(Photo Ian Waldie/Getty Images)
Scientists have developed a technique for retrieving fingerprints from bullet casings and bomb fragments after they have been fired or detonated. The new method, which relies on subtle corrosion of metal surfaces is already being applied for the first time anywhere in the world by two British police forces.
The patterns of corrosion remain even after the surface has been cleaned, heated to 600C or even painted over. This means that traces of fingerprints stay on the metal long after the residue from a person's finger has gone.Of course, all the bad guys would have to do to thwart this technique would be to use gloves before ever handling the cartridges from a new box of ammo and loading it into the magazine. For every technique, there is a counter.
"All other conventional techniques that the police anywhere in the world would use require some kind of either physical or chemical interaction with the fingerprint residue. So for example if you are using powder the powder sticks to the tackiness in the sweat," said Dr John Bond of the University of Leicester and the Scientific Support Unit of Northamptonshire Police. Instead, the technique he has developed relies on permanent physical changes to the metal.
To visualise the patterns, Bond's technique involves passing 2500 volts through the metal and sprinkling a fine conducting powder on it. While the electric field is applied, the powder sticks to the corroded areas.
"If the corrosion's there, it will stay there unless you abrasively clean the metal. The only way to get rid of it is to actually take the surface layer of the metal off," he said. "Some metals corrode very well and some not at all. One metal that works very well is brass and gun cartridge cases happen to be made of brass."