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.
Fractures by Projectiles
Which side was the bullet fired from?
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.
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?
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.
Learning Check
In the figure below & left, which impact occurred
first?
In the figure above & right, from which side did the
impact occur?
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.
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
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.
Nonoptical Physical Properties of
Glass
• Surface striations and markings
– Rollers leave parallel ream marks on sheet glass
– Markings may indicate the glass’s orientation
when pieces are missing
– Surface scratches, etchings, and other markings
may also be used to individualize evidence
• Other Properties
– Hardness=5-6 on Mohs scale; use a scratch test.
– Determinations of curvature can distinguish flat
glass from container, decorative, or ophthalmic.
Forensic Analysis of Glass
Quantitative Properties of Glass
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.
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.
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.
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.
Snell’s Law
N=1.52
N=1.33
The higher the n, the more the light bends
Refractive Index By Immersion
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.
Becke Lines
Glass has higher refractive index-note white line inside
• Glass has lower refractive index-note white line
outside
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
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
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.
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
Learning Check
1. 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.
Quantitative Properties of Glass
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