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Transcript

  • 1. Glass Evidence
  • 2. Glass
    • hard, brittle, amorphous substance
    • composed of
      • sand (silicon oxides)
      • various metal oxides
        • boric oxide
        • lead oxide
  • 3. Glass
    • formed
      • sand is mixed with metal oxides
      • melted at high temperatures
      • cooled to a rigid condition without crystallization
  • 4. Glass
    • additions
      • soda (sodium carbonates) lowers the melting temperature of sand and makes it easier to work with
      • lime (calcium oxide) prevents “soda-lime” glass from dissolving in water
  • 5. Glass
    • types
      • float glass
        • molten glass is cooled on top of a bath of molten tin
        • typically used for windows, bottles
  • 6. Glass
    • types
      • borosilicates
        • auto headlights, heat-resistant glass, Pyrex
        • boron oxide added to oxide mix
  • 7. Glass
    • types
      • tempered glass
        • made stronger than ordinary glass by introducing stress through rapid heating and cooling of the glass surfaces
        • when this type breaks, it does not shatter but fragments “dices” into small squares with little splintering
  • 8.  
  • 9. Glass
    • types
      • laminated glass
        • strengthened by sandwiching one layer of plastic between two pieces of ordinary window glass
  • 10. Glass
    • as evidence
      • chemical composition relatively uniform and therefore not “good” analysis
      • physically match pieces like a jigsaw puzzle to show individualization
      • other physical characteristics
        • density
        • refractive index
        • fracture analysis
  • 11. Glass
    • density
      • solid particles will either float, sink or remain suspended in a liquid depending on its density relative to the liquid
      • ascertained by a variety of methods
        • displacement method
        • floatation method
  • 12. Glass
    • density
      • floatation method
        • standard/reference glass particle is immersed in a mixture of bromoform and bromobenzene
        • composition of mixture is adjusted until the glass chip remains suspended
        • at this point, the glass chip and mixture have the same density
        • calculated from amounts of chemicals added
  • 13. Glass
    • as evidence
      • refractive index
        • light travels at a constant velocity of 300 million meters per second
        • when entering a medium (glass, water, etc) the velocity changes causing the light waves to bend
        • this is refraction
  • 14. Glass
    • as evidence
      • refractive index
        • different mediums will cause the light to bend at different angles
  • 15. Glass
    • as evidence
      • refractive index
        • refractive index is a measure
          • ratio of the speed of light in a vacuum to its speed in a given medium
        • when a transparent solid is immersed in a liquid with a similar refractive index, light is NOT refracted and the solid seems to disappear from view
  • 16. Glass
    • as evidence
      • refractive index
        • immersion method
          • glass particles immersed in liquid medium
          • refractive index of liquid is adjusted (using temperature) until it matches the sample
          • “ match point” obtained with disappearance of the Becke line
            • Becke line is a bright halo observed near the border of a particle that is immersed in a liquid of a different refractive index
            • halo disappears when medium and fragment have similar refractive indices
  • 17. Glass
    • as evidence
      • FBI has collected density and refractive indices values from glass submitted for examination
      • data bank emerged correlating these values to their frequency of occurrence in the glass population in the US
      • used to assess probability that fragments originated from same source
  • 18. Glass
    • glass fracture analysis
      • glass bends in response to any force that is exerted on any one of its surfaces
      • when the limit of its elasticity is reached, the glass fractures
        • reveals info on force and direction of impact
        • radial fractures are lines that radiate from the point of impact
        • concentric fractures are circular lines that encircle the point of impact
  • 19. Glass
        • a projectile creating a hole will leave a wider opening on the exit side of the glass than on the entrance side
  • 20. Glass
        • at too low a velocity the projectile will create an irregular shape –
          • this will make it difficult to determine direction
          • in this case, the pattern of the radial and concentric fractures stress mark can indicate direction
  • 21. Glass
        • sequencing of multiple impacts
          • radial fractures of second impact terminate when they intersect fractures from the first impact