Fiber Analysis
Physical Aspects of Forensic Science
.
Fiber Evidence
A fiber is the smallest unit of a textile material that has a length many times
greater than its diameter. A fiber can be spun with other fibers to form a yarn that
can be woven or knitted to form a fabric.
The type and length of fiber used, the type of spinning method, and the type of
fabric construction all affect the transfer of fibers and the significance of fiber
associations. This becomes very important when there is a possibility of fiber
transfer between a suspect and a victim during the commission of a crime.
Matching unique fibers on the clothing of a victim to fibers on a suspect’s clothing
can be very helpful to an investigation, whereas the matching of common fibers
such as white cotton or blue denim fibers would be less helpful.
The discovery of cross transfers and multiple fiber transfers between the suspect's
clothing and the victim's clothing dramatically increases the likelihood that these
two individuals had physical contact.
http://www.fbi.gov/hq/lab/fsc/backissu/july2000/deedric3.htm#Fiber%20Evidence
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
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.
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
Fibers
Fibers are very useful as trace evidence:
 Vary widely in class characteristics
color, shape, chemical composition, etc.
 Easily transferred from one source to another
(carpets, clothes, etc.)
 Significant persistence (won’t degrade)
Importance of Fiber
Evidence
• Perpetrators of crimes are not always aware
or able to control the fibers they have left
behind or picked up
Importance of Fiber
Evidence
• In contrast to hair, fibers offer much
greater evidential value because they
incorporate numerous variables
– Number of fibers in each strand, diameter of
strands and fibers, direction and number of
twists, type of weave, and dye content, as
well as foreign material embedded or
adherent to the fiber
Trace > Fibers
How are fibers used as evidence?
As with other trace evidence,
fibers can be transferred
to/from a person or objects
linking them to one another.
Trace > Fibers
How long do fibers persist?
Most fiber evidence is lost (fall off) a short
time after the transfer occurs.
The fibers that do remain will be persistent.
Trace > Fibers
Fibers can be classified into three main
categories:



Natural (animal, plant, mineral)
Manufactured
Synthetic
Textile Fiber Defined
• Defined as the smallest part of a textile
material
– Many objects in our environment (clothing,
ropes, rugs, blankets, etc.) are composed of
yarns made of textile fibers
Textile Fiber Categories
• Animal (hairs)
– Wool, cashmere, silk
• Vegetable
– Cotton, kapok, linen
• Mineral
– Asbestos
• Manmade
– Acetate, rayon, nylon,
acrylic, polyester, and
olefin
Trace > Fibers > Natural
Natural Fibers:


Found in nature
Can be artificially colored or treated
Cotton
Wool
Hemp
Animal Fibers
Wool - Hairs from sheep
Most common of animal fibers
 Hairs are spun to form thread

Silk - comes from silkworm
Spun as double filament (separated before use)
 Because of length, doesn’t shed easily

Other Hairs from Animals
Animal Fibers
• Woolen fibers occupy less than 1% of all
fibers used in production of textile
materials
• Wool has a microscopic structure that is
characteristic of hair
• The cuticle (outer covering) is made of
flattened cells, commonly called scales
Animal Fibers (continued)
• The scales resemble shingles of a roof and
are one of the most useful features to ID
an unknown textile fiber as wool
• Other animal hairs are not as frequently
encountered so they can be quite
valuable if they occur as evidence
– Include goat (cashmere, mohair), llama
(alpaca, vicuna, guanaco), and camel hair
Animal Fibers
• Cattle and rabbit hair are found in the
manufacture of certain kinds of felts
– Felts are made from water suspensions of
randomly arranged fibers. When the fibers
settle out, the water is removed and the
mass of fibers is pressed to form the felt
– Some modern felts are no longer made
exclusively from hairs but are mixtures with
other fibers
Animal Fibers
• Silk places a distant second to wool in
occurrence, and its use has decreased
since development of artificial fibers
• Silk fibers are not very often encountered
in crime investigations, probably because
silk fabrics do not shed very easily
Trace > Fibers > Natural
Plant Fibers
Cotton - seed hairs of cotton plant
by far most common fiber (find almost everywhere)
Under microscope,
fibers resemble
twisted ribbon
Vegetable Fibers
• Only cotton is found in any large extent
in items of clothing
• Approximately 24% of total US textile
fiber production was cotton in 1979
• Other plant fibers, such as jute and sisal,
are seen in various types of cordage and
baggings
Vegetable Fibers
• Cotton fibers have a distinctive flattened, twisted
microscopic appearance, which is quite
characteristic
• The fibers resemble a twisted ribbon
– In mercerizing process, fibers are treated with alkali,
making them swell up and become more rounded and
less twisted in appearance.
– This process results in improved texture and feel, but
the fibers are still recognizable as cotton under the
microscope
Vegetable Fibers
• Undyed cotton
fibers are so
common they
have little value as
physical evidence
• Almost any
surface or dust
sample will be
found to contain
white cotton
fibers
Household Dust
Trace > Fibers > Natural
Other Plant Fibers:
Linen - stem fiber from flax plant
Kapok - from seed hairs of kapok plant
Other fibers - Manila, hemp, sisal, jute
Mineral Fibers
Asbestos - crystalline material
Used to be used for insulation
 Fractures into thin rods that can
get into your lungs; can kill you
 Not used much anymore

Filament vs. Staple
Filament: Long continuous fiber (like silk)
Staple: Filament is cut into smaller pieces;
staples are spun together to form thread (like
cotton)
Manade Fibers
• Represent approximately 75% of total
textile fiber production in US
• Can be defined as a fiber of a particular
chemical composition that has been
manufactured into a particular shape and
size, contains a certain amount of various
additives, and has been processed in a
particular way
Manmade Fibers
• Within the 6 most seen of the 21 generic
classifications established by the US
Federal Trade Commission, there are well
over a 1,000 different fiber types
• Therefore, numerous fiber types can be
present in the composition of textile
materials
– This is true before even considering
differences in color
Manufactured Fibers
Regenerated Fibers


Cellulose is dissolved, then
resolidified to form the
polymer fiber
Can occur in filament or
staple form
Example: Rayon
Synthetic Fibers


Man made
Can also be filament or
staple
Examples:
Nylon and Polyester
Synthetic Fibers
Acrylics



More common as
evidence
Usually in staple form
Staples spun together,
similar to wool
Trace > Fibers > Analysis
Begin by identifying and comparing class
characteristics for unknown sample
(evidence) and known sample.
Unknown
Known
Trace > Fibers > Analysis
Fibers from rug in a van.
Fibers found on victim.
Trace > Fibers > Analysis
Class characteristics
Color: microscopic examination
Size: length and width can be measured
Shape: cross section is viewed
Class characteristics
Refractive Index – n. The ratio of the speed of light in air or in a
vacuum to the speed of light in another medium.
Other microscopic properties (PLM)
Class characteristics
Chemical Composition: determined by advanced
instrumentation
Threads, Yarn, Rope, Cordage
Smallest component is fibers (staple) twisted
together to form thread or is a filament.
This thread can then be twisted with other
threads to form a thicker thread (string, etc.)
This thicker cord can then be twisted with other
thicker cords, etc.
Threads, Yarn, Rope, Cordage
Small cords or fibers twisted together to
form larger cords


At each step, the
number of cords can
be counted.
At each step, the twist
direction is either “S”
or “Z”
Fiber
niso
nll
n
Biref
MP (ºC)
K1
1.518 to 1.528
1.544 to 1.551
1.505 to 1.516
0.035 to 0.039
Does not melt
K2
1.777 to 1.877
2.050 to 2.350
1.641 to 1.646
0.200 to 0.710
Does not melt
K3
1.512 to 1.521
1.510 to 1.520
1.512 to 1.525
-0.001 to
-0.005
Does not melt
K4
1.538 to 1.539
1.530 to 1.539
1.538 to 1.539
-0.000 to
-0.002
192 – 210
K5
1.533 to 1.545
1.568 to 1.583
1.515 to 1.526
0.049 to 0.061
210 – 230
K6
1.540 to 1.541
1.577 to 1.582
1.515 to 1.526
0.056 to 0.063
250 – 264
K7
1.522
1.553
1.507
0.046
182 – 186
K8
1.535 to 1.539
1.568 to 1.574
1.518 to 1.522
0.050 to 0.052
133 – 138
K9
1.567 to 1.575
1.632 to 1.642
1.534 to 1.542
0.098 to 0.102
282 – 290
K10
1.474 to 1.478
1.474 to 1.479
1.473 to 1.477
0.002 to 0.005
245 – 260
Q
1.520
1.515
1.513
-0.003
Does not melt
Important to Remember:
• It is important to collect evidence from
both complainants and suspects as soon as
possible
• Studies show that some 80% of fibers can
be expected to be lost in four hours, with
just 5-10% remaining at the end of 24 hours
Methods of Examination
• In the recent past, the ID and comparison of
fibers were at a relatively simple level which
relied heavily on microscopy
From Less than 1 cm of a 20 mm Diameter
Fiber It is Possible to Determine:
•
•
•
•
•
•
•
Generic class
Polymer composition
Finish--bright/dull
Cross-sectional shape
Melting point
Refractive Indices
Birefringence
•
•
•
•
•
Color
Fluorescence
Absorption spectrum
Dye class
Dye Components
Microscopy
• Microscopic examination provides the
quickest, most accurate, and least
destructive means of determining the
microscopic characteristics and polymer
type of textile fibers.
Microscopic View
Acetate
Dacron
Stereomicroscope
• Should be used first to examine fibers.
• Physical features such as crimp, length,
color, relative diameter, luster, apparent
cross section, damage, and adhering debris
should be noted.
• Fibers are then tentatively classified into
broad groups such as synthetic, natural, or
inorganic.
Comparison Microscope
• If all of the characteristics are the same
under the stereoscope, then the comparison
microscope is used.
• A point-by-point and side-by-side
comparison provides the most discriminating
method of determining if two or more fibers
are consistent with originating from the
same source.
Comparison Microscopy
• Side-by-side
Comparison
• Bright Field
Adjustment
Comparison Microscopy
• Characterization
• Fluorescence
– Chemical factors
– Environmental factors
Comparison Microscope
• Comparisons should be made under the
same illumination conditions at the same
magnifications.
• This requires color balancing the light
sources.
• A balanced neutral background color is
optimal.
Fluorescence Microscopy
Kevlar fibers in complex
composite material
strongly fluoresce.
• The sample is
illuminated by
ultraviolet light,
causing some
phases to fluoresce
so they can be
observed, counted,
sized and mapped.
Polarized Light Microscope
• Perhaps the most versatile
of all microscopes; allows
the analyst to actually see
and manipulate the sample
of interest.
• Refractive indices,
birefringence, and
dispersion can all be
quantitatively determined.
Microspectrophotometry
• To the unaided eye, 2 dyes
may be identical.
• Using a grating
spectrometer, light
absorbed by or reflected
from a sample is separated
into its component
wavelengths, and intensity
at each wavelength
plotted.
Microspectrophotometry
• Microscope linked to a
Spectrophotometer
– IR Absorption spectrum
– UV/VIS Absorption Spectrum
Microspectrophotometry
• IR spectography identifies generic subtypes
indistinguishable by microscopic exam
• Use of IR microscopes coupled with Fourier
transform infrared (FT-IR) spectrometers
has greatly simplified the IR analysis of
single fibers
Microspectrophotometry
• Advantages
– Nondestructive
– Not limited to sample size
• Disadvantages
– Reactive dyes
– Chemical composition
– Tentative identification
Scanning Electron
Microscopy
• SEM with energy dispersive
spectroscopy(EDS) is used as an imaging
and microanalytical tool in characterization
of fibers.
• Surface morphology can be examined with
great depth of field at continually variable
magnifications.
Thin-Layer Chromatography
• An inexpensive, simple, well-documented
technique that can be used (under certain
conditions) to complement the use of visible
spectroscopy in comparisons of fiber
colorants.
• Dye components are separated by their
differential migration caused by a mobile
phase flowing through a porous, adsorptive
medium.
TLC (continued)
• Should be considered for single-fiber
comparisons only when it is not possible to
discriminate between the fibers of interest
using other techniques, such as comparison
microscopy (brightfield and fluorescence)
and microspectrophotometry in the visible
range
TLC (continued)
• Technique
–
–
–
–
–
Extraction of dyes
Solid stationary phase
Liquid moving phase
Capillary action
Chromatogram
TLC (continued)
• Interpretation
–
–
–
–
Rf (retention factor)
Color
Proportions
Scanning densitometer
• peak height ratios
– Fluorescence
TLC (continued)
• Analysis of Chromatograms
– Positive association
– Exclusion
– Inconclusive