Forensics Unit 5 Part 1 Notes

Glass Composition

• Significance of Glass

  • Due to the brittle nature of glass, it is commonly found at crime scenes involving burglary and hit & run.
  • Small glass particles easily adhere to clothing.
  • Glass is very stable & not susceptible to environmental degradation.
  • A piece of glass can be individualized only if it is “jigsawed” into the original panel.

• Composition

  • Primary ingredient is silicon dioxide SiO2 (quartz mineral or sand) and traces of various metal oxides: sodium, calcium, magnesium, and aluminum oxides
  • The sand mixture is heated to extremely high temperatures and cooled rapidly before it can recrystalize.

• Types of Glass

  • SODA LIME GLASS
  • SODA (NaCO3): lowers the melting point & viscosity making the glass easier to work with
  • LIME (CaO): glass does not dissolve in water
  • OTHER METAL OXIDES
  • Examples: Most windows, jars, drinking glasses and bottles
  • BOROSILICATES
  • BORON OXIDE : heat resistant (Pyrex)
  • Examples: lab glassware, cookware, automobile headlights
  • LEAD GLASS
  • Lead (II) oxide :very high refractive index (easy to see through and sparkly)
  • Examples: lenses, decorative glassware (Swarovski crystals and Waterford crystal)
  • Colored Glass
  • Additional colored compounds may be added for color: Cobalt = blue glass Chromium = green tinted glass

• Window Manufacturing

  • SHEET GLASS: blown glass cylinder cut open, unrolled or flattened by gravity and cooled
  • PLATE GLASS: molten glass is forced through rollers, cooled, polished
  • FLOAT GLASS: Glass plate formed by floating the glass over molten steel,  that side of the glass fluoresces under UV light

• Safety Glass

  • TEMPERED GLASS: Used in side and rear windows
  • Rapid cooling of glass so the plate cools unevenly (annealing)
  • When it shatters, the glass dices
  • Tempered glass can be made from soda lime or pyrex glass
  • LAMINATED GLASS: Used in auto windshields
  • Plastic sheet sandwiched between two regular pieces of glass
  • BULLETPROOF GLASS: Several layers of laminated glass
  • May crack under pressure, but tends to remain integral
  • Multiple layers of laminated glass

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Glass Reconstruction

• Orienting Glass
  • Dirt and Surface Texture - the outside of the glass may be more dirty and worn due to nature
  • Ream Lines - Invisible lines from polishing the glass can be seen by placing glass on photo paper and exposing to light
  • UV light - float glass will fluoresce on the side that was in contact with the molten tin
  • Look for edges and similar sized pieces
• Radial and Concentric Fractures
  • RADIAL FRACTURES Fractures that radiate out from the center. Radial fractures happen first; immediately.
  • CONCENTRIC FRACTURES Circular fractures around the center. These fractures happen slowly.
• Determining the order of bullets
  • A fracture cannot cross an existing fracture
• Determining the direction of force
  • Examine the Crater: the entrance is smaller than the exit.
  • The side of the glass where the bullet is does not necessarily tell you the direction it came from
  • The side of the glass where the broken glass is does not indicate the direction of force. Sometimes it fractures and does not fall until the wind hits it
  • The size of the projectile cannot be accurately measured from the crater
  • Stress Marks: conchoidal fractures Examine the edge of a radial fracture. (concentric fractures will be reverse)

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