Tin pest

Tin pest is an autocatalytic, allotropic transformation of the element tin, which causes deterioration of tin objects at low temperatures. Tin pest has also been called tin disease, or tin leprosy (Lèpre d'étain).

It was observed in medieval Europe that the pipes of church pipe organs were affected in cool climates. As soon as the tin began decomposing, the process sped up, and seemed to feed on itself.

The allotropic transformation
At 13.2 degrees Celsius (about 56 degrees Fahrenheit) and below, pure tin transforms from the (silvery, ductile) allotrope of β-modification white tin to brittle, α-modification grey tin. Eventually it decomposes into powder, hence the name tin pest.

The decomposition will catalyze itself, which is why the reaction seems to speed up once it starts; the mere presence of tin pest leads to more tin pest. Tin objects at low temperatures will simply disintegrate.

Scott expedition to Antarctica
In 1910 British polar explorer Robert Scott hoped to be the first to reach the South Pole, but was beaten by Norwegian explorer Roald Amundsen. On foot, the expedition trudged through the frozen deserts of the Antarctic, making for caches of food and kerosene deposited on the way in. In early 1912, at the first cache, there was no kerosene; the cans &mdash; soldered with tin &mdash; were empty. Members of the expedition later died in the cold and blizzards, only eleven miles from a massive depot of supplies.

The cause of the empty tins is unknown. Some observers blame poor quality soldering, although tin cans over eighty years old have been discovered in Antarctic buildings with the soldering in good condition. In any case, the lack of kerosene was just one factor in the deaths. One source observes,

"The likely cause of death for Scott's polar party was some combination of scurvy, gangrene, starvation, dehydration, and hypothermia."

Napoleon's buttons
The story is often told of Napoleon's men freezing in the bitter Russian winter, their clothes falling apart as tin pest ate the buttons. Whether failing buttons were indeed a contributing factor in the failure of the invasion remains disputed; critics of the theory point out that the tin used would have been quite impure and thus more tolerant of cold temperatures. Laboratory tests provide evidence that the time required for unalloyed tin to develop significant tin pest damage at lowered temperatures is about 18 months, which is more than twice the length of Napoleon's Russian campaign.

Modern tin
Today tin casting and plating avoids tin pest. The tin is alloyed with small amounts of antimony or bismuth, which prevent the decomposition. Silver, indium, and lead have also been used.