fiber evidence. fibers example of locard’s exchange principle –all garment surfaces have loose...
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Fiber Evidence
Fibers
• Example of Locard’s Exchange Principle– All garment surfaces have loose fibers that
have been picked up through contact– Most common form of forensic evidence
Natural vs. Man-made
• Natural: Can be subdivided into:– Animal– Vegetable– Mineral
• Man-made: Manufactured (synthetic)– Began more recently– First showed up in WWII
Natural Fibers
• Animal -- All animal fibers are protein
• Vegetable -- Cellulose
• Mineral -- asbestos, mineral wool
Animal Fibers
• Wool: Can come from many different animals– Species-specific
characteristics
• Shaved off animals and spun into threads
• May also be specific hairs (fur coats etc.)
Animal Fibers
• Silk: unraveled cocoon of a silkworm caterpillar
• Cocoons are boiled, unraveled and spun into the fibers
Vegetable Fibers
• Include cotton, linen, and bamboo
• May be dyed or undyed– Dyed is more helpful as
dye can be analyzed
• Can be from seeds (cotton), fruit (coir) or stems (flax, jute, linen)
Mineral Fibers
• Primarily used for insulation
• Fiberglass: form of glass in short, brittle fibers
• Asbestos: naturally occurring mineral with long fibers
Man-made Fibers
• All are polymers– Repeating units of a
monomer
• Includes polyester, nylon, acrylic, acetate
• Includes regenerated (start with natural materials) and synthetic (made from chemicals) fibers
Comparison of Natural and Synthetic Fibers
Fiber Characteristics
Weave Patterns
• To weave, crosswise threads (weft) woven back and forth through lengthwise threads (warp)
• Pattern in which weft passes over and under warp is weave pattern
• Weave pattern can be used for forensic evidence
Fiber Evidence
• Scientists can use to identify origin of fiber or narrow possibilities
• If edges are torn and can be fit together, can show common origin
• Microscope analysis of fibers– Color– Shape: Cross-sectional, diameter– Striations on surface– Delustering particles to reduce shine
Fiber Cross-sectional Analysis
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Fiber Identification
• Burn Test: – Approaching Flame: Melts, shrinks or curls
away– In a Flame: Burns quickly, burns slowly,
Does not burn at all• Burning fibers may be self-extinguishing• May give off distinctive odor
– Residue: Ash-like, brittle, bead-like and hard
Fiber Identification
• Solubility: – Solvents include acetone, HCl and sodium
hypochlorite– Some fibers completely dissolve– Some solvents may cause structural
damage
Fiber Identification
• Staining: – Look at color and appearance– Different chemical nature of different fibers
causes each to react differently to the staining mixture
Fiber Identification
• Microscopic appearance– Striations or smooth– Ribbon-like or cylindrical– Transparent or opaque
Collection and Preservation of Fiber Evidence
• Look for minute strands of fibers• Relevant articles of clothing should be
packaged carefully in separate paper bags
• Use clean forceps to remove fibers.– Place in small sheet of paper– Fold and label paper– Place paper packet inside another container