Some special equipment is needed to make a perfect plaster cast. Here’s how the pros do it:
- Clean out any loose material without disturbing the impression.
- Spray a fixative onto the surface.
- Build a portable frame or wood ledge and place it around the track mark.
- Mix plaster of Paris or dental stone casting material and pour it over the marks. Another product, Snow Print Wax, can be used for impressions found in packed snow.
- Place reinforcement sticks or cheesecloth into the mixture.
- When casting is hard, carefully lift it out and put it in a cardboard box upside down to dry.
Latents (Not Readily Visible)
Latent shoe prints are found on counter tops, staircases, and walls, and footwear impressions can be lifted just like fingerprints using chemicals and powder. One bank robber climbed over the counter during a holdup—and his shoes matched the print made on the surface perfectly.
But for the flimsiest of all evidence, shoe prints found in dust, another system is needed. The best way is to turn out the lights and shine a light—a flashlight will do—against the hard, dusty surface and document that with a photograph. In order to preserve these footprints, analysts use an electrostatic dust lifter. A sheet of black lifting material, like foil, is laid over the surface. A high-voltage charge is run through the material. The electricity makes the dust stick to the sheer, and voilà—an image! The process is nondestructive and works on just about any surface.
Unusual Prints: A Case Story
A state trooper in Alaska was called in to investigate several cases of moose poaching. (Bet you didn’t know that moose poaching is a crime.) Some earlier cases had been solved due to the carelessness of the criminals, who had left behind incriminating evidence such as a boot print and, in one memorable case, a wallet complete with the poacher’s driver’s license! One culprit, though, seemed to have left no clues at all—until police discovered a clear print of his license-plate number where he had backed his vehicle up against a snow bank.
Photo courtesy of ©Ron Roberts
Excerpt from Andrea's Detective Notebook: Crime Scene Science (available on CD)