A crystal change, not ordinary freezing
Tin does not become gray tin simply because it gets cold in the way water freezes into ice. Below about 13°C (56°F), ordinary metallic tin—known as white tin—can gradually transform into gray tin, a different arrangement of the same tin atoms. Gray tin is a semiconductor with a diamond-like crystal structure, rather than the tightly packed metallic structure of white tin.
The change is slow because atoms must shift into new positions across the solid. Cold makes the gray form more favorable, but the transformation may take time to begin and spread. Impurities or tiny patches of gray tin can help it get started by providing places where the new structure can grow.
Why the metal falls apart
The two structures occupy different amounts of space. As gray tin forms, the material expands; neighboring regions strain and crack as the transformation advances. Gray tin is also brittle, so it cannot bend to relieve that stress. Once cracks expose more tin, the conversion can continue through the object, leaving it weak and crumbly.
This deterioration is often called tin pest. It is not corrosion: oxygen is not turning the metal into rust. The substance remains tin, but its crystal structure—and therefore its physical behavior—has changed.
Why the effect is not inevitable
Temperature is only part of the story. The transformation is much more troublesome when tin stays cold for a long time, and alloying tin with other metals can inhibit it. That is why tin objects do not automatically disintegrate whenever winter arrives: composition, duration, and whether the new crystal form gets a foothold all matter.
The striking consequence is that a metal object can fail from within without losing its material to the surroundings. Its atoms remain tin; their new arrangement simply makes the once-solid object unable to hold together.
