“SO HOW DOES this work again?”
Nick watched with interest as Mandy Webster set up her experiment. She placed the shell casing at the end of a short wooden wand. The rest of her apparatus was set up in a cardboard box lined with aluminum foil. A metal contact plate was wired to a 2,500-volt battery. Nearby a metal scoop, with a wooden handle, rested atop a tray of fine black powder.
“It’s pretty ingenious,” Mandy enthused. “Ordinary fingerprinting techniques require some sort of sweaty residue to be left on the metal, but this British inventor, Dr. John Bond, has developed a technique that can sometimes retrieve prints even after they’ve been wiped or burned away. It’s based on the idea that the salt in the original fingerprints actually corrodes the metal underneath. That corrosion remains even after the sweaty prints are gone.”
“And the sweatier the hands, the deeper the corrosion?” Nick remembered reading something about the process in a forensic journal.
“Yep,” Mandy said. “Good thing most people tend to get a little nervous when they’re planning to murder someone.”
She flicked a switch and placed the casing against the electrical terminal. The wooden wand kept her from being shocked herself, although she was also wearing rubber gloves just to be safe. “The corroded areas are too small to be seen by the naked eye, as in microns,” she continued, “but they pick up less of an electrical charge than the clean brass.”
She scooped up a small quantity of the black powder and sprinkled it over the casing, rolling the 9mm cylinder across the electrode as she did so to make sure all its surfaces were exposed to the powder. “These are actually tiny ceramic beads, about a half a millimeter in diameter, coated with a very fine conducting powder. In theory, they should cling to the microscopic corrosion pattern left behind by the fingerprint.”
Nick was impressed by how relatively low-tech the apparatus was, as opposed to some of the expensive DNA scanners that busted the crime lab’s budget. “You built this set-up yourself?”
“You bet,” Mandy said proudly. “Not really that hard. The original inventor constructed his prototype out of cardboard, masking tape, and popsicle sticks.”
“Ecklie’s going to love that,” Nick observed. The stingy undersheriff was forever complaining about the cost of keeping the lab’s hardware up-to-date.
Mandy flicked off the current and held the cartridge up for inspection. Sure enough, inky black whorls and ridges stood out against the brass exterior of the casing. The uncorroded metal, which had held a stronger charge, gleamed by comparison.
Mandy subjected the prints to her expert eye. “Probably a forefinger or thumb. From when the shooter loaded the gun.” She passed it over to Nick, so he could admire her results. “Now we just need to heat the casing to bake the powder in place and compare it against our exemplars.”
Nick couldn’t wait to compare the fingerprint to Roger Park’s. The smarmy TV producer was possibly responsible for at least two deaths. It would be great to pin him to at least one of them.
“Thanks, Mandy. You may have just cracked this case.”
“Don’t thank me,” she said. “Thank our friends in Scotland Yard.”
“Right.” Nick looked forward to reporting their success to Catherine and Greg. “What was the name of that inventor again?”
Mandy took off her glasses and affected her best Sean Connery impression.