Enhanced Fingerprints on gun after being wiped

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English physicist John Bond has developed a technique for analyzing fingerprints on a gun after it's been wiped clean! Dr John Bond, who works with Northamptonshire Police, realised that because sweat corrodes metal surfaces, fingerprints could be 'seen' on bullet casings and other metal surfaces even after they were wiped clean. Bond applied an electrical charge and a fine carbon powder to a gun's corroded part, revealing a fingerprint pattern. Police are already using the four-month-old technology to reopen some cases.

An excerpt from an interview with him:
Reporter - How does your new technique work?
John - What we’ve been looking at is a phenomenon we’ve found that fingerprint deposits will tend to corrode metal surfaces. There’s some constituents in the fingerprint deposit that on metals like brass and copper will corrode the metal to an extent that even when you’ve then got rid of the residue totally you can sometimes actually see an image of where the fingerprint was in the metal or, where that’s not possible, we’ve developed a technique to actually enhance that corrosion and make the fingerprint become visible again.
Reporter - So how do you visualise the fingerprint in the form of its corrosion pattern on that surface?
John - We take the metal and apply an electrical potential to it at the order of 2500V. We then apply a very fine conducting powder, very similar to photocopying toner powder. What we’ve discovered is that that will preferentially adhere to the metal at the points where the corrosions occurred which are coincident with the original fingerprint ridge pattern. You get an image of where the fingerprint was in this black powder.

Read in greater detail: http://www.thenakedscientists.com/HTML/content/interviews/interview/926/