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MIT Technology Review

Clearer Evidence

A household chemical rejuvenates faded fingerprints.

All fingerprints are not created equal. A print left by an adult can last for months, for instance, while those left by children are notoriously quick to degrade. But only recently has the chemistry behind these differences come to light, and scientists at the Y-12 National Security Complex, a National Nuclear Security Administration facility in Oak Ridge, TN, have devised a surprisingly simple chemical technique that renders even exceedingly faint prints more readable.

Good fingerprints, researchers at neighboring Oak Ridge National Laboratory discovered several years ago, are made primarily of sebum, the oil secreted by glands on the scalp, face, back, and chest. This material is transferred to the fingertips whenever a person touches his or her scalp or face, and it is transferred once again-as fingerprints-when the person touches a smooth surface. The fatty acids in sebum seal in moisture and give fingerprints longevity. By contrast, the worst fingerprints are left by those who have just washed their hands and by young children who have not yet begun to secrete sebum. Their fingerprints contain mainly salts and protein molecules dissolved in water, which dries up within hours, leaving virtually unreadable prints.

Last year Y-12 scientists Linda Lewis and Bob Smithwick discovered that by exposing such fingerprint ghosts to acetic acid, a chemical found in vinegar, they can regenerate the prints, making it possible to apply the chemical-fuming and dusting techniques employed to highlight and preserve good prints. Using their method, Lewis and Smithwick have revitalized previously unreadable prints after more than a month of dehydration.

The acetic acid technique “may be a very viable method of regenerating older prints,” says Bill Doyne, a forensic expert with the U.S. Army Criminal Investigation Laboratory. And for crime scene investigators, that find of a better method could be a boon that leads to more busts.