Chapter 4: Physical Evidence
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Chapter 4
Physical Evidence
Learning Objectives
1. Distinguish between class and individual characteristics.
2. Explain questioned samples.
3. Identify two major categories of impression evidence.
4. Describe how to cast a shoe impression using dental stone “from scratch.”
5. Explain how a fracture match is made.
Introduction
If physical evidence is to be useful in exonerating the innocent and pursuing suspects, it must be
treated in accordance with its importance. The evidence must be located, its position
documented, be collected, be identified by marks/writing on it and/or on the package in which it
is placed, and be transmitted to the evidence room or the crime laboratory. The chain of custody
Lecture Notes
I. Class versus Individual Characteristics
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Characteristics of physical evidence that are common to a group of objects or persons are termed
class characteristics. Regardless of how thoroughly examined, such evidence can be placed
II. Comparison Samples
Much of the work of forensic science involves comparing various types of samples. Special
terms are used to refer to these samples, and one must know what they mean to communicate
A. Unknown or Questioned Samples
There are three subcategories of unknown or questioned samples:
• Recovered crime scene sample whose source is in question: This evidence may have
been left, for example, by either victims or suspects.
• Questioned evidence that may have been transferred to an offender during the
commission of a crime and been taken away by him or her: When compared with the
B. Known Samples
There are also three subcategories of known samples:
• Standard or reference sample: This is material from a known or verifiable source. It is
compared to similar material from an unknown source to determine whether an
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association or linkage exists between a crime scene, a victim, and the offender. For
example, a sample of blood is taken under medical conditions from the suspect so that it
III. Soil and Pollen
The study of soils is within the purview of forensic geology and is called “geoforensics.” Soil
consists of loose aggregates of accumulations of natural elements, such as rocks, bacteria,
minerals, pollen, fungus, and plant material. Examinations are performed to characterize,
identify, and compare soil evidence, which may help to establish an association between
individuals, objects, and/or locations.
Although long thought of as class characteristic evidence and compared based on color,
Forensic palynology is the budding discipline that studies pollen and spores. Despite the fact it
can be a powerful investigative tool, it is extremely underutilized because it is labor intensive
and requires considerable expertise and experience. The value of pollen and spores lies in their
abundance and the diversity of plants, even in microhabitats.
A. Locating and Handling Soil Evidence
Soil evidence may be important when the suspect drives or walks on unpaved areas because it
is picked up by tire treads or the bottom of shoes and the cuffs of pants. It may also be
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permits.
• Collect soil not only from the crime scene but also from the logical points of access to,
and escape from, the scene.
IV. Impression Evidence: Footwear and Tires
Shoe and tire impressions are often present at crime scenes and are frequently overlooked by
investigators. Although European agencies report collecting footwear evidence in over 70% of
A. Categories of Impression Evidence
Impression evidence is divided into two major categories: surface impressions and three-
dimensional impressions.
Surface impressions (SIs) are two-dimensional. They have a length and width and by
definition lack any appreciable height. They are formed when the soles of footwear or tires
The following crime scene guidelines apply to preserving SIs:
• If they can be collected without damage, photograph them with and without a two-
dimensional scale. Package them so SIs cannot rub against their containers.
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the surface being examined may help find them. Oblique lighting should also be used to
enhance details when making photographs.
• Take overlapping photographs if the surface impression is very long.
• There are several well-established methods for collecting surface impressions:
o An electrostatic dust print lifter (See Figure 4-3 in the text) can be used to recover
an SI. It must be photographed as soon as recovered because the image is not
o Mikrosil Forensic Silicone is used for casting powdered SIs, particularly on
textured or uneven surfaces. Like fingerprint dust and lifters, Mikrosil Silicone
comes in different colors. It also produces excellent details when casting tool
impressions, which are three dimensional.
o Electronic shoeprint scanners are positioned directly over the SI and capture
detailed digital images for comparison use.
Three-dimensional impressions (3DI) are those that have a significant depth, in addition to
having a length and width. 3DI may be found impressed into soil, sand, snow, or other
materials (See Figure 4-6 in the text). The level of detail is affected by the material in which
B. Preserving Three-Dimensional Impressions
The first step in casting is the preparation of the impression. The rule is that the impression
cannot be disturbed. Thus, if twigs, leaves, or other materials are stuck in the impression, they
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over the impression, being careful not to disturb the details of the impression.
In Chapter 3, Shake-N-Cast was mentioned. It has all the needed materials preassembled in
one bag increasing convenience and accuracy of use. To prepare and apply dental stone, mix
If the impression is frozen in snow or ice, use a colored spray paint to highlight the
impression for photographs. Make sure to protect the impression from heat until photographed
and cast to prevent melting. If the snow is soft or slushy use Snow Print Wax to keep the
Sulfur has long been used to cast impressions in snow but requires additional preparation and
safety measures compared to dental stone. Specifically, sulfur must be used in a well-
ventilated area due to the fumes, ideally with a respiratory mask on. Sulfur requires
V. Paint
Paint evidence may be encountered in many types of investigation, but perhaps most commonly
it is transferred from one source to another in burglaries and hit-and-run accidents. Paint is
transferred as fresh smears, dried chips, or “chalking” from old, dry paint. Many burglaries
involve a tool that was used to pry a door or window open, transferring paint from the building to
the suspect’s tools or clothing.
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Usually, paint is class-characteristic evidence, although in some cases it can reach the level of
individual evidence. If the chips are large enough, it may be possible to make a fracture match
between a questioned and a known source. If a building or car has been painted multiple times,
then the layered structure of a questioned sample can be compared to a known source as to the
number of layers, their colors, the sequence of colors, the relative thickness of each layer, their
texture, and the chemical composition of each layer of paint.
VI. Glass
Generally, glass is class-characteristic evidence, although in some situations it may become
individual-characteristic evidence. Glass is important as physical evidence because it is
Laboratory examination of glass may be able to determine:
• The type of glass, such as tempered or container.
• The direction of force used to break the glass, for example, inside or outside (see Figure 4–
When a glass window is broken by a shot, both radial fracture and concentric fracture lines may
develop. It is essential that the crime scene technician and investigator understand the ways in
which glass reacts to force. This knowledge may be critical in determining whether a crime has
been committed, establishing the sequence of events and evaluating the credibility of statements
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A. Handling Glass Evidence
The physical properties used for comparison of known and questioned glass specimens are
If glass is being submitted for comparison purposes, that is, to determine whether the particles
may have originated from a common source, they are ordinarily small, and the following
precautions should be taken:
• Glass in hair may be shaken or combed out over clean paper.
• Clothing and shoes should be packaged separately, and in no case should the victim’s
and suspect’s items be comingled in any way.
• If a vehicle is involved, use a vacuum to collect the glass from each area of the vehicle;
evidence collected from different areas should be packaged separately, and a new
vacuum bag should be used for each new area processed.
• Known and questioned glass samples and evidence from victims and suspects should be
shipped separately to avoid contamination.
o These types of evidence should be wrapped in clean paper and sealed in a rigid
VII. Fibers, Cloth Fragments, and Impressions
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Fibers are of value as evidence because they incorporate such variables as material type, number
of fibers per strand, number of strands, thickness of fibers and strands, amount and direction of
VIII. String, Cord, Rope, and Tape
String, cord, rope, and tape evidence is most commonly found in robbery, murder, rape, suicide,
and kidnapping cases. It may also be associated with accidental hangings and sexual
IX. Fingerprints
Fingerprint identification collection occur in essentially two distinct ways. Most fingerprint
work is done by a single officer who may be investigating something like a home burglary where
someone raised a window of a home, stole some items, and left the way they entered. The officer
is going to select a fingerprint powder, dust the promising areas, perhaps lift prints with Scotch
Tape, place it on a card, make some notations, and leave after talking with the victims. The
officer has done a good job.
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A. Basis for Identification
Fingerprint examiners follow the ACE-V guidelines: Analysis, Comparison, Evaluation,
Verification. The general rule is that identifications are checked by a second analyst to
determine their accuracy.
B. Plastic, Latent, and Patent Fingerprints
Fingerprints are classified into three major groups
• Plastic prints are finger impressions left in pliable, soft surfaces, such as wax, soap,
C. Conditions Affecting the Quality of Latent Fingerprints
The following types of surfaces on which fingerprints are deposited affect the quality of
fingerprints:
• Porous surfaces including paper, cardboard, cigarette butts, and untreated wood.
Some of the other conditions affecting the quality of fingerprints include the following:
• Physical or employment produced infirmities.
D. Locating Latent Prints
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Latent prints are such valuable evidence that extraordinary efforts should be made to recover
them. It is imperative that the investigator thoroughly search all surface areas in and around
E. DNA Swabbing and Developing Latent Fingerprints on Bodies
Recovering DNA and a latent fingerprint from the same site on a body is problematic. If the
initial attempt is to visualize the print, the DNA may be contaminated. On the other hand, if a
The development of identifiable LFPs on the skin of dead bodies and living victims can be
done, but it is difficult. Two volunteers placed 150 fingerprints on 10 dead bodies as
part of a clinical study. Thereafter, attempts were made to develop them using black
F. Collecting and Preserving Latent Prints
Occasionally, items such as beer cans or glasses that have condensation on them need to be
processed for prints. Heat lamps or any other source of artificial heat should not be used to
G. Marking and Identifying Print Lifts
When a latent print has been developed, lifted, and placed on a card, the card must be