FORENSIC SCIENCE
Trace Evidence
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Introduction
Trace Evidence--any small pieces of material,
man-made or naturally occurring
Most common examples:
 Hair
 Fiber
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Test Questions for Trace
Evidence

What is it?

Is it man-made or natural?

What is its source?

How common is it?

Can it be identified to a single source?
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Fiber Evidence
NOTE: Fabric is the type of material and fibers are the
“threads” that make up the fabric
The use of fiber evidence in court cases is used many
times to connect the suspect to the victim or to the
crime scene. In the case of Wayne Williams,
fibers were the entire case. Williams was
convicted in 1982 based on carpet fibers that were
found in his home, car and on several murder
victims. Although this case is unusual, fibers are
generally considered of greater value as evidence
than that of rootless hairs since they may contain a
greater number of variables, thus showing more
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individual characteristics.
Polymers
Synthetic fibers are made of polymers which are
long string of repeating chemical units.
By varying the chemical structure of the monomers
or by varying the way they are weaved together,
polymers are created that have different
properties.
As a result of these differences, forensically they can
be distinguished from one another.
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Analysis
of
Fibrous
Material
U.S. Department of Justice
FBI, April 1999
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Types of Fibers
Synthetic
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Polyester
Rayon
Nylon
Acetate
Acrylic
Spandex
Natural
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Silk
Cotton
Wool
Mohair
Cashmere
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Classification
Classified according to their origin:
 Vegetable or cellulose
 Animal or protein
 Mineral
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Cellulose Fibers

Cotton--vegetable fiber. Strong, tough, flexible;
moisture absorbent; not shape retentive

Ramie--vegetable fiber. Less flexible than cotton
so its often blended with cotton

Rayon--first man-made fiber; soft, lustrous,
versatile fiber
Cellulose esters--cellulose is chemically altered to
create an entirely new compound not found in
nature.

Acetate--less expensive, less polluting than rayon
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Fiber Comparison
Can you tell the difference(s) between the cotton on
the left and the rayon on the right?
Petroleum Plastics
(Made from derivatives of petroleum,
coal and natural gas)
Nylon--most durable man-made fabric;
extremely light weight
 Polyester--most widely used man-made
fiber
 Acrylic--provides warmth from a
lightweight, soft and resilient fabric
 Spandex--extreme elastic properties

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Protein Fibers

Wool--animal fiber
coming most often from
sheep but may be goat
(mohair), rabbit (angora),
camel, mink, beaver

Silk--animal fiber that is
spun by a silk worm to
make its cocoon; fiber
reflects light and has
insulating properties
Wool Fibers (400X)
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Mineral Fibers
Asbestos--a natural fiber that was used in
fire-resistant substances
 Metallics (mylar)--a manufactured mineral
fiber
 Fiberglass--another manufactured mineral
fiber

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Woven Fabric
PLAIN
Simplest and most common
weave
Warp and weft pass under
each other alternately
Create even patterns of 1/1
and 2/2
Design resembles a
checkerboard

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Woven Fabric
TWILL
Create by passing the warp
yearn over one to three weft
yearns before going under
one
Makes a diagonal weave
Design resembles a stair
steps
Denim is the most obvious
example

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Woven Fabric
SATIN
The yarn interlacing is
not uniform
Creates long floats
Interlacing weave passes
over four or more yarns
Satin is the most obvious
example

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Knitted Fabric
Knitted fabrics are made by interlocking loops into a
specific arrangement. It may be one continuous
thread or a combination. Either way, the yarn is
formed into successive rows of loops and then
drawn through another series of loops to make the
fabric..
Diagram:
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Identification and
Comparison of Fibers

Microscopic examination
– Color--compositional differences in the dyes
– Fibers surface--delustering particles that may
be added by manufacturers

Microspectrophotometer--compares fiber
colors through spectral patterns

Chromatography--gives a more detailed
analysis of the dye composition
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Identification and
Comparison of Fibers (cont.)

Polarizing microscope
can be used to determine the refractive indices
of various fibers. The fiber is immersed in a
fluid that has a comparable refractive index.
The disappearance of the Becke line is
observed under the microscope.
 In addition, fibers will absorb infrared light in a
characteristic pattern. This can be observed
through the use of an infrared
microspectrophotometer and a microscope.
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
Collection of Fiber Evidence
Bag clothing items individually in paper
bags. Make sure that different items are not
placed on the same surface before being
bagged.
 Make tape lifts of exposed skin areas of
bodies and any inanimate objects
 Removed fibers should be folded into a
bindle and stored in a paper bag

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Uniqueness
Establishing Individual Characteristics
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If there is only one source for the transfer
material with a controlled environment where
the contact took place
If there is contamination of several different
materials from surface onto surface two
If there is a method available to characterize the
material, such as applying DNA
Otherwise, trace evidence would have only
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class characteristics.