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Forensic Analysis of Glass

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Title: Forensic Analysis of Glass


1
Forensic Analysis of Glass
HW Ch 4 1,2,4,6,11-15,24,26,27 Due 10/26
2
Forensic Examination of Glass
  • Goals in examining glass evidence
  • Determine the types of glass at the scene
  • Determine how the glass was fractured
  • Use physical characteristics to classify it
  • Individualize the glass to a source
  • Compare physical and chemical characteristics
  • Optical properties color and refractive index
  • Non-optical properties surface wear, striations
    from manufacturing, thickness, surface film or
    dirt, hardness, density
  • Chemical properties additives or trace elements

3
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

4
What is Glass?
  • Fusion of sand (SiO2), soda (Na2CO3) lime (CaO)
    that produces a transparent solid when cooled.
  • A 3D network of atoms which lacks the repeated,
    orderly arrangement typical of crystalline
    materials.

5
  • Physical properties
  • hard
  • elastic
  • brittle
  • non-conductor of electricity
  • density
  • refractive index
  • Chemical
  • resistant to all but fluorine and very strong
    bases.

6
What Types of Glass Are There?
  • The primary uses for glass are in windows,
    containers, light bulbs and eyewear.
  • Borosilicate Glass (pyrex) 5 borax (Na2B4O7) is
    added to resist breaking when heated or cooled.
  • Colored Glass metal oxides or colloidal iron
    (Fe) sulfur (S) are added to change its color.
  • Lead glass Pb increases refractive index
    density

7
What Types of Glass Are There?
  • Flat glass made by a float glass process
    molten glass is floated on a pool of tin while
    cooling. Commonly found in doors and windows.
  • Laminated glass used in windshields, two sheets
    of glass with plastic between them.
  • Tempered safety glass used in car side windows
    and designed to break into tiny pieces potassium
    (K) replaces sodium (Na) on the surface.

8
Rolled Glass
Float Glass
Float Glass Process
9
How Do Glass Windows Break?
  • Each force causes a deformation that may leave a
    visible mark or fracture the glass. This can be
    used to determine the direction and amount of
    force.
  • Glass acts initially as an elastic surface and
    bends away when a force is applied. When the
    force increases beyond its tensile strength, it
    cracks.

10
How Does Glass Break?
  • Radial cracks form first and are propagated in
    short segments on the side opposite the force.
  • Concentric cracks come later from continued
    pressure on the same side as the force applied.

11
Which Bullet Hole Was First?
  • The sequence of impacts can be determined since
    crack propagation is stopped by earlier cracks.

In the figure above, which impact occurred first?
12
How Does Glass Break?
  • 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 arrow shows the side that
    received the impact.
  • 3R rule
  • Radial cracks give rib marks that make
  • Right angles on the
  • Reverse side from where the force was applied

13
3R Rule
  • Radial cracks give rib marks that make
  • Right angles on the
  • Reverse side from where the force was applied

14
Which impact happened first?
  • Explain why in your notes.
  • HW Notes on p125-139 due 10/29

15
Exceptions to the Three R Rule
Tempered glass dices without forming
ridges Very small windows held tightly in
frame cant bend or bulge appreciably Windows
broken by heat or explosion no point of
impact curved, smooth edges at break points
16
Types of Fractures by Projectiles
  • Bullets are a projectile force (load) that can
    pass through glass.
  • Load side is the entrance side unloaded side is
    the exit side.
  • Low-speed projectiles rib marks may indicate
    where breaking force was applied
  • As the bullets velocity increases, the central
    hole becomes smaller, cracking patterns become
    simpler, and the exit hole becomes wider than the
    entrance hole.

17
constructed using polycarbonate, thermoplastic,
and layers of laminated glass. The aim is to
make a material with the appearance and clarity
of standard glass but with effective protection
from small arms.
18
  • usually made from a combination of two or more
    types of glass, one hard and one soft. The softer
    layer makes the glass more elastic, so it can
    flex instead of shatter.
  • strong but optically transparent material that is
    particularly resistant to being penetrated when
    struck by bullets, but like all other known
    materials, is not completely impenetrable.

19
Which side was the bullet fired from?
Fractures by Projectiles
Exit (unloaded) side is wider than entry (load)
side.
  • Stress lines on the glass edge of radial cracks
    form a right angle on the reverse side from the
    force.
  • Stress lines on the glass edge of concentric
    cracks form a right angle on the same side as the
    force.

20
Putting it Back Together Again?
  • Examiners can fit together two or more pieces of
    glass that were broken from the same object.
  • Because glass is amorphous, no two glass objects
    will break the same way.

21
Learning Check
In the figure below left, which impact occurred
first?
22
Glass Transfer Evidence
  • When glass objects are broken, glass flies
    backward from all parts of the object where
    cracks appear not just from point of impact.
  • This creates a shower of minute glass particles
    and a transfer of evidence.
  • Glass fragment comparison depends finding and
    measuring properties that will associate one
    glass fragment with another while eliminating
    other sources.

23
Collection of Glass Samples
  • The glass fragments should be packaged in boxes
    to avoid further breakage.
  • If evidence is to be examined for glass
    fragments, it should be taken whole and each item
    individually wrapped in paper and boxed.
  • If even the remotest possibility exists that
    glass fragments may be pieced together, every
    effort must be made to collect all glass
    fragments.
  • Submit glass evidence along with a representative
    sample of each type of glass from the crime scene.

24
Forensic Examination of Glass
  • Goals in examining glass evidence
  • Determine the types of glass at the scene
  • Determine how the glass was fractured
  • Use physical characteristics to classify it
  • Individualize the glass to a source
  • Compare physical and chemical characteristics
  • Optical properties color and refractive index
  • Non-optical properties surface wear, striations
    from manufacturing, thickness, surface film or
    dirt, hardness, density
  • Chemical properties additives or trace elements

25
Optical Properties of Glass
Make side-by-side comparisons using similar-sized
fragments. Place samples on a white surface
using natural light. Use both fluorescent and
incandescent light to determine the glasss
color. Visual color analysis is very
subjective. Dyes and pigments can be almost
impossible to extract.
26
Nonoptical Physical Properties of Glass
  • Surface striations and markings
  • Rollers leave parallel ream marks on sheet glass
  • Markings may indicate the glasss orientation
    when pieces are missing
  • Surface scratches, etchings, and other markings
    may also be used to individualize evidence
  • Other Properties
  • Hardness5-6 on Mohs scale use a scratch test.
  • Determinations of curvature can distinguish flat
    glass from container, decorative, or ophthalmic.

27
Forensic Analysis of Glass
28
Quantitative Properties of Glass
29
Why Measure Density?
  • Can be used as a screening technique with large
    numbers of fragments.
  • Useful in identifying multiple sources present in
    the known and/or questioned samples.
  • It is nondestructive and an intensive property
    (not dependant on sample mass).
  • Need to measure very precisely in parts per
    hundred or thousand or better.

30
Glass Density
  • Density can be measured by
  • directly determining mass and volume (usually by
    displacement)
  • comparison by flotation
  • comparison using a density gradient column
  • Density gradient column method
  • Fragments of different densities settle at
    different levels in the column of liquid of
    varying density.
  • Technique is not accurate for fragments that are
    cracked or contain an inclusion.

31
Density by the Flotation Method
  • A glass particle is immersed in a liquid. The
    density of the liquid is adjusted by the addition
    of small amounts of another liquid until the
    glass chip remains suspended.
  • At this point, the glass will have the same
    density as the liquid medium and can be compared
    to other relevant pieces of glass which will
    remain suspended, sink, or float.

32
Why Measure Refractive Index?
  • Refractive index ratio of the velocity of light
    in a vacuum to the velocity of light in any other
    medium
  • For example, at 25oC the refractive index of
    water is 1.333. This means that light travels
    1.333 times faster in a vacuum than it does in
    water.
  • Like density, refractive index is an intensive
    property but it can be measured very precisely
    (0.0002) and does not destroy the sample.
  • Refractive index of glass varies with small
    changes in composition or by how it is
    manufactured.

33
Snells Law
N1.52
N1.33
The higher the n, the more the light bends
34
Refractive Index By Immersion
  • Immersing a glass particle in a liquid medium
    (silicone oil) whose refractive index can varied
    with temperature until it is equal to that of the
    glass particle.
  • At this point, known as the match point, the
    Becke line disappears and minimum contrast
    between liquid and particle is observed RI oil
    RI glass.
  • The Becke line is a bright halo near the boarder
    of a particle that is immersed in a liquid of a
    different refractive index.

35
Refractive Index By Immersion
36
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.
37
Becke Lines
Glass has higher refractive index-note white line
inside
  • Glass has lower refractive index-note white line
    outside

38
Problems with Refractive Index
The measurement of RI alone can be of limited use
because RI distributions of flat glasses and
container glasses overlap.
Histogram flat glasses Curve container glasses
39
Problems with Refractive Index
A rough statistical estimate of the likelihood of
finding glass of that refractive before 1970
1964-1979
Manufacturing changed in late 1970s making glass
more uniform
1980-1997
40
FBI Refractive Index vs Density Data
  • The FBI has compiled density and refractive index
    data for glass from around the world.
  • The FBI has identified a relationship between
    their refractive indices and densities for 1400
    glass specimens that is better at classification.

41
Chemical Analysis of Glass
  • Fluorescence
  • Under UV radiation, many glasses exhibit
    fluorescence (glow)
  • Caused by heavy metals (including tin) from
    float process or organic coatings
  • Scanning Electron Microscopy Energy Dispersive
    X-ray Analysis
  • Can determine many elements simultaneously
  • Surfaces of samples (gt50 mg) can be analyzed
  • Atomic Absorption Spectroscopy
  • You must first know which elements are present
  • Can analyze ppm levels of elements present

42
Learning Check
  • Which unique chemical component would be found in
    each type of glass shown below?
  • (beaker) (windshield)
    (crystal)

2. Use Table 2.3 to determine if lead
borosilicate glass can be distinguished from
borosilicate glass by density, refractive index,
or both.
43
Quantitative Properties of Glass
44
The Wave of the Future Laser Ablation
  • Laser Ablation Inductively Coupled Plasma Mass
    Spectrometry (LA-ICP-MS)
  • Laser burns off a microscopic sample
  • Elements are ionized by plasma
  • Detects 46 trace elements and their isotopes
    simultaneously in glass at lt 1 ppb

45
  • LA-ICP-MS (Laser Ablation Inductively Coupled
    Plasma Mass Spectrometry) is a powerful
    analytical technology that enables highly
    sensitive elemental and isotopic analysis to be
    performed directly on solid samples.
  • powerful analytical technology that enables
    highly sensitive elemental and isotopic analysis
    to be performed directly on solid samples.

46
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