Electrostatic technique retrieves prints etched into brass by sweaty fingers

Forensic scientists can now find fingerprints on metal surfaces that have been wiped clean. John Bond, Scientific Support Manager at Northamptonshire Police, and colleagues at the University of Leicester, UK, have developed a way of enhancing the patterns that fingerprint residues corrode in metal surfaces - leaving detectable marks even when the metal is painted, swabbed, or heated. 

Bond’s team hope their research might be used to pin suspects to a crime by retrieving fingerprints from bullet cartridges after they have been fired from a gun - though early attempts have not proved that successful.

Conventional fingerprinting techniques enhance the marks left wherever fingers touch surfaces and transfer ridges of natural oils and sweat. But these prints can easily be smeared, washed off or obscured by dirt. Previous work has showed, however, that the combination of water and inorganic salts - particularly, chloride ions - in fingerprint residue can also permanently etch fingerprints directly into metal surfaces. 

To reveal the hidden prints, the metal is first washed with hot soapy water to remove any grime and all traces of fingerprint residue. Next, an electrical charge of about 2500 volts is applied to the metal, and a fine conducting powder - similar to photocopier toner - is added, which preferentially adheres to the corroded ridges in the metal. Bond suggests this works because corrosion leads to both impurities and lattice imperfections on the metal’s surface, increasing its resistivity. 

’Importantly, the corrosion is accelerated by heat, for example when discharging a firearm,’ Bond says. ’And most bullet casings are made from brass, a metal which works particularly well for the corrosion process.’ 

The technique may be particularly helpful in solving gun crime. ’It is difficult to imagine putting bullets into a gun without pressing quite hard on them,’ says Rob Hillman, who worked on the project. ’And the fingerprinting infrastructure is so good that as soon as we find an image we can use it immediately.’

The team are not yet involved in current crimes, but two police forces in the UK and a federal prosecutor in the US have shown interest in the technology and sent in old bullet casings for analysis. 

Bond also believes this technique may find roles in assisting investigations into bombings and arson attacks. Fingerprints could be found on all sorts of metal surfaces that have been exposed to heat, he says, such as metallic bomb fragments, copper piping or brass doorknobs. 

’This is interesting research, but they were working on fresh fingerprints and there may be problems investigating older cases,’ says Neil McMurray, who works on detecting fingerprint residues from metals at the University of Swansea. 

Indeed, Bond admits that while the technique works well for brass and copper disks, a real-life test to retrieve fingerprints from fired cartridges was not so successful. 

’We fired off 40 cartridges on which we’d deposited fingerprints pre-firing and could retrieve prints from only 2 of them,’ he told Chemistry World. He speculates that this is because rough metal-metal abrasion as a cartridge is loaded and fired may introduce fresh corrosion marks; or could move around fingerprint residues which corrode again from their new position, blurring the original pattern.  

Lewis Brindley

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