Answer these Q’s
•
What is glass made of?
•
How many types of glass can you name?
•
Name as many objects as you can that are composed
of glass.
• How can glass be transferred?
•
How do you think forensic scientist use glass in an
investigation?
Forensic Analysis of Glass
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
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.
• Physical properties: hard, elastic, brittle, nonconductor of electricity, density, refractive index, etc.
• Chemical: resistant to all but fluorine and very strong
bases.
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
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.
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.
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.
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
Exceptions to the Three R Rule
Tempered glass
“dices” without forming ridges
Very small windows held tightly in frame
can’t bend or bulge appreciably
Windows broken by heat or explosion
no “point of impact”
curved, smooth edges at break points
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 bullet’s velocity increases, the central hole
becomes smaller, cracking patterns become
simpler, and the exit hole becomes wider than the
entrance hole.
Rolled Glass
Float Glass
Float Glass Process
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.
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.
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.
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
dependent on sample mass).
• Need to measure very precisely in parts per
hundred or thousand or better.
Density
Type of Glass
Density
window
2.46-2.49
headlight
2.47-2.63
pyrex
2.23-2.36
lead glass
2.9-5.9
porcelain
2.3-2.5
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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.
Flotation Method
precise and rapid method for comparing glass
densities.
1. a glass particle is immersed in a liquid.
1. The density of the liquid is carefully adjusted
–
–
by the addition of small amounts of an appropriate
liquid
until the glass chip remains suspended in the liquid
medium.
1. At this point, the glass will have the same
density as the liquid medium
–
can be compared to other relevant pieces of glass
which will remain suspended, sink, or float.
• A shattered glass jar was found at a burglary
scene and was determined to be made of sodalime glass (density = 2.47 g/mL). A glass
fragment recovered from a suspect’s home had
a mass of 4.652 g. When the glass fragment
was placed into a graduated cylinder with
20.00mL of water, the level of the water rose to
21.53 mL.
• Does the glass fragment link the suspect to the
crime scene?
Determination of
Refractive Index
 Immersion method—lower fragments into liquids
whose refractive index is different.
 Match point—when the refractive index of the
glass is equal to that of the liquid
 Becke line—a halo-like shadow that appears
around an object immersed in a liquid. It
disappears when the refractive index of the
liquid matches the refractive index of the glass
fragment (the match point)
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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 match point, temperature is noted and refractive index of
the liquid is read from a calibration chart
• 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.
The Becke Line
The Becke line is a “halo” that can be seen on the
inside of the glass on the left, indicating that the
glass has a higher refractive index than the liquid
medium. The Becke line as seen on the right is
outside of the glass, indicating just the opposite.
Kendall/Hunt Publishing Company
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Refractive Index By Immersion
Becke Lines
Glass has higher refractive index-note white line inside
•
Glass has lower refractive index-note white line outside
Refractive Index
Liquid
RI
Glass
RI
Water
1.333
Vitreous silica
1.458
Olive oil
1.467
Headlight
1.47-1.49
Glycerin
1.473
Window
1.51-1.52
Castor oil
1.82
Bottle
1.51-1.52
Clove oil
1.543
Optical
1.52-1.53
Bromobenzene
1.560
Quartz
1.544-1.553
Bromoform
1.597
Lead
1.56-1.61
Cinnamon oil
1.619
Diamond
2.419
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• refractive index = velocity of light in vacuum
velocity of light in medium
Snell’s Law
N=1.52
N=1.33
The higher the n, the more the light bends
Fracture Patterns
 Radial fracture lines radiate out from the
origin of the impact; they begin on the
opposite side of the force
 Concentric fracture lines are circular lines
around the point of impact; they begin on
the same side as the force
 3R rule—radial cracks form a right angle
on the reverse side of the force.
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Glass as Evidence
 Class characteristics; physical and
chemical properties such as refractive
index, density, color, chemical
composition
 Individual characteristics; if the fragments
can fit together like pieces of a puzzle, the
source can be considered unique
Kendall/Hunt Publishing Company
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Considerations
for Collection
 The collector must consider that fragments within a
questioned sample may have multiple origins. If
possible, the collector should attempt an initial
separation based on physical properties.
 The collector must consider the possibility there may be
a physical match to a known sample (e.g., a piece of
glass to a fractured vehicle headlamp). When an attempt
to make a physical match is made at the site of
collection, the collector should take precautions to avoid
mixing of the known and questioned samples.
 Any glass samples collected should be documented,
marked (if necessary), packaged, and labeled.
Collecting the Sample
 The glass sample should consist of the largest amount that can be
practically collected from each broken object and packaged separately.
The sample should be removed from the structure (e.g., window frame,
light assembly). The inside and outside surfaces of the known sample
should be labeled if a determination of direction of breakage or
reconstruction of the pane is desired.
 When multiple broken glass sources are identified, it is necessary to
sample all sources.
 A sample should be collected from various locations throughout the
broken portion of the object in order to be as representative as possible.
 The sample should be collected with consideration being given to the
presence of other types of evidence on that sample (e.g., fibers, blood).
Important Physical Properties
• Light waves travel in air at a constant velocity
until they penetrate another medium, such as
glass or water, at which point they are
suddenly slowed, causing the rays to bend.
• The bending of light waves because of a
change in velocity is called refraction.
• Refractive index is the ratio of the velocity of
light in a vacuum to that in the medium under
examination.
Important Physical Properties
• 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 and will serve to characterize a
substance.
• Refractive index of glass varies with small
changes in composition or by how it is
manufactured.
Why use RI?
• 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
Problems with Refractive Index
1964-1979
1980-1997
A rough statistical
estimate of the
likelihood of finding
glass of that refractive
before 1970
Manufacturing
changed in late
1970’s making glass
more uniform
Density vs RI
• The glass fragment must be scrupulously clean
and free of inclusions
• accurate density measurements require a
sample that is 2-3 mm in diameter
• refractive index determination are more rapid
and can be performed with smaller samples so
most laboratories choose to preform
refractive index first.
Optical Properties of Glass
Make side-by-side comparisons using similarsized fragments.
Place samples on a white surface using natural
light.
Use both fluorescent and incandescent light to
determine the glass’s color.
Visual color analysis is very subjective.
Dyes and pigments can be almost impossible to
extract.
Learning Check
1. Which unique chemical component would be
found in each type of glass shown below?
(beaker)
(windshield)
(crystal)
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 (>50 mg) can be analyzed
• Atomic Absorption Spectroscopy
– You must first know which elements are present
– Can analyze ppm levels of elements present
Glass Composition Analysis
• most methods of glass composition analysis
are destructive
• require glass samples larger than those found
in forensic casework
• many of the techniques used require
expensive equipment
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 < 1 ppb
• 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.
Use the table to determine if lead borosilicate glass can
be distinguished from borosilicate glass by density,
refractive index, or both.