| Abstract: | SUMMARIES. The deterioration of artefacts may involve a number of different decay mechanisms and produce a variety of morphologies dependent on the mode of formation of the deterioration product and the type of artefact concerned. Deterioration can be considered as derived from reconstructive, epitactic, or topotactic events, but does this categorization help us to explain why it is that some deterioration processes are random, chaotic, structured or periodic? One of the approaches that can help to explain the complex structures observed in ancient artefacts is that of fractal geometry. Examples are given of bronze and iron artefacts whose heavily corroded structures illustrate fractal morphologies, layered or banded structures, which represent self organizing systems. These phenomena may produce layers of precipitation, or periodic microstructures whose genesis is not related to annual or fluctuating burial conditions, but to the type of growth phenomena of which the Liesegang precipitation is one example. Fractal and related models which help to explain some of these deteriorated structures are discussed, together with illustrated examples of several different types of morphologies.
CONCLUSION. We know that some forms of deterioration of works of art give rise to chaotic structures that have no relation¬ship to the original artefact and create problems in the preservation of a variety of different types of artefacts. This review suggests that there are a large number of cases in which feedback and self-ordering systems at work in deteriorating artefacts lead to structures or micromorphologies that can be considered to be fractal. Self-ordered systems are situated at the delicately balanced edge between order and disorder, and small perturbations in the parameters of the system may have major consequences for the types of structures that are observed. Scientists differ on the relative value of fractal analysis in their examination of different types of non-equilibrium phenomena. Many patterns and morphol¬ogies have been identified as fractal, but despite this success, its actual utility is often debated. A valid criti¬cism is that the most important consideration is not the resultant morphology of the deterioration process as such, but the underlying mechanistic description. Only in cases where fractal dimensions and other scaling laws can be related to a specific mechanism has there been viable progress made in relating the type of structures observed to the processes that are involved in a causal manner. It seems probable that there may be many different mechanisms at work, which result in the types of structures illustrated in this paper. The more detailed application of fractal geometric analysis may be useful in helping to understand how some of these structures arise. The deterioration of artefacts may proceed in ways which give rise to totally random structures, which preserve vestiges of the original surface or micromorph-ology of the object, or which create entirely new struc¬tural forms. The layered structures observed in weathered crusts found on ancient glass, for example, were already an object of scientific study in 1853 when Sir David Brewster first began to carry out research into their nature. They later led some researchers to suggest that corroded glass could be dated by counting the number of weathered layers, by analogy to the rings of a tree [25]. We can now explain these layers by the types of mechanisms discussed here which are unrelated either to the age of the glass or to the original microstructure of the vessel or window glass from which it evolved. In discussing chaotic structures, the notion of self-organization and of the Liesegang phenomena, we are able to place our understanding today on a much sounder theoretical footing, thanks to the developments and understanding that fractal geometry provides. The further elaboration of these types of decay mechanisms will undoubtedly give rise to greater insight into the deterioration of works of art in the future. In order to examine a quantitative mathematical model, a consider¬able amount of manipulation of the primary data is required in order to pursue a fractal analysis of the kind of morphology shown in Figure 5. In the opinion of Didier Sornette, who is a professor in the Institute of Geophysics and Planetary Physics at the University of California, Los Angeles, and an expert in the field of fractals [26], this appears to represent a good example for further study. Funding tor this kind of study is not yet available, but should it become so, further work on this interesting topic can then be undertaken. |
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