Forensic Analysis of Glass

1
Chapter 5

• Forensic Analysis of Glass

2
Objectives

• Students should gain an understanding of
• The composition of different types of glass
• The optical and nonoptical properties of glass
• Techniques to determine the way in which glass
  has fractured
• Techniques to match glass fragments
• The use of scanning electron microscopy and X-ray
  fluorescence to determine the elemental
  composition of glass

3
Introduction

• Glass is often encountered in criminal
  investigations.
• Fragments may be found on a suspect as transfer
  evidence.
• Key considerations for investigators
• Which types of glass are found at the scene
• How glass fragments can be classified
• How to individualize glass fragments

4
Types of Glass (1 of 3)

• Glass is a solid with an amorphous structure its
  atoms have a random, disordered arrangement.
• Glass softens over a wide temperature range.
• Soda-lime glass is common in windows and bottles
• The addition of metal oxides can give a special
  appearance to glass.

5
Types of Glass (2 of 3)

• Tempered glass is four times stronger than window
  glass.
• Manufacture of tempered glass
• Hot glass is rolled into sheets.
• The upper and lower surfaces are cooled rapidly
  with jets of air.
• Tempered glass does not shatter it breaks into
  dices.

6
Types of Glass (3 of 3)

• Windshield glass is a kind of laminated glass.
• It consists of two layers of glass with
  high-strength vinyl plastic film in between the
  layers.
• The film holds the glass in place when it breaks.

7
Forensic Examination of Glass Evidence An
Overview (1 of 2)

• Goals in examining glass evidence
• Determine the class to which the glass belongs
• Individualize the glass to one source

8
Forensic Examination of Glass Evidence An
Overview (2 of 2)

• Two samples are needed for forensic analysis
• Glass from the crime scene
• Glass on items belonging to the suspect
• Compare characteristics of the samples
• Color, fracture pattern, scratches, striations
  from manufacturing, unevenness of thickness,
  surface wear, surface film or dirt, weathering
  patterns

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Nonoptical Physical Properties of Glass (1 of 5)

• Nonoptical physical properties include the
  glasss surface curvature, texture, and any
  special treatments.
• Such properties are useful in proving two pieces
  of glass are not associated.

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Nonoptical Physical Properties of Glass (2 of 5)

• Surface striations and markings
• Rollers leave parallel ream marks on sheet glass
• Markings may indicate the glasss orientation
  when pieces are missing
• Relative spacing of marks may be used for
  individualization
• Surface scratches, etchings, and other markings
  may also be used to individualize evidence

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Nonoptical Physical Properties of Glass (3 of 5)

• Surface contaminants
• Contaminants show the presence of impurities.
• Patterns of adhering materials might suggest how
  pieces fit together.
• Chemical analysis of adhering materials might
  further individualize the pieces.

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Nonoptical Physical Properties of Glass (4 of 5)

• Thickness
• The thickness of glass must be measured with a
  high degree of accuracy.
• The investigator cannot assume the glasss
  thickness is constant.
• Determinations of curvature can distinguish flat
  glass from container, decorative, or ophthalmic
  glass.

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Nonoptical Physical Properties of Glass (5 of 5)

• Hardness measured on the Mohs scale, which
  indicates a substances hardness relative to
  other substances
• Talc (softest) assigned a value of 1
• Diamond (hardest) assigned a value of 10
• Glass tends to fall in the 56 range
• For unknown minerals, relative hardness is
  determined by using the sample to try to scratch
  benchmark minerals

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Glass Fractures (1 of 8)

• Elasticity the ability of a material to return
  to its previous shape after a force is exerted on
  it
• Use in forensic investigation may be able to
  analyze fractured window panes at a crime scene
  and determine what happened

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Glass Fractures (2 of 8)

• Three types of forces are distinguished
• Compressive squeezes the material
• Tensile expands the material
• Shear slides one part of material in one
  direction and another part in another direction
• Each force causes a deformation.

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Glass Fractures (3 of 8)

• Glass breaks when a tensile strain is applied
  that overcomes the materials natural tensile
  stress limit.
• Cracks grow from the unloaded side to the loaded
  side.
• Radial cracks radiate outward, away from the load
  point.
• Tangential (concentric) cracks form if a load
  persists, leading to a spider web appearance.

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Glass Fractures (4 of 8)

• Bullets are projectile loads through glass.
• Load side is the entrance unloaded side is the
  exit.
• As the bullets velocity increases, the central
  hole becomes smaller, cracking patterns become
  simpler, and the exit hole becomes wider than the
  entrance hole.

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Glass Fractures (5 of 8)

• Edges of broken pieces of glass will show rib
  (stress) marks.
• In a radial crack, the rib marks are
  perpendicular to unloaded side and parallel to
  loaded side the opposite is true for a
  tangential crack.

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Glass Fractures (6 of 8)

• 3R rule
• Radial cracks give rib marks that make
• Right angles on the
• Reverse side from where the force was applied

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Glass Fractures (7 of 8)

• High-speed projectiles exit hole will be wider
  than entrance hole
• Low-speed projectiles rib marks may indicate
  where breaking force was applied

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Glass Fractures (8 of 8)

• Contaminants such as paint or window putty might
  help identify the inside and outside sides of
  glass.
• Analysis can even determine the sequence of
  impacts for multiple bullet holes.
• Glass fragments from suspects will be very small
  and lost fairly rapidly.

22
Glass Density Tests

• Density a class characteristic
• Density gradient column method used to determine
  glass density
• Fragments of different densities settle at
  different levels in the column
• Technique is not accurate for fragments that are
  cracked or contain an inclusion
• Density tests can exclude fragments that do not
  match the known specimen

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Optical Physical Properties of Glass (1 of 11)

• Color
• Make side-by-side comparisons using similar-sized
  fragments
• Place the samples over a white surface using
  natural light
• Use both fluorescent and incandescent light to
  determine the glasss color

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Optical Physical Properties of Glass (2 of 11)

• Refractive index measure of how much light is
  bent as it enters the glass
• Velocity of light in the air ? velocity of light
  in the glass
• The temperature of the sample affects its
  density the density change affects the velocity
  of light as it passes through the sample
• Single sheets of plate glass do not usually have
  a uniform refractive index value
• The index of refraction can vary as much as
  0.0002 from one side of the glass to another

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Optical Physical Properties of Glass (3 of 11)

• Oil immersion method
• The forensic examiner places the questioned glass
  fragments in specialized silicone oils with known
  refractive indices.
• The immersion oil is heated to change its
  refractive index.
• The refractive index of the oil decreases as
  temperature increases until the Becke line
  disappears.
• At the match point, the refractive indices of the
  oil and the glass fragment are the same.

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Optical Physical Properties of Glass (4 of 11)

• Emmons procedure uses a hot stage microscope
  plus different source lamps
• Measures the index of refraction for the sample
  at multiple wavelengths
• Correlates the refractive index and the
  wavelength at fixed temperatures for the silicone
  oil into which the glass sample is placed

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Optical Physical Properties of Glass (5 of 11)

• Emmons procedure
• Step 1 crush the glass and place it in silicone
  oil on the hot stage
• Step 2 measure the glasss index of refraction
  with a sodium lamp and with a hydrogen lamp
• Step 3 as the temperature of the hot stage
  increases, take measurements at three different
  wavelengths
• Step 4 record a line for the refractive index of
  the glass at each wavelength for each temperature
• Step 5 superimpose the lines to create a
  Hartmann net

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Optical Physical Properties of Glass (6 of 11)

• Annealing is used to distinguish tempered glass
  from nontempered glass.
• Glass fragments are heated in a furnace at a
  temperature gt 600 C.
• The change in refractive index reveals whether
  the glass is tempered or nontempered.

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Optical Physical Properties of Glass (7 of 11)

• Properties of glass are more often used to
  exonerate suspects than to prove an association.
• Glass evidence is not always individualized
  Fragments from different sources may have similar
  indices of refraction and similar densities.
• Once the refractive index is known, measurement
  of the glasss density will improve
  discrimination capability twofold.

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Optical Physical Properties of Glass (8 of 11)

• The FBI has compiled density and refractive index
  data for glass from around the world.
• The FBI has also identified the relationship
  between their refractive indices and densities
  for 1400 glass specimens.

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Optical Physical Properties of Glass (9 of 11)

• Elemental analysis of glass
• Elemental analysis should be performed only after
  all nondestructive methods of examination are
  complete and when additional discrimination is
  necessary.
• Scanning electron microscopy may be used to
  analyze glass, but precise quantitative
  determination of the element concentrations is
  not possible.

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Optical Physical Properties of Glass (10 of 11)

• Techniques for elemental analysis of glass
• X-ray fluorescence focuses a beam of X-rays on
  the glasss surface and measures the energy of
  the X-rays emitted
• Flameless atomic absorption spectrophotometry
  (FAAS)
• Inductively coupled plasma (ICP) when used with
  an optical emissions spectrometer, can identify
  10 elements in glass (aluminum, barium, calcium,
  iron, magnesium, manganese, sodium, strontium,
  titanium, zirconium)

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Optical Physical Properties of Glass (11 of 11)

• Disadvantages of the elemental analysis of glass
• The fragment must be dissolved in acid, so the
  original sample is destroyed
• Techniques require the use of hazardous chemicals