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Bureau of Criminal Apprehension

A Division of the Minnesota Department of Public Safety

Serial Number Restoration

Law enforcement agencies submit firearms and other items to the BCA Forensic Science Services that have obliterated serial numbers, serial numbers that have been ground, filed or removed by other means. Forensic scientists can use physical and chemical techniques to restore or enhance altered characters.  Restoring a serial number can provide law enforcement agencies with information about a firearm or other item and may offer a lead in an investigation.
A frame of a revolver containing an obliterated serial number.
Generally, scientists will polish the metal with a sand-paper type material to remove chips of metal from the surface.  Next they will apply chemical reagents, typically acids, to etch the metal until the number is restored.  The scientist will select the proper chemical based on the variety of metal surface.  For example, different chemicals are used to etch aluminum, zinc and steel alloys. 
Obliterated characters restored on a wood gun stock.
Although the BCA routinely restores serial numbers on the metal surfaces of firearms, examiners also have procedures for obliterations on wood and plastic surfaces. 
The BCA Firearm Section’s firearms reference collection is useful for determining the serial number locations and numbering conventions used by the manufacturers.  This information allows firearms examiners to identify partially obliterated or restored characters in a firearm’s serial number.