2. CONTENTS
• Glass definition
• Types of glass
• General properties of glass
• Scope of glass examination
• Types of cracks and fractures in glass samples and
their interpretation
• Forensic examination of glass samples
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3. SCOPES
• Glass, as a physical clue, is frequently
encountered in various crimes; such as
burglary, road accidents, murder, sexual
assaults, shooting incidents, arson and
vandalism.
• The chips of broken glass window may be
lodged in suspect’s shoes or garments during
the act of burglary/crime; particles of
headlight glass found at the crime scene may
offer clues that confirm the identity of a
suspected vehicle; glass may also be found on
the clothing of an alleged assailant, where a
bottle is used as weapon.
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4. CONTINUE…
• Whenever there is violence, bottles, window
pane glass, mirrors, eye glasses and other
glass objects can be accidently scattered and
fragments of these can also adhere to the
criminal’s clothing or shoes.
• Thus, glass forms one of the evidentiary
materials in many criminal investigations.
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5. •Glass can be found in most localities. It is produced in a
wide variety of forms and compositions.
•It can occur as evidence when it is broken during the
commission of a crime.
•Broken glass fragments ranging in size from large pieces
to tiny shards may be transferred to and retained by
nearby persons or objects.
•The mere presence of fragments of glass on the clothing
of an alleged burglar in a case involving entry through a
broken window may be significant evidence if fragments
are found.
•The significance of such evidence will be enhanced if the
fragments are determined to be indistinguishable in all
measured properties from the broken window.
GENERAL
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7. •The closer something is to the breaking glass, the
more likely it is to have glass fragments transferred to
it.The number of fragments transferred decreases with
distance from the break (Pounds and Smalldon 1978).
•The person breaking a window will have more glass
on him or her than a bystander, and the more blows
required to break out the glass, the more glass that
will be transferred (Allen et al. 1998b).
•The number of glass fragments generated by a
break is independent of the size and thickness of the
window but increases with greater damage to the glass
(Locke and Unikowski 1992).
NUMBER OF GLASS FRAGMENTS THAT CAN BE
TRANSFERRED IS CONTROLLED BY A NUMBER OF
FACTORS:
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8. RECOVERY OF GLASS FRAGMENTS FROM
CLOTHES BY A FORENSIC EXAMINER DEPENDS
UPON ADDITIONAL FACTORS:
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•Less glass is retained on slick clothing, such as
nylon jackets, than on rough clothing, such as wool
sweaters.
•Wet clothing retains more glass than dry clothing.
•Glass fragments fall off clothing over time, and
larger pieces fall off before smaller pieces.
•Glass falls off faster if the person wearing the
clothing is active.
9. GLASS
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Glass is technically defined as
“The inorganic product of fusion which has cooled
to a rigid condition without crystallizing”.
In contrast to crystalline solids, which have an
ordered internal arrangement of atoms, the
internal structure of glass consists of a network
of atoms lacking long-range symmetry; This
condition is referred to as the vitreous, or
glassy, state
10. • An extended, 3D network of atoms which
lacks the repeated, orderly arrangement
typical of crystalline materials.
• Glass is made by heating silica sand with soda
and lime--and sometimes other materials--to a
molten mass, then cooling it so quickly that
there is no time for crystals to form in the glass.
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11. •Even though glass is a
liquid, to us it appears solid
it is not viscous like other
liquids, but it looks rigid.
•The viscosity is such a high
value that the amorphous
material acts like a solid.
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12. COMPONENTS
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Formers
– forms the glassy, non-crystalline structure
fluxes
– improve melting properties but impart low
chemical resistance
• typically alkali or alkaline earth oxides
modifiers (stabilizers or intermediates)
– a material that improves stability
• typically oxides of Ca, Al, or Zn
14. TYPES OF GLASS
A)On the basis of manufacturing process:
Ordinary sheet glass
Float glass(plate)
B) On the basis of composition:
Oxide glass
Non oxide glass
C)On the basis of market application:
Commercial/soda lime glass
Lead glass
Borosilicate glass
Laminated glass
Tempered glass
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19. GLASS FRACTURE
• When force is applied on any surface of glass
it bends but since the elasticity of glass is
limited ultimately, it gets fractured after the
threshold force application.
• An investigator often has to decide wheather a
pane of glass was broken from outside or from
inside, wheather it was broken with a bullet or
with a blunt object.
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20. IMPACT OF FORCE ON GLASS
e
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Impact causes a pane
of glass to bulge
– Side opposite the
impact will stretch
more & rupture first
Radial cracks are
rapidly propagated in
short segments from
the point of impact
21. Elasticity permits bending until
radial cracks form on the opposite
side of the force
Continued force places tension
on the front surface (force side),
forming the concentric cracks
RADIAL AND CONCENTRIC CRACKS
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22. TYPES OF CRACKS
1) Radial cracks:
When an object has been thrown through a
glass pane, a fracture forming a pattern
somewhat like a spider-web will be seen.
The cracks will appear radiating outwards
from the point of impact making a star shaped
fracture known as radial fracture.
The radial fracture originates on the surface
opposite to that on which force was applied.
This type of fracture is always the first to
appear on glass.
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24. 4 R RULE
Ridges on Radial
cracks are at Right
angles to the Reverse
side of impact.
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25. TYPES CONTINUE..
2)Concentric cracks:
A series of broken circles originate on the surface,
on which force is being applied around the point of
impact.
These are the secondary fractures as they always
appear after radial fractures.
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26. TYPES CONTINUE…
3)Cone fractures:
When a high projectile(like bullet), penetrates the
glass, it makes a round crater shaped hole.
It is surrounded by radial & concentric cracks.
The hole is usually wider on the exit side and gives
appearance like a cone.
Thus the narrower side of a cone fracture indicates
the direction from which the bullet entered.
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29. SIGNIFICANCE AND USE OF STUDY OF GLASS
FRACTURES
• Fracture patterns are unique;
Pieces from the broken glass pane or hole often show
marks that are characteristics of the type of injury and
direction of force.
If correctly interpreted, these findings gives useful
information about the object used for breaking and
velocity of breaking object.
Fracture examinations can provide information as to the
direction of the breaking force and the sequence of
multiple impacts.
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30. DETERMINATION OF THE DIRECTION OF FORCE
IN BREAKING A WINDOW PANE:
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-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
33. POINTS TO REMEMBER WHILE ANALYZING
GLASS FRACTURES
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✓Radial cracks are formed first, commencing on the side
of the glass opposite to the destructive force
✓Concentric cracks occur afterward, starting on the
same side as the force
✓As the velocity of the penetrating projectile decrease,
the irregularity of the shape of the hole and of its
surrounding cracks increase
✓Fracture always terminates at the existing line fracture
✓Stress marks occur on the edge of a radial glass
fracture.
34. CONTINUE…
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✓Stress marks run perpendicular to one edge
and parallel to the other edge of glass.
✓Stress’ perpendicular edge is always located
opposite from which the force of impact
occurred.
✓Concentric fractures, the perpendicular end
always faces the surface on which the force
originated.
✓Radial cracks form a Right angle on the
Reverse side of the force (4 R rule).
35. •If the pieces of broken glass can be made to fit together in
the manner of a jig-saw puzzle, positive association can be
made.
•Even glass fragments as small as the head of a pin can be
compared. However, even if unusual properties are present,
only a strong indication of common origin can be given, not
an absolute identification.
•If a window has been struck with a blunt instrument such
as a rock, stick or fist, it is possible to determine the side of
impact and the nature of the force involved.
•If a window has been penetrated by a bullet, it is possible
to determine the direction from which it was fired.
•If two or more bullet holes are in close proximity, it is
possible to determine the sequence of firing
RESULTS POSSIBLE FROM LABORATORY
EXAMINATION OF GLASS
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36. OBJECTIVES
To be able to identify, classify & individualize the
piece/pieces of glass fragments found at
suspect/victim’s clothing or at crime scene and to
use it, if possible, as an element to aid
reconstruction of events or as an evidence to
prove/disprove…..
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37. # Visual Inspection of Known/ Questioned for
Fracture Matches
# Comparison of Glass:
•Physical Properties
•Optical Properties
•Chemical Properties
# Classification of Glass into End Use
Category
# Discrimination between glass samples
# Interpretation and Value of Results
GLASS ANALYSIS
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38. PROBLEM TO BE SOLVED???
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Classification:
The ability to use some measured characteristics of a
questioned object to place it into a product use class.
Discrimination:
The ability to distinguish between two or more objects
within the same product use class.
40. FORENSIC GLASS EXAMINATION
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•A forensic glass analysis is typically a comparison
of two or more glass fragments in an attempt to
determine if they originated from different sources.
•These analyses require the determination of class
characteristics that may associate objects with a
group of similar objects such as containers, but
never to a single object.
•Only physically matching of two or more broken
glass fragments allows for their association with
each other to the exclusion of all other sources .
41. POINTS TO REMEMBER….
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•Every analytical test available is not always
performed on each specimen.
•The aim of a comparative glass analysis is to exclude
possible sources. When a difference is detected, no
further comparison is necessary.
•It is not always possible to assess every potential
point of comparison in each glass specimen.
•A glass fragment may be too small to be analyzed
with reproducible results even when a feature is
preserved.
•Consequently, the actual tests performed on a set of
specimens depend on the size and shape of the glass
fragment, as well as analytical considerations.
42. PLASTIC IDENTIFICATION AND SN SURFACE
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Plastic can be eliminated by
testing for indentation by a needle
point.
Fluorescence upon short wave
(254nm) illumination of an original
surface can detect the Sn
contamination on one side of float
glass.
43. TABLE SALT & GLASS SAMPLE
Table salt can be differentiated by
their shape; they are crystallized
particles and thus have a regular
and ordered shape unlike glass
which is amorphous and has an
irregular shape.
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44. PHYSICAL MATCHING
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This is most conclusive proof of source
correspondence, since no two fractures will
ever be identical over any appreciable
length.
A complimentary lateral fit along the
broken edges over a length of quarter inch
(1/4) or more establishes that the two glass
fragments were continuous before
breakage.
45. INITIAL EXAMINATION
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The physical properties used
for comparison include glass
color, fluorescence,
thickness, surface features,
and curvature, observance
of conchoidal fracture,
determination of hardness,
reaction to a hotpoint,
microscopy.
48. COLOR
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Materials can be added to the batch to
produce glass in practically any color.
Impurities present in the raw materials
used to produce glass can impart
unintentional color.
Differences in color represent a change
in glass chemistry and can be used to
differentiate specimens.
49. COLOR CONTINUE..
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Typically not possible to reliably
perform colorimetry on glass fragments
in forensic casework due to too small
size and too low color density of
samples.
Color assessment is performed
visually against a white background in
natural light with the particle on edge.
Side-by-side comparison should be
used with similarly sized particles.
50. DETECTION OF CURVATURE
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An Interferometer can be used to detect
the most minimal curvature on the glass
surface.
Curvature indicates possible sources:
•windshield
•containers
•other non-flat glass source
51. CURVATURE:
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A spherometer is used to measure the radius
of curvature of the glass fragments having curved surface.
The radius of curvature of the fragment is calculated
using the formulae.
R= (l2/6h)+(h/2)
Where
1 = the mean distance between the legs of
the spherometer.
h = height of the curved surface
52. FLUORESCENCE
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•fluorescence can be used as a basis to
differentiate glass specimens.
•The glass surface that was in contact with the tin
bath during the manufacturing procedure will
fluoresce when exposed to short-wave (~254 nm)
ultraviolet light.
•Fluorescence examinations can also be
performed using fluorescence spectroscopy on
specimens as small as 0.05 mm2 .
•Fluorescence on a glass surface will be detected
only if the surface that will fluoresce is preserved,
collected, and analyzed.
54. DENSITY
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The ratio of the mass of an object
to the volume occupied by that object
– g/cm3 (solids); g/mL (liquids)
d = m/V
Densities of solids & liquids are
often
compared to the density of water
– sink or float
Varies with temperature
55. DENSITIES OF VARIOUS GLASSES
AND RELATED MATERIALS:
Window glass, flat- 2.47 to 2.56
Head light glass- 2.47 to 2.63
Mica- 2.6 to 3.2
Quartz- 2.65
Glass, flint- 2.9 to 5.9
Diamond- 3.01 to 3.52
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56. GLASS DENSITY CAN BE MEASURED BY:
Displacement method
Floatation method
Density gradient column method
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58. REFRACTIVE INDEX (ND)
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•The ratio of the velocity of light in a
vacuum to the velocity of light in a given
Medium
– ND (water) = 1.333
i.e light travels 1.333 time faster in
vacuum than in water
•An intensive property
•Varies with temperature and the light
frequency
59. R I OF COMMONLY ENCOUNTERED GLASSES
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Automobile head light
glass
Bottles
Window glass
Opthalmic glass
1.47 – 1.49
1.51 – 1.52
1.51 – 1.52
1.52 – 1.53
60. REFRACTIVE INDEX IS THE MOST COMMONLY
MEASURED PROPERTY IN THE FORENSIC
EXAMINATION OF GLASS FRAGMENTS BECAUSE:
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•Precise refractive indices can be measured
rapidly on the small fragments typically found in
casework.
•It can aid in the characterization of glass.
•It provides good discrimination potential.
61. METHODS TO CALCULATE REFRECTIVE
INDICES OF QUESTIONED GLASS SAMPLES
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Immersion Methods
Becke line method,
Dispersion staining method
Emmons Double Variation method
Automated Method
62. DENSITY MEASUREMENTS ARE PERFORMED LESS
FREQUENTLY THAN REFRACTIVE INDEX
DETERMINATIONS BECAUSE:
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•The glass fragment must be scrupulously clean
and free of inclusions.
•Accurate density measurements require a
sample that is two to three millimeters in
diameter.
•density measurements required the use of
hazardous liquids, such as bromoform.
63. BECKÉ LINE METHOD (1892)
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•When the objective of the
microscope is raised (focus
up), a bright line moves into
the direction of the material
of higher R.I.
•Once the line disappears or
doesn’t move,the R.I. of the
oil can be measured by a
refractometer.
•The Becké line is best
observed with contrast
microscopy.
64. OIL IMMERSION AT THE MATCH
TEMPERATURE
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65. ELEMENTAL ANALYSIS
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Glass composition analysis can be used to
differentiate between:
glasses made by different manufacturers,
glasses from different production lines of the same
manufacturer,
glasses made over a period of time in a single
production line.
66. GLASS COMPOSITION ANALYSIS IS PERFORMED
INFREQUENTLY BECAUSE:
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•Most methods of glass composition analysis are
destructive.
•Most methods require glass samples larger than those
routinely encountered in forensic casework.
•Most of the instrumentation used to measure glass
composition is expensive to purchase and maintain, and
much of the instrumentation has few other applications.
•Because of the complexity of the calculations, Bayesian
statistical analysis including compositional data is
extremely difficult to apply.
67. DESPITE THESE DRAWBACKS, CHEMICAL ANALYSIS
REMAINS THE BEST MEANS FOR DIFFERENTIATING
GLASS SPECIMENS.
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