fiber evidence. fibers example of locard’s exchange principle –all garment surfaces have loose...

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