Contributions by: Jeremy Borrelli
Tin pest, or tin disease, is a rare form of deterioration that occurs as a result of the allotropic conversion of white tin into grey tin.
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Some elements exist in more than one chemical form called allotropes; allotropic conversion is the process when the chemical elements of an object change from one molecular structure to another. Solid tin exists in two allotropic forms, white and grey, each of which contains different chemical properties. White tin has a body centered tetragonal crystalline structure and a density of 7.28, while grey tin has a cubic structure and a density of 5.84 (Faulkner 1965). This transformation is generally accepted to occur around 13∫C (Gilberg 1991:3). Below this temperature, grey tin is more stable, while white tin is more stable at temperatures above 13C. Because the volume of grey tin is almost 25% of that of white tin, the transformation from white to grey results in the loss of structural cohesiveness of the metal and therefore, increased deterioration of the tin within an artifact (Gilberg 1991:4). The resulting effect is the formation of a discrete, powdery efflorescence of the grey tin on the surface of the object, eventually reducing the metal to powder, effectively destroying the object. In museum settings this can be especially damaging to artifacts as the process is autocatalytic and will perpetuate itself and infect other tin objects around one that is infected, hence the moniker, tin disease.
Tin pest was first recognized in academic literature in 1851 by O.L. Erdmann, and later in 1868 by Russian chemist Fritzsche (Erdmann 1851; Fritzsche 1868). Both scientists used visual observation to examine the phenomenon and came to the conclusion that cold temperatures of about 30C or below would cause the tin to disintegrate. Scientists would not know the cause for tin pest until the turn of the century when Ernst Cohen discovered the allotropic properties of tin (Cohen 1911). In his studies Cohen experimented and refuted earlier notions that tin pest was solely caused by low temperatures and could be caused by other factors such as subjecting white tin to prolonged contact with powdered grey tin. Ultimately he is responsible for the designation of the phenomena as ìtin pestî (Cohen 1929). His research eventually led him to analyze the process in museums where he published many findings that would be used in early conservation manuals (Gilberg 1991:14). This led to an outburst of reporting the phenomena among the conservation community. It wasnít until Organ and Plenderleith (1953) challenged many of these reports that tin pest became recognized as a rarity and much of the deterioration of tin artifacts was rather due to more localized corrosion. The process has been reported on maritime sites (e.g. Lipcsei et al 2002) but it is still subject to much debate and the difference between terrestrial and marine tin pest has yet to be determined.
References[edit | edit source]
Cohen, E. 1911. The Allotropy of Metals. Transactions of the Faraday Society 7:122-135.
--1929. The Physical Chemist in Search for Purity in an Impure World. Chemisty and Industry:162-169.
Erdman, O.L. 1851. Ueber eine merkwurdige Structur-veranderung bleihaltigen Zinnes'. Journal fur praktisch Chemie 52:428-451.
Faulkner, C.J. 1965. The Properties of Tin. London: International Tin Research Institute.
Fritzsche, T. 1868. Sur un phenomeme de rupture au mileau de blocks d'etain sous l'action d'un froid intense. Comptes Rendus 67:1106-1107.
Gilberg, M. 1991. The History of Tin Pest: The Museum Disease. Australian Institute for the Conservation of Cultural Materials Bulletin 17(1-2):3-20.
Lipcsei, L., A. Murray, R. Smith and M. Savas. 2002. An Examination of Deterioration Products Found on Tin Ingots Excavated from the 14th Century B.C., Late Bronze Age Shipwreck, The Ulu Burun, Near Kas, Turkey. MRS Proceedings, 712, II6.4.
Organ, R., and H.J. Plenderleith. 1953. The Decay and Conservation of Museum Objects of Tin. Studies in Conservation 1:63-72.