Fingerprint residue contains natural salt secretions that corrode metals at the points where the salt contacts the metal. This microscopic fingerprint corrosion leaves a permanent mark on metal that cannot be washed away. The heat of the firing of the gun also does not alter these marks.

The new technique that has been established to visualize these types of prints relies on running electrical current over the metal object , such as a bullet casing or a gun, that has been coated with a fine toner like powder. When a charge is run down the metal object the fine conducting powder is attracted to these areas of corrosion revealing a powdered fingerprint.

This technology can also be used in fires to recover prints of metals exposed to very high temperatures.

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