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

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)