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.

Once the University of Leicester scientists knew that fingerprints could corrode metal, they applied a very large electrical charge–2,500 volts–to the corroded area. They then applied to the metal a very fine, black conducting powder similar to photocopier toner, which adhered to the areas of corrosion. “You could see the outline of the fingerprint in the black powder, thereby rendering the fingerprint visible,” says Bond.

Johnson thinks that the technique is exciting but warns that the surface area of cartridges is so small that the entire print may not be obtained. “One of the major issues in fingerprint analysis is how much of the print is necessary before you can reliably say it is someone’s fingerprint,” says Singer. In general, though, Singer is impressed with Bond’s research. “The more methods we have to develop invisible fingerprints, the better off we are.”

Bond says that the technique has been extensively tested in the lab and will be applied in more cases in both the United States and the United Kingdom. Furthermore, he has been in contact with the U.S. military, which is eager to use the technology for roadside bombs or improvised explosive devices. “Traditional bomb-making metals are ones like copper, which we know corrode with fingerprints,” says Bond. “The fingerprint on metal from an exploded bomb should work the same way it does on a bullet with a fingerprint.”