Fiber Analysis
Hair and Fiber 3
Fiber Evidence
• Fiber
– The smallest unit of a textile material that
has a length many times greater than its
diameter
– Can be spun with other fibers to form a yarn
(rope) that can be woven or knitted to form
a fabric
• Can be natural (plant or animal) or manmade
– Man-made fibers now account for over ½ of
all textile production
Forensic Value
• Are considered class evidence
• Are common trace evidence at a crime
scene
• Can be characterized based on
comparison of both physical and
chemical properties
Fiber Classification
• Natural fibers are classified according
to their origin
– Vegetable or cellulose
– Animal or protein
– Mineral
• Some natural fibers can be altered into
artificial ones
– Cotton and rayon
– Rayon is chemically-altered cellulose
Chemical Alteration
Note the differences between the cotton fiber (left)
and rayon (right). Can you detect what happens in
the chemical transformation process?
Fiber Evidence
• Important characteristics
– Type and length of fiber
– Spinning method
– Fabric construction
• Above characteristics can greatly affect
transfer of fibers and significance as
evidence
• Can originate from more than clothing
– Furniture, upholstery, etc.
Fiber Evidence
• Significance
– Based upon clear relationships between
objects associated with crime
– Relative value of evidence can be based upon
several factors
• Type of fiber
• Number of fibers
• Color or variation of color
• Location of fibers
• Number of different fibers
• Likelihood of transfer based on fiber construction
Fiber Evidence
• Useful in similar crimes to hairs
• Since DNA is not found, often limited to
class characteristics due to mass
production
• Transfer usually found through physical
contact
• Routinely found in vehicle accidents or on
glass/screens
Animal Fibers
•Silk
–Technically a protein
secretion
–No longer found in
nature
–Among the most
expensive fibers
–Fibers are altered by
changing diet of silkworm
–Does not resemble
typical non-human hair
–Highly light reflective
Mineral Fibers
•Fiberglass
–Artificially produced
fiber
•Asbestos
–Natural fiber from the
mineral serpentine
Plant Fibers
•Cotton
–Most common plant
fiber
–Virtually worthless
as evidence if white
•If dyed, the dye is
more valuable than
the cotton
–Distinctive twisted,
ribbon-like shape
Plant Fibers
• Flax
– Used to make linen
– Distinctive lateral structures
through fiber
– Can be expensive
Plant Fibers
Ramie fiber, common in SE Asia
Hemp fiber, the strongest
natural fiber
Plant fibers are based on the
polymer cellulose, the
chemical that forms the cell
wall of plant cells
Jute fiber, also common
in Asia
Synthetic Fibers
• First introduced in 1911 (rayon) followed
by nylon in 1939
• Hundreds of names currently used for
essentially the same artificial fibers
– Generally grouped into generic names
• Example: Polyester - Sold under 28 different names
Synthetic Fiber Chemistry
• Based on polymers
– Long chain molecules
• Come in natural and man-made varieties
• Synthetic polymers were first produced in 1909
– Originally discovered by accident
• When a glass rod was removed from contact with
polymer, it stretched and stuck to the rod. It
hardened when cooled and would stretch into long
filaments
Synthetic Fiber Chemistry
• Polymers
– Well known natural polymers
• Starch
• Cellulose - nature of polymer provided structure to
plants
• Proteins - form animal hairs
– Well known man-made polymers
• Nylon
• Teflon
• Silicone
• Vulcanized rubber
Synthetic Fiber Chemistry
• Man-made polymers
– Basic chemical substance of all artificial fibers
– Also include most paints, plastics, adhesives,
rubbers
• Polymers are routinely formed from
thousands of individual atoms
• Composed of repeating units called
monomers that link like chains
Synthetic Fiber Chemistry
“Classic” polymer structure of the synthetic fiber nylon.
Note the long chain of repeating atoms.
Synthetic Fibers
• In spite of common names, practically no
two of the same type of fiber are
manufactured the same way all the way
through the production process
• Positive ID almost always involves
microscopy and chemical analysis
Three different scanning electron microscope
images of nylon carpet fibers. Note clear
structural differences at this magnification.
Synthetic Fiber Analysis
• Identification and Comparison
• Color
– Based on introduced combinations of dyes
– Different dyes may identify manufacturer
• Diameter
– Typically very little variation due to precise
machinery
– Cross section usually helpful as well
• Production characteristics
– Striations - almost always lengthwise
– “Pitting” - occurs from particles added to
fiber to reduce shine
Typical cross
section of
synthetic
carpet fibers.
Synthetic Fiber Analysis
•Production method can be single largest identifier
•Cross sections are exceptionally important
–Synthetic fibers are forced out of a nozzle when
they are hot
–Holes of the nozzle are not always round
Synthetic Fiber Analysis
• Burning
• Chemistry of fibers can cause fiber to burn
in different ways
• Odor, color of flame, smoke and the appearance
of the residue can also be an indicator
• Thermal decomposition
• Gently heating can break down polymers to
basic monomers
• Chemical testing
• Solubility and decomposition
Synthetic Fiber Analysis
• Chemical composition
– Most companies have different
“formulas” for product
– Product “fingerprint” can be
determined if chemicals are
identified
• Require spectroscopy
– Fibers can also be melted down
into crystals and then identified
• Light reflectance will be different
for unique crystals
• Known as crystallography
Common nylon
(above) and rayon
(below) fibers under
polarized light
Synthetic Fiber Analysis
Spectroscopic analysis of two apparently identical red,
cotton fibers. Note the clear chemical differences
between the two dyes.