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 Fabrics are composed of individual
threads or yarns made of fibers
 Fibers are knitted, woven, bonded,
crocheted, felted, or knotted together
 Most are either woven or knitted
 Individual qualities of fabrics include:
 Degree of stretch
 Absorbency
 Water repellence
 Softness
 Durability
 Yarn
 Continuous strand of fibers or filaments that may
be twisted together
 Warp
 Lengthwise yarn in fabric
 Weft
 Crosswise yarn in fabric
 Blend
 Fabric made up of two or more different types of
fibers
 Plain
 Simplest & most common
pattern
 Warp & weft yarns pass under
each other alternately
 Design resembles a
checkerboard
 Satin
 Yarn interlacing isn’t uniform
 Interlacing weave passes over
four or more yarns
 Creates long floats
 Twill
 Warp yarn is passed over one
to three weft yarns before
going under one.
 Makes diagonal weave
pattern
 Denim is most common
example
 Knitted fabrics are made by
interlocking loops into a specific
arrangement
 May be one continuous thread or a
combination
 Yarn is formed into successive rows
of loops & then drawn through
another series of loops to make the
fabric
 All fabrics are long chains of monomers
called polymers
 Limited number of natural fibers but
1000s of artificial fibers
 Polymer properties vary
 Chemical structure of monomers vary
 Chemical bond of monomers vary
 Because of these differences, polymers
can be distinguished form one another
forensically
 Characteristics that can be used to identify & compare
fibers as forensic evidence:
 Fiber cross section
 Chemical structure
 Microscopic observation
 Burn tests
 Thermal decomposition
 Chemical tests
 Density
 Refractive index
 Fluorescence
 Synthetic fibers are forced out of a
nozzle when they are hot & then
woven
 Holes of the nozzle are not always
round so fiber filament may have a
unique shape in the cross section
 Microscopic observation
 Compare color, diameter, twist,
number of strands, etc
 Thermal Decomposition
 Use heat to break down the fiber to basic
monomers
 Example: acetate fibers decompose to
form acetic acid
 Burn Tests – how fibers burn
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Odor
Color of flame
Color of smoke
Appearance of residue
 Chemical Tests
 Solubility & decomposition
 Separates waft & weft
 Density
 Mass of an object divided by its volume
 Especially useful for single fibers
 Nondestructive to fibers
 Refractive index
 Measurement of the bending of light
Becke line outside fiber
edges means liquid has
higher refractive index
than fiber
as it passes from air into a solid or
liquid
 Becke line is halo-like shadow
appearing around an object immersed
in a liquid of different refractive index
 If liquid & fiber have same refractive
value, fiber becomes invisible because it
bends light at the same angle as liquid
 If liquid has lower or higher refractive
value, the fiber can be seen
 Fiber has “halo” of a dark boundary
Becke line inside fiber
image means liquid has
lower refractive index
than fiber
 Fluorescence
 Absorption & reemission of light when exposed to UV
radiation
 Expose to both long & short wave radiation for comparison
 May be caused by chemical properties of fabric itself or
dyes/optical brighteners added to fabric
 Used for comparing fibers as well as spotting fibers for
collection
 Component of dyes can be separated & matched to
unknown
 More than 7,000 different dye formulations
 Chromatography is used to separate dyes for
comparative analysis
 The way a fabric accepts a particular dye can be used to
identify & compare samples
 Put 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 & any inanimate
objects
 Removed fibers should be folded into a small sheet of
paper & stored in a paper bag
 Fiber evidence in court cases can be
used to connect the suspect to the
victim or to the crime scene
 In the case of Wayne Williams,
fibers weighed heavily on the
outcome of the case
 Williams was convicted in 1982
based on carpet fibers that were
found in his home, in his car, & on
several murder victims