Preserving Impression Evidence -- Plaster Casts
When approaching a crime scene with the intention of recording impression evidence, the first thing forensic investigators are taught to do is to secure and preserve the area. Because impressions are easily disturbed and often overlooked, a scene filled with too many people walking around could quickly become worthless to someone looking for footprints.
Once a perimeter is secure, investigators walk inward, looking for impressions and reconstructing the events of the crime to the best of their knowledge. They try to determine important facts such as the direction of travel or the number of suspects at the scene. Special lighting techniques can uncover hidden impressions, including the use of oblique lighting. By shining a light source diagonally at the ground, not vertically, the ridges of an impression create shadows which alert investigators to disturbed and uneven surfaces. Photographs of discovered impressions are also taken for visual documentation.
Although impression evidence is extremely fragile, if left undisturbed it can remain for long periods of time. As long as a crime scene area isn't harmed by weather (mainly wind, rain and snow) or disturbance from other shoes, tires or tools, there are two major techniques experts use to gather impression evidence:
- Latent (two-dimensional) impressions - Latent impression recovery is very similar to basic fingerprinting. This technique is used for difficult-to-preserve, two-dimensional impressions on materials such as tile floors, wood floors or chairs. Powder is applied with a brush to make the print more visible, and then tape or a lifting machine records a visual of the impression.
- Casting - To recover larger, three-dimensional impressions such as tire marks or footprints left in muddy conditions, experts use casting. The process works in very much the same way as an orthodontist makes a model of a patient's teeth: A substance is poured into the impression, hardened, and then removed, providing a cast of the print on the ground. Investigators use a variety of products to create casts, but dental stone, in fact, is widely considered the best casting material due to its strength, accuracy and ease of use.
As available equipment and techniques for forensic science improve, many in the field are realizing the importance of impression evidence. Although easy to overlook and difficult to locate, footprints and other impression evidence left at the scene of a crime are typically even more prevalent than fingerprints, and they can provide important clues to mysterious cases. To learn more about the shadowy world of criminal investigation, step lightly to the next page.