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Shattered: Forensic Glass Analysis. What is Glass?. “An inorganic product of fusion which has cooled to a rigid condition without crystallizing” Uniform amorphous solid No specific m.p. Softens over a temperature range. 'Ordinary' sheet glass

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what is glass
What is Glass?
  • “An inorganic product of fusion which has cooled to a rigid condition without crystallizing”
  • Uniform amorphous solid
  • No specific m.p.
  • Softens over a temperature
  • range

'Ordinary' sheet glass

-made by passing the molten glass through rollers; process gives an almost flat finish but with distortions.

-glass is used in glazing greenhouses and garden sheds where the visual distortions do not matter.

Float glass (plate)

Float glass gets its name from the method of production used to manufacture it.

-molten glass is 'floated' onto a bed of molten tin

-produces a glass which is flat and distortion free

-process imparts a fluorescence to the glass [UV light]


Rolled Glass

Float Glass

Float Glass Process


Toughened (Safety glass)

-glass is produced by applying a special treatment to ordinary float glass after it has been cut to size and finished.

-treatment involves heating the glass so that it begins to soften (about 620 C) and then rapidly cooling it.

-produces a glass which, if broken, breaks into small pieces without sharp edges.

Laminated glass

-glass is made up of a sandwich of two or more sheets of glass (or plastic), bonded together by a flexible, normally transparent material.

-if cracked or broken, the flexible material is designed to hold the glass fragments in place.

what s in glass
What’s in Glass?
  • Network Components-Formers:

SiO2, B2O3, P2O5, GeO2, V2O5, As2O3, Sb2O5

  • Fluxes–Softeners [lowers melting point]:

Na2O, K2O, LiO, Al2O3, B2O3, Cs2O

  • Stabilizers–Chemical/Corrosion Resistance:

CaO, MgO, Al2O3, PbO, SrO, BaO, ZnO, ZrO

major types and uses
Major Types and Uses
  • Soda lime silicate glass

SiO2 + Na2O / K2O + CaO / Al2O3 / MgO

    • Flat glass, container glass, electric light bulbs
  • Borosilicate glass

> 5% B2O3 (replaces Na2O)

Lab glassware, thermometers, cookware, sealed-beam headlights

Aluminosilicate glass
    • Higher percentage of aluminum
    • higher temperatures than borosilicate
    • Aluminoborosilicate – 50:50 Al/B
    • Labware, cookware, and glass fibers
  • Lead alkali silicate (leaded glass)
    • Up to 80% PbO
    • high refractive index
    • “Crystal” tableware,

Different glasses-domestic windows, car headlamps, light bulb glass-are produced differently from different chemistries.

forensic individualization of glass
Forensic Individualization of Glass
  • Flat Glass–Soda lime silicate

-Rolling or Floating

    • Tempered: Rapid Cooling
      • Adds Strength
      • Dices when broken
      • Automotive windows & security windows
      • Float Glass floresces when excited at 254 nm.
Coated: Surface modification
    • Mirrors
  • Laminated: Sandwiched around plastic
    • Automotive windshields
  • Headlights: often borosilicate
  • Light bulbs: soda lime glass
  • Heat absorbing/ UV filtering
    • tinting
  • Eyeglasses: prescription lenses/photosensitive
Container Glass
    • Lower magnesium, higher sodium
    • Clear vs. greenish (window)
  • Glass Fibers
    • Fiberglass insulation
    • Alumino-borosilicate
    • Binder (red or yellow) to hold fibers in bundles
forensic analysis of glass
Forensic Analysis of Glass

Fundamental Question: Is it Glass?

  • Isotropic vs. Anisotropic
    • Glass: Isotropic
      • dark when rotated under crossed polarizers
    • Crystalline solids and plastics: Anisotropic
      • Change retardation when rotated under crossed polarizers
  • Hardness
  • Solubility
examination of glass chips
Examination of Glass Chips
  • Physical – thickness, uniformity, color, curvature, surface conditions, soiling, density
    • Sink-Float Method
      • CHBr3, C2H2Br4, Sodium Polytungstate
    • Density Range:

2.465-2.540 g/cm3

  • Optical – Refractive Index
  • Elemental – Composition of Glass
determine the refractive index in a liquid medium or solid sample
Determine the Refractive Index(in a liquid medium or solid sample)
  • Oblique Illumination
  • Dispersion staining color
  • Movement of the Becke Line: most popular; see a halo or brightness near the edge of sample due to increase of distance between sample & microscope objective (Fig 16.11, p327)

When 2 substances have different RI, light passing through them produces a Becke Line

A Becke line is a band or rim of light visible along a grain/crystal boundary in plane-polarized light.


Glass analysis is often

performed using our old

friend the PLM

-the goal of glass analysis

by PLM is to individualize an

evidence glass fragment to an

exemplar from a crime scene

[primary or secondary]


How does Glass break?

Forensic Fracture Analysis


Begin at impact point and radiate

outward from point of impact

-initial crack is on the side

opposite the applied force


-Cracks seeming to make a circle around point of impact

-Motion of the projectile through the glass

puts tension on the front surface of the glass, causing concentric fractures.

-high-speed projectile [a rock] penetrates the glass, it leaves an exit hole larger than it‘s entrance hole.

-helps to determine the direction on impact.


Conchoidal Lines – stress marks shaped like arches that are perpendicular to 1 glass surface and curved nearly parallel to the opposite surface; perpendicular surface faces side where crack originates; primarily found with bullet holes

  • A determination of the direction of force in breaking a window pane:
  • -direction of the rib marks
  • [stress marks on broken edges of glass that are perpendicular to one side of glass]
  • For radial fractures (radiating from the center):
  • - the direction of the force is on the same side as the tangential parts of the rib marks.
bullets and glass
It’s possible to determine

the bullet's direction by

noting the side of the cone-

shaped hole left by the


-small opening is on the



Bullets and Glass
counting bullet holes
Counting Bullet Holes

Determination of the sequence of bullet holes can be made by noting the radial fractures.

Radial fractures caused by the passage of a bullet stop at pre-existing fracture.