Shattered: Forensic Glass Analysis

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Shattered Forensic Glass Analysis
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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

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'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
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Rolled Glass
Float Glass
Float Glass Process
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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.
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Whats in Glass?

• Network Components-Formers
• SiO2, B2O3, P2O5, GeO2, V2O5, As2O3, Sb2O5
• FluxesSofteners lowers melting point
• Na2O, K2O, LiO, Al2O3, B2O3, Cs2O
• StabilizersChemical/Corrosion Resistance
• CaO, MgO, Al2O3, PbO, SrO, BaO, ZnO, ZrO

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Major Types and Uses

• Soda lime silicate glass
• SiO2 Na2O / K2O CaO / Al2O3 / MgO
• Flat glass, container glass, electric light bulbs
• Borosilicate glass
• gt 5 B2O3 (replaces Na2O)
• Lab glassware, thermometers, cookware,
  sealed-beam headlights

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• Aluminosilicate glass
• Higher percentage of aluminum
• higher temperatures than borosilicate
• Aluminoborosilicate 5050 Al/B
• Labware, cookware, and glass fibers
• Lead alkali silicate (leaded glass)
• Up to 80 PbO
• high refractive index
• Crystal tableware,

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

• Flat GlassSoda 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.

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• 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

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• 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

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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

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Comparison of Physical Characteristics

• Color
• Flatness
• Thickness
• FluorescenceFloat Glass
• 254 nm

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Physical Properties

• Density
• Sink-Float Method
• CHBr3, C2H2Br4, Sodium Polytungstate
• Density Range
• 2.465-2.540 g/cm3

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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.
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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
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Populations of Glass RIs
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GRIM II System -automated system for matching
glass RI -uses a step-heating stage and a RI
liquid that varies with temperature
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GRIM II identifies the RI match by monitoring
a video image of the glass fragment in the
liquid -as it is heated/cooled the contrast is
measured until a minimum is reached-the match
point
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• Chemical Composition
• Inductively Coupled Plasma (ICP)
• SEM/EDS
• Neutron Activation Analysis
• X-Ray Fluorescence

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How does Glass break? Forensic Fracture Analysis
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Radial
Begin at a point and radiate outward from point
of impact -initial crack is on the side opposite
the applied force
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Concentric

• 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.

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• 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.

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Bullets and Glass

• Its possible to determine
• the bullet's direction by
• noting the side of the cone-
• shaped hole left by the
• bullet.
• -small opening is on the
• entrance
• side

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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.
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