Forensic Science: Fundamentals & Investigations, Chapter 15 1 Kendall/Hunt Publishing Company 1 Trace Evidence, Casts, and Impressions “Breadth of view is one of the essentials of our profession. The interplay of ideas and the oblique use of knowledge are often of extraordinary interest.” —Arthur Conan Doyle’s Sherlock Holmes, in The Valley of Fear
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Forensic Science: Fundamentals & Investigations, Chapter 15 1 Kendall/Hunt Publishing Company1 Trace Evidence, Casts, and Impressions “Breadth of view.
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“Breadth of view is one of the essentials of our profession. The interplay of ideas and the oblique use of knowledge are often of extraordinary interest.”
Trace evidence is physical evidence found in small amounts at a crime scene. Common examples would be hair, fiber, paint chips, body fluids, stains, powders, explosive residue, glass particles, vegetative matter, metal particles, and soil. It may also include more unusual types of evidence.
Physical property: A characteristic that does not involve a change in the identity of a substance, such as odor, color, boiling point, density, refractive index.
Chemical property: A characteristic that determines how a substance will change into another substance with different physical properties.
When investigators find substances at the scene of a crime and send them to the laboratory for identification, the forensic chemist uses several techniques or lab tests to identify them. One of these techniques is qualitative analysis.
A number of white powders that appear the same can be identified by their physical and chemical properties.
In 1912, Emile Gourbin was a bank clerk in Lyons, France. He came under suspicion of strangling his girl friend, Marie Latelle. Gourbin was arrested but had what appeared to be an air-tight alibi. Edmond Locard went to Gourbin’s cell and removed scrapings from under his fingernails. The scrapings contained tissue possibly from Marie’s neck, but this was not provable. Locard noticed that the tissue was coated with a pink dust, which he identified as rice starch. On the particles he found bismuth, magnesium stearate, zinc oxide, and a reddish iron oxide pigment called Venetian red. Examination of the face powder used by Marie revealed that a powder prepared for her by a Lyons druggist was similar in composition. In these days of mass-produced face powder, this evidence would have far less significance. However, in 1912, because of the special preparation, it led to the confession of Gourbin.
Different methods to make latent prints visible include: Luminol will make bloody footprints visible. Dusting the latent print reveals an impression. Electrostatic lifting and gel lifting (impression
shown above) techniques also can capture hidden impressions.
Tire Treads and Impressions Tire Treads and Impressions
To channel water away and provide traction, a tire’s tread is divided into ridges and grooves.
Tires can leave patent, latent, or plastic tread patterns.
Tire evidence usually indicates the type of vehicle that left the mark, may be used to link a suspect or victim to a crime scene, and can reveal the events that took place.
Identifying tread patterns may not be enough to link a suspect with a crime scene. Other things can help with this: – Front and rear track widths. – Wheelbase measurement. – Turning diameter.
Databases can be checked to find the vehicle with these specifications.
Note “yaw” marks on roadway. Marks on the right are from the rear tires and marks on the left are from the front tires just prior to the rollover. Tire yaw marks occur when a vehicle slides sideways while still moving forward. In a true yaw, where the vehicle’s rear is attempting to pass the vehicle’s front, each rear tire tracks outside the corresponding front tire. (Photo Courtesy of County Sheriff’s Office)
A bank robber was startled by an alarm just as the teller handed her the money. She grabbed it and in her haste to get away, ran smack dab into a glass door. Nevertheless, she recovered and got away. Subsequent examination of the door revealed a red lipstick imprint of the perpetrator’s mouth. Later police picked up a suspect, but needed evidence to link her to the robbery.
A prosecutor’s witness was having a drink in a busy restaurant. A waiter noted that he was soon joined by a woman with dark hair, wearing sunglasses. The waitress brought her a drink and was surprised to see her leave shortly afterwards.
The waitress noticed that the witness looked like he had fallen asleep, but she soon discovered that he was dead. A key piece of evidence was a lipstick smudge on a napkin at the scene.
Seven women were picked up shortly afterward near the restaurant. Two women had red lipstick, one had purple lipstick, and two wore no lipstick, but had tubes or red in their purses. Three of them were wearing dark glasses.
Can we match the lipstick left at the scene to one of the suspects?