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Tin Disease

Dateline: 04/03/00

By Alan Bruzel

What Is Tin Disease?

First, the word allotrope needs to be explained. An element existing in more than one form is said to exhibit allotropy (from the Greek allos, other, and tropos, way). The element oxygen, which may be found in two allotropic forms, diatomic oxygen and triatomic oxygen, provides one example. We breathe the diatomic O₂, and we try not to breathe the triatomic O₃, also known as ozone. Carbon, with its two allotropes, graphite and diamond, provides another example.

Tin also has two allotropes: the familiar white metal, called beta-tin, and a crumbly, gray non-metallic powder known as alpha-tin. At temperatures below 13.2^oC (55.8^oF) the gray powder allotrope begins to show more stability than the white metal allotrope. As temperatures decrease below 13.2^oC, the shift from the metallic allotrope to the non-metallic allotrope progresses, reaching a maximum at about -30^oC (-22^oF).

The gradual disintegration of shiny tin metal into a crumbly, gray powder was observed hundreds of years ago on tin organ pipes housed in unheated northern European cathedrals. With allotropy unknown, the phenomenon was attributed to the work of the Devil and was variously denoted by the terms tin disease, tin pest, tin blight, or tin plague. The reaction is autocatalytic; in other words, the presence of the gray alpha-tin accelerates the process. Tin disease manifests itself in pure tin. Antimony, bismuth, or lead retards this transformation. Tin alloys are thus more resistant to tin disease.

Tin Disease and History

The transformation of tin into its gray powdery allotrope has presumably occurred enough times in history to provide suitable examples for chemistry textbooks. Many of these accounts may be apocryphal. Many may simply be corrosion processes romanticized into bona fide cases of tin disease. (Tin by itself resists corrosion because of the oxide layer on its surface. If used to coat other metals, tin's corrosion-preventing properties become compromised when its surface is damaged. For example, the corrosion-prone steel of a "tin can" will quickly rust when its thin tin coat is punctured.) In any event, the following list may include valid illustrations of the temperature-dependant transformation of tin allotropes:

Tin-plated funeral memorials of the Aegean Late Bronze Age (about 1400-1200 B.C.) fell victim to tin disease not too long after their consecration.

Tin farthings manufactured in England from 1684 to 1692 are prized by numismatists if they show no tin disease. (The tin coins made in Thailand in the 1940's would not be expected to suffer from this ailment because of the milder weather conditions.)

Tin ingots stored in St. Petersburg became worthless gray powder following a Russian winter. (Chemists restored the valuable white tin metal by melting the gray allotrope.)

Because of the extreme cold, the tin buttons fastening the clothes of Napoleon's troops became unusable gray powder, thereby exacerbating Napoleon's already disastrous invasion of Russia.

What the Web Has to Say about:
Tin Disease

Allotropes
Discussion of carbon, oxygen and tin allotropes. Article by Nigel Bunce and Jim Hunt, University of Guelph.

Chemical Energetics
Energy and conversion of allotropic forms of elements. From the University of Waterloo.

Coins of England and Great Britain: Farthings
Tony Clayton presents details on United Kingdom coins and the metal used to fashion them.

Short History of Metals and Metallurgy
Introduction to some metallic elements and their properties. From Alan W. Cramb, Carnegie Mellon University.

Thermodynamics Exam
Calculating the temperature at which pure white tin converts to gray tin. From Christopher Alan Parr, University of Texas at Dallas.

Tin Pest
Barry Knight suspects corrosion, not tin disease, is the process occurring in museum collections. From Conservation Online.

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