fabric analysis november 20, 2014. why study fibers? hair and fibers are the most common evidence...

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 threadFilaments are single strands of a

polymer.Polymers are chains of simple

molecules called monomers.

What are fibers?

Fabric, unmagnified Magnified fabric, with visible fibers (threads) and filaments (make up fibers)

Chemical structure of silk – long chain of three amino acids

fiber

filament

Types of fibersIn 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 originsNatural 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 originsChemically 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 originsSynthetic 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?

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