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Glass Fragment Identification
Forensic Materials Science
• Scientific Working Group for Materials
Science
• SWGMAT provides Guidelines/Best Practices
that have application to Fibers, Glass, Paint,
Hair, and Tape.
• SWGMAT also develops standards to
accredit laboratories and expert witnesses.
• http://swgmat.org/
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Evidence – Class vs. Individual
• Class evidence shares physical
characteristics with a group of objects or
individuals.
• Individual evidence originates with a single
person or source.
• Some categories of physical evidence can
be only be class evidence while others can
be either class or individual evidence,
including glass.
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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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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.
• 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.
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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.
• 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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Measuring Refractive Index
• The Refractive Index (RI) of a substance is a
measure of the speed at which light travels (v)
through that medium.
• 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.
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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
Normal line
n2
n1
θ1
Snell’s Law
N=1.52
N=1.33
The higher the n, the more the light bends
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2. Theory of Refraction, (Angular)
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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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Becke line:
nglass >nmedium
nglass < nmedium
nmedium = 1.525
nglass = 1.60
nmedium = 1.525
nglass = 1.34
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Becke Line
Kendall/Hunt Publishing Company
22
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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Refractive index of household
liquids
Refractive index at 20°C
• Baby oil: 1.45
• Canola oil: 1.465-1.467
• Olive oil: 1.467-1.4705
• Soybean oil: 1.470-1.472
• Grape Seed Oil: 1.471-1.478,
• Castor Oil: 1.4750 - 1.4850
• Corn Oil: 1.4735 - 1.4785
• Xylene: 1.505
• Clove Oil: 1.543
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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.
Measuring Refractive Index
• Glass Refractive Index Measurement (GRIM)
system employs a phase-contrast microscope
with a temperature-controlled hot stage that
allows for precise heating and cooling of the
sample being analyzed.
• The GRIM method uses a microscope slide
containing glass fragments placed in special
heatable immersion oil. This slide is placed on
the heated stage and the microscope is focused
and aligned.
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Measuring Refractive Index
• The temperature of the microscope stage is set so the
RI of the oil is higher than that of the glass sample.
• The temperature is then automatically lowered and the
contrast between the glass shard and the oil is
monitored. The match point (temperature of minimum
contrast) is recorded.
• This is then repeated by gradually heating the slide and
this match point is also recorded.
• The two numbers are averaged and this number can be
compared between two fragments of glass to determine
if they match or are differ.
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X-ray Diffraction (XRD)
• In XRD analysis a glass sample is bombarded
with X-rays, and the atomic composition of the
glass is determined through detection of the
characteristic scattering of those X-rays by the
electron clouds of the individual atoms
comprising the sample.
• The scattering pattern is then analyzed by
specialized computer software to determine
these atoms.
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Elemental Analysis
• Laser Ablation- Inductively Coupled Plasma-Mass
Spectrometry (LA-ICP-MS)
• A small portion of the sample is removed from the
surface by laser irradiation.
• This removed sample portion is then converted to atomic
ions (charged atomic particles).
• An instrument called a mass spectrometer is then used
to separate and analyze the characteristic ions produced
based upon the mass-to-charge ratio of those ions.
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Elemental Analysis
• The elemental analysis of the evidence glass is then
compared to the known glass from a crime scene
(such as a broken window).
• If no known glass exists, the evidence can be
compared with other types of glass (headlamp,
window, drinking glasses).
• It is also possible that the company that made the
glass or even the geographical location where the
glass was made can be determined from the
composition.
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