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Glass

Glass. Physical Properties. How can glass be useful at a crime scene?. What is glass, really?. Hard, brittle, amorphous Made of sand(silicon dioxide) and metal oxides Melted and cooled Soda(NaCO3) Lime(CaO) Cooled on molten tin  flat glass  float glass.

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Glass

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  1. Glass Physical Properties

  2. How can glass be useful at a crime scene?

  3. What is glass, really? • Hard, brittle, amorphous • Made of sand(silicon dioxide) and metal oxides • Melted and cooled • Soda(NaCO3) • Lime(CaO) • Cooled on molten tin  flat glass  float glass

  4. Different types of glass • Heat-resistant glass • (i.e. Pyrex) • boron oxide • borosilicates • Tempered glass • Stronger • Made through rapid heating and cooling • Breaks into tiny cubes • Side and rear windows, foreign windshields • Laminated glass • US windshields • Plastic sheet sandwiched between two pieces of glass

  5. Fragment Matching • Fragment edges • Striations and scratches • Frequently too fragmentary or tiny

  6. Other Comparisons • Density • Refractive Index • Could at least exclude evidence

  7. Tests • Flotation • bromoform or bromobenzene • Immersion method • refractive index adjusted to match – match point • disappearance of Becke line (bright halo) • temp control • GRIM 3 • glass refractive index measurement • automate previous two tests

  8. FBI Database • Frequency of occurence for different glass types • Probability

  9. Glass Fractures • Penetration of window glass by projectile by bullet or a stone • Cracks • Radial fractures: radiate outward • Concentric fractures: encircle hole

  10. Glass Fractures • Round, crater shaped hole • Symmetrical radial and concentric cracks • Wider on exit side • As velocity decreases, irregularity of hole and cracks increase

  11. 3 R’sRadial cracks form a Right angle on the Reverse side of the force

  12. Wallner lines

  13. Analyzing Cracks • Sequence of impacts: • Successive penetrations of glass • Fracture always terminates at existing line of fracture

  14. Which hole formed first?

  15. Collection and Preservation • Collect all glass found for fragment matching • Collect glass on suspect • Representative sample of broken glass remaining at the crime scene (if matching can’t occur) • Packaged in solid containers to avoid further breakage • Suspect’s shoes and/or clothing is individually wrapped in paper

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