Stone throwers might toss fingerprints into police hands

He who throws the first stone might now be identified by his fingerprints. A new study of fingerprinting methods overturns the common wisdom among police and forensic experts that much like blood, you can’t get prints from a stone.  

By matching fingerprint-finding techniques to the local geology, it is possible to get latent fingerprints from at least some stones, according to a study published February 6 in the Journal of Forensic Sciences. The researchers are forensic scientists working for the police force in Israel, where stone-throwing is a common part of Israeli-Palestinian conflict.

“Stones have been a weapon of opportunity for centuries,” says chemist Rob Hillman of the University of Leicester in England. So it’s surprising, he says, that little effort has been put into methods to identify fingerprints on stones. The new study “makes substantial progress in filling this notable gap.”

Apart from eyewitness testimony, it’s usually impossible to figure out who threw a stone after the fact. But that could change if police investigators can lift fingerprints from the evidence. “Fingerprints are important evidence linking a person to an object that was used during the offense,” says study coauthor Amit Cohen of the Division of Identification and Forensic Science of the Israel Police. “Stones might be used for breaking windows during burglaries or, as mentioned in our work, stones thrown during riots. Therefore it can be assumed that new means of reaching truth and solving crimes will be used by law enforcement agencies.”

Until now, most forensic teams haven’t bothered fingerprinting stones used in a crime, for the same reason that they usually don’t fingerprint clothing. Fingerprints are left behind by oils and sweat carried on the hands. Prints don’t form neatly on rough surfaces, and porous surfaces tend to absorb and spread fats and oils.

“It has always been thought that we cannot recover finger mark impressions from items such as rocks and stones,” says Dennis Gentles of Abertay University in Scotland, a forensic scientist who has recovered fingerprints from other challenging surfaces, including foods and bird feathers. Gentles once had a student experiment with fingerprinting smooth stones collected from a river and a beach, with little success.

Some of the most common methods for developing fingerprints, or making them visible, involve dusting the surface with fine powders, often containing magnetic iron. Exposing prints to fumes from cyanoacrylate, or superglue, is another popular method that works through a reaction of the chemical fumes with amino acids, fatty acids and proteins in the fingerprints along with moisture in the air. Ninhydrin is another compound that reacts with prints to reveal pink or purple fingerprints.

In the new study, Cohen and colleagues tested those three common fingerprint development techniques on stones made of several kinds of rock common in Israel. (I’m using “rock” here to refer to the original mineral material and “stone” as a smaller chunk, such as what a person might throw by hand.) They tried each method on pieces of granite, basalt, scoria, limestone, chert, marl and on bricks (also called curbstone).

The researchers got visible fingerprints from the limestone, chert, granite and brick, with the best results on limestone and chert. The best technique for fingerprinting a stone depended on how porous the rock was, the team found. When rocks were more porous, like limestone, the best methods were those also used to lift prints from other porous surfaces such as wood: powder followed by ninhydrin, for example. For chert and other nonpermeable rocks, methods for nonporous surfaces such as glass should be used, the team suggests, such as powder followed by superglue.

Detailed prints, good enough to run through the Automated Fingerprint Identification System, or AFIS, emerged for 40 percent of the limestone samples and nearly 60 percent of the chert samples that were tested with magnetic powder within an hour of being handled.

But time is of the essence. As fingerprints sit, fats spread and saturate the rock surface, and amino acids can break apart. The Israeli researchers were able to find prints identifiable either by comparison to a suspect or through AFIS for 64 percent of all limestone samples and 80 percent of chert after an hour, but those numbers dropped to 30 percent for limestone and 10 percent for chert after 24 hours.

“Fingerprint development is a complex domain that requires a lot of work to get to full maturity of the method,” Cohen says. More advanced methods using nanotechnology or other chemical and imaging techniques may bring even better results, he says.