Chapter 6: Fibers
Fibers
• Fiber: is the smallest unit of a textile material
woven or twisted together to form a thread or
yarn.
• Fibers (like hair) are among the most
common items left at a crime scene.
• Class evidence- mass produced by textile
difficult to trace back
• Probative value- can create connection btwn
a victim and suspect.
Activity 6.1 pg. 129
• Collection and observing fibers from your
shirt and pants.
Sources & Types of Fibers
• Fibers can occur naturally (plant and
animal fibers)
Or
• Fibers can also be man-made
(synthetic).
Types of Fabric Weave
In a weave, the lengthwise yarn is called the warp.
The crosswise yarn is called the weft or woof.
Types include:
 Plain
 Twill
 Satin
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Woven Fabric
PLAIN
Simplest and most
common weave
Warp and weft pass under
each other alternately
Design resembles a
checkerboard

6
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.
Diagram:
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Blends
use of different fabrics & colors to create the warp and weft patterns.
Fabric Observation
• Laboratory Activity 6.1
• Pg. 131
Fibers
All fibers are made of polymers which
are long chains of repeating units.
The word polymer means many (poly)
units (mer).
The repeating units of a polymer are
called monomers.
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Fiber Morphology
Investigators use:
1. fiber cross section
2. chemical structure
3. synthetic polymers
for forensic analysis
Fiber Cross Section
• The cross section of a manmade fiber can be
manufacturer-specific
• Unusual cross sections
increase fiber association.
Cross-sectional views of nylon carpet
fibers as seen with a scanning electron
microscope (SEM)
Types of Fibers
•
Synthetic






Polyester
Rayon
Nylon
Acetate
Acrylic
Spandex
•
Natural





Silk
Cotton
Wool
Mohair
Cashmere
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Natural Fibers
Many different natural fibers that come from plants and animals are used in the
production of fabric.
Cotton fibers are the plant fibers most commonly used
in textile materials
The animal fiber most frequently used in the
production of textile materials is wool, and the most
common wool fibers originate from sheep.
http://www.fireflydiapers.com/articles/diaperarticle_naturalfibersabsorb.htm
Natural Fibers
• Wool--animal fiber from
sheep, may also be goat
(mohair), rabbit (angora),
camel, mink, beaver
• wool: composed of polypeptide
keratin chains of amino acid.
Wool Fibers (400X)
• main bond is sulfur which
accounts for the smell of wool
when burned
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Chemical structure: Natural fibers
• Silk: composed of proteins, only 2 amino acids
• Silk--animal fiber produced by silkworms
(cocoon)
Cotton
• #polymers is glucose
# smell like burning leaves
# 40% of all fabric is cotton
# 60% of all clothes and furniture
# low probative value because its too
common
• Linen: made from flax plant
# Contains cellulose (fibers are longer than
cotton)
# often blended with other fibers because it
is brittle.
Synthetic Fibers
More than half of all fibers used in the production
of textile materials are synthetic or man-made.
Nylon, rayon, and polyester are all examples of
synthetic fibers.
*nylon and polyester are the most common
Cross-section of a
man-made fiber
Fibers under a microscope
Images: http://www.trashforteaching.org/phpstore/product_images/YarnWS.JPG
http://www.fbi.gov/hq/lab/fsc/backissu/july2000/deedric3.htm#Fiber%20Evidence
http://www.jivepuppi.com/images/fiber_evidence.jpg
Synthetic Fibers
Made from cellulose
• Rayon--first man-made fiber;
• Acetate:– made from a reaction with acetic
acid.
Synthetic Fibers
(Made from derivatives of petroleum, coal and natural gas)
• Acrylic- used in carpets
• Spandex--elastic properties
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Forensics of Fiber Analysis
• Cross transfers of fiber often occur in cases in
which there is person-to-person contact
• More contact= more fiber transferred
Forensic Analysis
• What can be used to identify and compare fibers as
forensic evidence? See book pg. 135
1. fiber cross section
2. View through microscope
3. Burn test
4. Thermal decomposition
5. Refractive index
6. Chemical test
Microscopic Examination
• A compound microscope: uses light
• The comparison microscope (two compound
microscopes joined by an optical bridge) is used for
more precise identification.
• phase-contrast microscope, reveals fiber structure
• Scanning electron microscope converts the emitted
electrons into a photographic image for display. This
affords high resolution and depth of focus.
Fiber Comparison
Can you tell the difference(s) between the cotton on
the left and the rayon on the right?
Spectrometer
• The spectrometer, which separates light into
component wavelengths.
• By passing light through something to produce a
spectrum, the analyst can read the resulting
lines, called "absorption lines."
• specific wavelengths are specific to molecules of
the substance.
Fiber Evidence Collection
• Fibers are gathered at a crime scene with
tweezers, tape, or a vacuum.
• They generally come from clothing,
drapery, wigs, carpeting, furniture, and
blankets.
• For analysis, they are first determined to
be natural, manufactured, or a mix of both.
Steps of Fiber Analysis
• The first step in fiber analysis is to
compare color and diameter.
• Dyes can also be further analyzed with
chromatography, which uses solvents to
separate the dye's chemical constituents.
Fiber Color
• Color influences the value given to a particular fiber identification.
• Often several dyes are used
• How color is applied and absorbed along the length of the fiber are
important comparison characteristics.
• Color-fading and discoloration can also lend increased value to a
fiber association.
Fiber Analysis
Testing Fibers
1) Burn Test: look @ how a fiber burns, its
odor, and appearance of ash.
2) Thermal decomposition: how a fiber
breaks down when heated.
*it returns to its monomer (initial building
block)
* ex: Acetate
acetic acid
litmus test will turn paper blue to red
• Thermal Decomposition of fiber
activity
3) Chemical test: test the solubility and
decomposition of a fabric using a strong
acids (Hydrochloric acid or sulfuric acid) or
strong bases (NaOCl, acetone, NaOH)
– this determines the fabrics polymers.
4) Density: m/v (pg. 148).
Density of water is 1.00 g/ml. Olefin is the
only fabric that will float in water.
5) Refractive index: bending of light as it
passes from air into a solid or liquid
-- investigators measure the refractive index
of an unknown sample with liquids of a
known refractive index.
--place sample in different liquids until Becke
line is no longer visible
liquid
w/ high index
A
same index as fabric
B
liquid w/
low refractive
index
C
6) Fluorescence: some fibers will fluoresce
when exposed to UV light
--- laundry soap and some bleach has
whiting agents that cause blue light to be
reflected making it appear whiter.
7)Dyes: investigators use a fabric to see if it
accepts a particular dye to identify and
compare it to an unknown sample
8) Chromatography:
separation of dyes by thin
layer chromatography
(TLC)
Rf value =
distance of pigment
distance of solvent front
Other test used in Fiber Analysis
9) FTIR: Fourier Transform Infrared
• based on the absorption and wavelength of
light in a fabrics polymer.
• can be used on a single fiber
• Non-destructive
10) PGS-MS: Pyrolysis Gas Chamber-Mass Spectrometry
• Burns and separates each combustion product of sample
• Match results of chromatogram & products to known
• Can be used in short length fibers but is destructive.
Fiber Forensics
FYI- do not copy
• analyst gets only a limited number of fibers
to work with—sometimes only one.
• Fibers are sent to the lab for analysis.
• Fibers from scene are compared to victim
Copy###
• Any inconsistency (one property does not
match) is sufficient to cancel association
Fiber Transfer and Persistence
• Fibers can be used as trace evidence due to
fiber transfer.
• How easy the fiber is transferred is affected by:
•
•
•
•
•
•
•
Area of contact
the amount of pressure used
Any friction due to side to side contact
Number of passes or contacts
Kind of clothing donor/recipient was wearing
Fiber type, length and texture
History of the garment.
• How long transfer fibers remain on victim
is fiber persistence.
• Time of wear and movement
• What is covering the fabric
• Type of activity
• Weather condition
** fiber persistence decreases exponentially
with time of wear.
Types of Fibers - Key
Acrylic Yarn
Cotton Yarn
Nylon Rope
Polyester Yarn
Rayon Rope
Wool Yarn