Tech

Forensic scientists could soon be pulling fingerprints from fired bullet casings, a technique not unlike one used by Batman to track down the Joker in this year's summer blockbuster.

Despite advances in DNA technology, fingerprint identification remains an important forensic tool. In the UK, twice as many car thieves are caught by fingerprint identification as by DNA evidence.

Fingerprinting dust clings to organic compounds like amino acids and urea from skin to reveal prints. But any of those residues on a bullet are likely to be burned away when it is fired, typically reaching temperatures above 200°C.

Now John Bond, a scientist with the Northamptonshire Police and fellow at the University of Leicester, both in the UK, has discovered that fired bullet casings can reveal fingerprints. And it is the high temperatures they reach that make it possible.

Etched prints

When a gunman loads a cartridge into their gun tiny quantities of salty sweat from their fingers are transferred onto it, recording an impression of the fingerprint.

Firing the gun rapidly heats the casing, vaporising the water in the sweat. "You're left with non-volatile salts," Bond says. Established fingerprinting techniques ignore those residues, but they can reveal prints, he has discovered.

"At high temperature, those salts are molten and you get a chemical reaction with the metal." Those reactions chemically etch the fingerprint into the surface of the bullet casing when the cartridge is fired - and no amount of washing or wiping will remove it.

"Even if heat vaporises normal clues, police will be able to prove who handled a particular gun," says Bond.

Brass rubbing

However, the technique only works with certain metals, Bond explains. Reactive metals like zinc and aluminium oxidise naturally in the air, becoming coated in a layer of oxide that prevents the fingerprints from corroding the metal.

"At the other end of the spectrum, gold and platinum are so unreactive that fingerprints don't react with them at all." The best results are found with copper, a key component of brass. "Fortunately most shell cases are made from brass," says Bond.

Bond says gunmen without sweaty fingers can also be traced from heated bullet casings. "There might not be any salty sweat present, but the fatty sebaceous glands will deposit fats and waxes on the metal," he says.

Those fatty deposits also record an impression of the prints. When the metal is heated up, its surface becomes oxidised. "But the fatty deposits inhibit that oxidation, so you end up with nice shiny brass where the fingerprint was deposited, and dull brass surrounding it."

Bond's team has already processed bullets provided by the US and UK police, and the technique could also be crucial in identifying terrorist bombers. The researchers have already talked to the military about using the technique in Afghanistan. "Bombs are likely to be made from dense metals like copper," says Bond. "The science should work in the same way."

Journal reference: Journal of Forensic Sciences (DOI: 10.1111/j.1556-4029.2008.00738.x)

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