long, it is obviously incomplete because it does not include decay related to biological agents or air pollution. Since the field is vast, in this article we focus our attention on one important problem: the crystallization of salts. First, we examine in detail the mechanisms by which growing crystals do harm, and show how a clear understanding of the thermodynamics of the problem opens new avenues for treatment. We then consider two different approaches to conservation: consolidants, which are polymers or sol-gel materials that penetrate the damaged stone and bind its wounds, and interfacial modification, which includes treatments to prevent ingress of water and treatments that change the salt/stone interfacial energy in such a way as to prevent stress development. Finally, we outline areas in stone conservation where materials research is urgently needed. If this problem is neglected, the damage to * We occasionally use the word “stone” in this article as shorthand for porous materials including brick, concrete, and mortar.
our cultural heritage, whether natural or accele
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