Transcript Fabric Analysis - Uplift Education

Fabric
Analysis
November 20, 2014
Why study fibers?
 Hair
and fibers are the most common
evidence found at a crime scene
 The chemical and physical properties of
fibers can be studied and compared
 Matching fibers can link a suspect to a
scene, victim, or weapon
Are fibers class or individualized evidence?
Why study fibers?
 Hair
and fibers are the most common
evidence found at a crime scene
 The chemical and physical properties of
fibers can be studied and compared
 Matching fibers can link a suspect to a
scene, victim, or weapon
Fibers are class evidence, because fabrics
are mass produced.
What are fibers?
 Fabrics
are made of fibers woven
together in a distinctive pattern.
 Fibers are made up of twisted
filaments; aka thread
Filaments are single strands of a
polymer.
Polymers are chains of simple
molecules called monomers.
What are fibers?
fiber
filament
Fabric, unmagnified
Magnified fabric, with visible
fibers (threads) and
filaments (make up fibers)
Chemical structure of silk –
long chain of three amino
acids
Types of fibers
In a fabric, the length-wise fibers are the warp
and the crosswise fibers are the weft or the
woof.
The warp and the weft often
differ in color, diameter,
material, number of twists,
number of filaments, etc.
Blends are fabrics made of
more than one material,
often as warp and weft.
Identifying physical characteristics
Many identifying characteristics can be
determined with a microscope
• Color
• Cross sectional shape
• Diameter
• Number of filaments
• Twisting
Identifying physical characteristics
Other physical characteristics:
•
Density (mass / volume)
•
Refractive index (how much light bends
when it passes through a substance)
•
Flourescence (the absorption and
reemission of light at another wavelength)
•
Chromatography (the separation of dyes)
Identifying chemical characteristics
•
Burn tests – different materials burn
differently
•
Thermal decomposition tests – when gently
heated, fibers break down into monomers,
which have different chemical properties.
•
Chemical tests – different materials react
differently with other chemicals.
We will do these tests!
Common fibers & their origins
Natural fibers
•
Cotton: plant fiber, made of cellulose
•
Linen: plant fiber (flax), made of cellulose
•
Wool: sheep hair, made of protein (primarily
keratin)
•
Silk: thread for cocoon; made of protein
Common fibers & their origins
Chemically treated natural fibers
•
Rayon: wood pulp is broken down, cellulose is
purified and re-formed.
•
Acetate: cellulose is reacted with acetic acid
Common fibers & their origins
Synthetic fibers
•
Nylon
•
Polyester
•
acrylic
Fun fact: nylon was the first
synthetic fiber; manufactured in
1935 by a chemist working for
DuPont
Probative value of fiber evidence
A person is killed during a home burglary. Two
days later, police arrest a burglar. On the
burglar’s gloves are fibers that are the same
color and material as the victim’s clothing.
Think, pair, share …
How can this evidence be used in court?
Is the evidence strong?
What additional evidence might you seek?
Probative value of fiber evidence
A woman tells her family that she is going to
break up with her boyfriend. Her body is later
discovered along side a road. Police search
her boyfriend’s car and find fibers that match
the sweater she was wearing when she died.
The fibers match in terms of material, color,
fiber diameter, fiber cross-sectional shape,
and twist.
How does this case differ from the previous?
In what ways is the evidence stronger?
In what ways is it weaker?
Lab Safety Refresher!
Video
Safety contract
Quiz next class
Read Lab,
determine jobs