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Glass Evidence


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Glass Evidence

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