Download Glass Fragment Identification

Glass Fragment Identification
Glass Evidence: Class or Individual?
• Individual: Broken glass pieces can be fitted
together like a puzzle. A specific fragment
can be uniquely placed at a crime scene.
• Class: Small fragments of glass can transfer
to a victim or perpetrator of a crime or their
vehicle.
• After a hit and run accident, glass fragments
consistent with a vehicle class can be
identified, even if the specific vehicle is not
known.
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Glass Chemistry
• Glass is made by heating silica (sand) with soda
ash (sodium oxide, Na2O) and lime (calcium
oxide, CaO) to a molten mass, then cooling it so
quickly that large crystals do not form.
• Glass is processed by rolling it into sheets or by
blowing or molding to desired shapes.
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Specialty Glass
• Metal oxides are added to make colored
glass.
• Frosted glass has surfaces treated with acid
or a plastic film.
• Tempered glass is stronger than normal
glass. It is made by a rapid heating and
cooling process.
– Pyrex® baking dishes
– Corelle® dinnerware.
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Polymer Glass
• Polymer glasses are strong transparent
plastics which can replace silica glass in
eyeglasses, drinking glasses, windows or
vehicle tail lights.
• Polymer glass is molded from several
different plastics
– Acrylic
– Polycarbonate
– Polyethylene terephthalate
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Windshield Glass
• Car windshields are made with laminated safety
glass.
• Safety glass has a layer of plastic between two
pieces of ordinary glass.
• Windshields are placed in cars using gaskets to
keep them rigidly in place.
• Modern windshields are designed not to fall out of
the vehicle even if they shatter.
• The laminated glass can break if an object is
thrust into the windshield.
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Shattered Windshield
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Glass in Car Accidents
• As a result of hit-and-run or other vehicle
accidents, headlights, tail lights and other
lamps are frequently broken.
• Windshield glass, tempered glass, and
mirrors break less commonly, but are also
possible.
• Both the scene of the accident and the
clothing of the victim can be sources of glass
fragments.
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Collection of Glass at Crime Scene
• Any person standing close to glass when it is
broken can pick up fragments of the broken
glass, particularly on clothing.
• Glass fragments can travel forward and
backward from the direction of the breaking
force.
• If a window was broken in a crime, clothing
worn by the suspect should be collected and
examined.
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Collection of Glass at Crime Scene
• Every effort should be made to collect all
the glass found if any possibility exists that
glass fragments may be pieced together.
• When the pieces are too small to
individually fit, the crime scene investigator
must submit all glass evidence found in
the possessions or clothing of the suspect
along with a representative sample of
broken glass obtained at the crime scene.
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Collection of Glass at Crime Scene
• Glass fragments should be packaged in
solid containers (boxes or vials) to avoid
additional breakage.
• Avoid packaging in cotton. Plastic wrap
can cause static electricity and make
glass fragments difficult to handle. Glass
can cut through paper envelopes, so
handle carefully.
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Collection of Glass at Crime Scene
• If the suspect’s shoes or clothing are to be
checked for glass fragments, they should
be individually wrapped in paper and
transferred to the laboratory.
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Analyzing Cracks
• Window glass penetrated by a projectile
(e.g. bullet or stone) can show cracks
which radiate outward (radial fractures)
and encircle the hole (concentric
fractures).
• Forensic scientists analyze these radial
and concentric fracture patterns to
determine the direction of the projectile
that impacted the glass.
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Analyzing Cracks
• Bullets and other high-velocity projectiles often
leave a hole that is wider at the exit side (like a
cone). This is important in determining the
direction of impact.
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Analyzing Cracks
• Direction of impact follows the “3 R’s” Rule:
Radial cracks form a
Right angle on the
Reverse side of the force.
• If several bullets break a sheet of glass, it can be
important to determine the sequence of impacts.
It is possible to determine the sequence
because a fracture always terminates at an
existing line of fracture.
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Analyzing Cracks
• Intense heat can also cause thermal
fractures.
• In non-tempered glass, a typical heat crack is
curved, has a smooth edge, and has no
indication of the point of origin of the crack.
• Hot glass that is too quickly chilled can also
have thermal fractures.
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Next Step…
• Read and OUTLINE the Intro Glass
Examinations article
• Make sure you turn in your pre-lab/post
lab questions from yesterday!
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Forensic Glass Analysis
• Typically two or more glass fragments are compared
to determine if they originated from different
sources.
• Unless there is an exact fit between two pieces of
broken glass, it isn’t possible to prove the glass
pieces came from the same source.
• Once a single test shows two pieces don’t match,
further tests are unnecessary.
• Evidence can show the fragments are consistent
with a match to a class of glass.
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Forensic Glass Analysis
• Less frequently, the forensic examiner
determines the end use of the glass or its
manufacture (e.g. a lightbulb, eyeglasses).
• The physical properties used for comparison
include glass color, fluorescence, thickness,
surface features, curvature and optical
properties.
• These can be assessed using various rapid
and nondestructive tests.
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Forensic Glass Analysis
• It is critical to initially determine if the
fragments are silica or polymer glass.
• Silica glass is much harder and has a
higher melting point than polymer glass.
• Some glass fragments may be too small
for analysis.
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Analyzing Glass Fragments
• Forensic glass comparison requires the
scientist to identify and measure
properties that will match one glass
fragment with another while minimizing or
eliminating other glass sources.
• Forensic scientists primarily examine two
physical properties:
1. Density
2. Refractive index
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Comparing Glass Densities
• Density is mass per unit volume (g/cm3).
• When two samples have the same volume, their
weights will differ if the chemical elements that
make up the material are different.
• Higher atomic numbers (# of protons) tend to
mean heavier atoms for those elements.
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Comparing Glass Densities
• One cubic centimeter of silver (atomic number
47) weighs less than one cubic centimeter of
gold (atomic number 79).
• Glass with different elemental compositions will
have different weights.
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Comparing Glass Densities
• The flotation method is a precise and rapid
method for comparing glass densities.
• A glass fragment is immersed in a series of
liquids of varying densities. The glass chip will
neither sink nor float in the liquid medium of the
same density.
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Comparing Glass Densities
• If the glass sinks, it is more dense than the liquid
and if floats it is less dense.
• By comparison, two glass fragments can be
shown to share the same density or have
different densities, eliminating a match. The
density of a suspending liquid mixture can be
accurately determined using a density meter.
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Measuring Refractive Index
• The Refractive Index (RI) of a substance is a
measure of the speed at which light travels (v)
through that medium.
• RI is defined as the ratio of the speed of light in
a vacuum to the speed of light traveling through
the medium being analyzed.
• RI is denoted with the unitless variable n.
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Measuring Refractive Index
• When light travels through two media with
differing RIs, the light becomes refracted, or
bent.
• This occurs because when the speed of the
wave of light changes, the direction of that wave
also changes.
• Snell's law is used to determine the direction of
light rays through refractive media with varying
RIs.
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Measuring Refractive Index
The Angle of
Incidence is the angle
θ from an imaginary
line perpendicular to a
surface, called the
normal line (dashed).
θ
Measuring Refractive Index
• As light passes the border between media, depending
upon the relative RIs of the two media, light will either be
refracted to a lesser angle, or a greater one. These
angles are measured with respect to the normal line.
• In the case of light traveling from air into water, light is
refracted towards the normal line, because the light is
slowed down in water; light traveling from water to air
refracts away from the normal line, since light speeds up.
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Measuring Refractive Index
• Snell's Law states:
For a given pair of materials,
sine of angle of incidence θ (in material 1)
sine of angle of refraction θ (in material 2)
is equal to v1 / v2, and equal to n2 / n1.
• Sine (sin) is a trigonometric function. It is the
ratio of the length of the side opposite an angle
in a triangle to the length of the hypotenuse.
• A scientific calculator will easily convert an angle
into its sine.
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Measuring Refractive Index
θ2
n2
n1
θ1
Measuring Refractive Index
The Becke line is a
bright halo near the
border of a particle that
is immersed in a liquid
of a different refractive
index.
When the two RI are
the same (the match
point) the Becke line
disappears and
minimum contrast
between liquid and
particle is observed.
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Measuring Refractive Index
A glass particle can
be sequentially
immersed in a series
of different liquid
media whose
refractive indices are
known.
When the refractive
index of the liquid
matches that of the
glass particle, the
Becke line
disappears.
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Refractive Indices for Common Items
Vehicle Headlight
1.47 – 1.49
Window
1.49 – 1.51
Bottle
1.51 – 1.52
Contact Lens
1.52 – 1.53
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Next Steps…
• Read Glass Fragment Identification
background and Lab Procedure
– Answer Pre-lab questions
• Get started on LAB 1
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