Several months ago, undercover police in Western Maryland bought a Mossberg shotgun, Model 500A, from a local drug dealer. The gun’s serial number, a forensic science staple for tracing guns, had been ground away. So the police sent the shotgun to the federal Bureau of Alcohol, Tobacco, Firearms and Explosives’ National Laboratory in Beltsville, where Gregory Klees, the lab’s go-to guy when it comes to the forensic science of un-erasing serial numbers, knew he had a challenge on his hands.
“Only two of the seven serial number characters could be restored” by his usual acid-etching techniques, Klees said, which meant police had to do a time-consuming search of gun-theft reports and interviews with victims to establish the weapon’s ownership.
An alternative process under development since 2013 may fix this problem in the future. Using a technique called electron backscatter diffraction, or EBSD, researchers at the Boulder, Colo., campus of the National Institute of Standards and Technology have been developing a novel way to bring back to life numbers that have been completely sanded away. While the method, which uses an electron microscope to examine atoms in metal, still requires refining, NIST materials scientists Ryan White and Robert Keller say it should be capable of restoring a full serial number, even in such difficult cases as the Mossberg shotgun, in about an hour.
Federal law requires gun manufacturers to stamp, etch or otherwise imprint a unique serial number on every firearm they produce. It is possible to scratch off those numerals and letters, but the mechanical or laser energy it takes to embed the number into the gun’s metal leaves a hidden record of damage around and beneath each digit. It is this trail of damage that White and Keller can find using EBSD equipment.
Almost all metals are composed of grains that pack together the way a 3-D jigsaw puzzle would. Look closely enough inside each grain and you will see a crystalline arrangement of atoms and notice that the orientation of the crystals in adjacent grains is like clashing sets of stripes running at different angles from each other.
To test whether they could reconstruct a thoroughly erased number in a piece of metal by examining grains with their EBSD technique, White and Keller punched some X’s into stainless-steel test strips and then ground the symbols away.
Looking at the metal through an electron microscope, they scanned the grains to record how the electrons bounced away and scattered. The resulting pattern of “backscattering” electrons from each grain is known as a diffraction pattern. This process allowed the researchers to determine whether a grain’s crystalline order had been jostled from a violent event such as a blow from a symbol-bearing die or a shot of heat from a laser. By looking at the diffraction patterns from many grains in the test metal, the scientists identified a trail of altered grains that essentially revealed the sanded-away X’s.
White says he is confident that the results, though preliminary, prove that the EBSD technique could serve forensics professionals trying to trace guns.
For now, though, investigators such as Klees must rely on standard methods, such as acid etching, which eats away at the grains in damaged regions of metal faster than in undamaged ones, a process in which lost letters or numerals re-form in reverse relief. Another common method can reveal traces of obliterated numbers through differences in the alignment of tiny magnetic particles atop undamaged and damaged grains. Which method works best, if at all, depends on a variety of factors, including the specific alloy of the metal and the extent of the damage done while erasing the serial number.
Klees says the EBSD technique holds promise, but he cautions that even if the NIST researchers can improve it into a fast and efficient method of number restoration, it would still have to scale the high hurdle of being ruled sound enough to stand up in court.
White and Keller say their EBSD technique should “work for any and all metallic materials which have undergone a change to the material structure beneath the imprint.” Besides finding serial numbers on firearms, for example, the method might recover identification numbers erased by car thieves from engine blocks, doors and trunk lids. The NIST researchers’ next round of experiments takes a step toward more realism, tackling a set of numbers stamped into aluminum and three different steel alloys.
Amato, a writer based in Silver Spring, runs DC Science Cafe.