Fingerprints are crucial evidence in many criminal investigations because they can tie a suspect to the scene of a crime with almost indisputable accuracy. Now crime-scene investigators have a new technique for finding fingerprints left on metals, like the cartridge from a spent bullet or fragments of an improvised explosive device, even if the perpetrator tries to wash the evidence clean.

Forensic scientist John Bond of the Northamptonshire Police, in the United Kingdom, developed the technique after discovering that certain metals, including copper and brass, corrode very slightly when touched, leaving behind a faint but indelible fingerprint. Already, the technique has been used to provide fingerprints in a nine-year-old double-homicide case in Kingsland, GA, after conventional fingerprinting methods were unable to identify any prints on a shell casing, says Bond.

Traditional fingerprinting techniques involve triggering a physical or chemical reaction with the deposits left behind by a finger to make a print visible. If these deposits are removed, the techniques will fail. This seriously limits what forensic scientists can do to identify fingerprints in spent cartridge cases and at arson scenes where normal prints have been removed, says Hazel Johnson, a specialist advisor at the Forensic Science Service, based in Birmingham, in the U.K. "We will look at the metals under a laser for potential fingerprints, but rarely is the technique able to spot the print," she says.

The new technique makes use of a physical change that occurs to metal when a person touches it. This is due to the salt in human sweat: ionic salt molecules present in the fingerprint residue corrode the metal surface to produce an image that can only be removed by abrasive cleaning of the metal. Bond, also a fellow at the University of Leicester, in the U.K., found that the fingerprint can be made visible by applying a voltage to the metal and coating it in a metallic powder.

"The advantage of the new technique is its permanence," says Ron Singer, crime-laboratory director for the Tarrant County Medical Examiner Crime Lab, in Fort Worth, TX. "It is looking for the minute amount of etching that takes place in the metal--the physical change that has occurred to the surface." Singer says that the technique could prove more resilient than conventional methods. "If you don't get it right the first time, you can do it again," he adds.