Allotropic Transformation
At 13.2 degrees Celsius (about 56 degrees Fahrenheit) and below, pure tin transforms from the silvery, ductile metallic allotrope of β-form white tin to brittle, nonmetallic, α-form grey tin with a diamond structure. The transformation is slow to initiate due to a high activation energy but the presence of germanium (or crystal structures of similar form and size) or very low temperatures ~-30 degrees Celsius aids the initiation. There is also a large volume increase of about 27% associated with the phase change. Eventually the α-form decomposes into powder, hence the name tin pest.
The decomposition will catalyze itself, which is why the reaction speeds up once it starts; the mere presence of tin pest leads to more tin pest. Tin objects at low temperatures will simply disintegrate.
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