Chapter 4 A Study Of Fibers And Textiles

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Scenario: A Thread of Evidence

Back 1

String of murders of black youth rocked Atlanta in the1980s.
Police gathered a number of unusually shaped fibers.
C. Fiber match to the rug was the key to conviction.

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amorphous

Back 2

without a defined shape; fibers composed of a loose arrangement of polymers that are soft, elastic, and absorbing (for example, cotton)

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crystalline

Back 3

regularly shaped; fibers composed of polymers packed side by side, which make it stiff and strong (for example, flax)

Front 4

direct transfer

Back 4

the passing of evidence, such as a fiber, from victim to suspect or vice vers

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fiber

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the smallest indivisible unit of a textile, it must be at least 100 times longer than wide

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mineral fiber

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a collection of mineral crystals formed into a recognizable pattern

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monomer

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a small molecule that may bond to other monomers to become a polymer

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natural fiber

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a fiber produced naturally and harvested from animal, plant, or mineral sources

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polymer

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a substance composed of long chains of repeating units

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synthetic fiber

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a fiber made from a man-made substance such as plastic

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secondary transfer

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the transfer of evidence such as a fiber from a source (for example, a carpet) to a person (suspect), and then to another person (victim)

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textile

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a flexible, flat mate- rial made by interlacing yarns (or “threads”)

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yarn

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fibers that have been spun together

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How Forensic Scientists Use Fibers

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1. Type of fiber – composition, uniqueness, and so on.
2. Fiber color – often key to matching techniques.
3. Number found – usually the more found the easier the match.
4. Where found – can you place the suspect at the scene?
5. Textile fiber originated from – carpet, upholstery, car, and so forth.
6. Multiple fibers – several types of fibers can be more conclusive.
7. Type of crime – can be the key to fiber transfer
8. Time between crime and fiber discovery – passage of time greatly reduces the effectiveness of fiber evidence.

Front 15

Gathering evidence

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special vacuums
sticky tape
tweezers

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Analysis
Nondestructive

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polarizing light microscopy
infrared spectroscopy.

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Destructive

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burning
dissolving in solvents

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A. Fiber classification
1. Natural

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a. Animal
i. fur
ii. hair
iii. silk
b. Plant fibers

i. seed fibers, example: cotton
ii. fruit fibers. example: coir
iii. stem fibers, example: hemp
iv. leaf fibers, example manila
c. Mineral fibers or asbestos

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Man-made fibers

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rayon, acetate, nylon, acrylics, and polyesters

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Regenerated fibers

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i. derived from cellulose
ii. mostly plant in origin
iii. example: rayon

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Synthetic polymer fibers

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i. petroleum products
ii. non-cellulose based fibers
iii. polyester, nylon, acrylic, olefins

Front 22

Comparison of natural and synthetic fibers

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under magnification, all synthetic fibers have very regular diameters hairs have cuticles

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Yarns

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Fibers too short to be used to make textiles in their raw state may be spun together to make yarns


Any given yarn will have a direction of twist.



Forensic scientists will identify the twist direction as part of their identification.

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Weave pattern

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warp & weft
pattern determined by number of threads jumped as weave over and under

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Thread count

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= The number of threads that are packed together for any given amount of fabric

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Case Studies

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Case Studies
A. The murder of George Marsh (1912)
B. Roger Payne (1968 U.K.)
C. John Joubert (1983 Nebraska)

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A. Irene Good

Back 27

textile expert