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Chapter 14 Glass Evidence
By the end of this chapter you will be able to:
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Explain how glass is formed
List some of the characteristics of glass
Provide examples of different types of glass
Calculate the density of glass
Use the refractive index to identify different types of
glass
Describe how glass fractures
Analyze glass fracture patterns
All Rights Reserved South-Western / Cengage Learning © 2012, 2009
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Forensic Science: Fundamentals & Investigations, Chapter 14
Introduction and History of Glass
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Humans have used obsidian (volcanic glass)
since prehistoric times – cutting tools,
arrowheads
Egypt circa 2500 B.C.—The earliest known
human-made glass objects (beads)
1st Century B.C.—glass blowing begins
13th Century—specialized glass production was
an art, a science, and a state secret in the
republic of Venice
Forensic Science: Fundamentals & Investigations, Chapter 14
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Forensic Science: Fundamentals & Investigations, Chapter 14
Introduction and History of Glass
14th Century—glass-making spreads through
Europe
The industrial revolution applies mass
production to many types of glass
Analysis of glass found at a crime scene can
yield trace evidence
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Fibers or blood on glass shards
Sequence and direction of bullets
Glass particles may be transferred to suspect’s shoes
or clothing
Forensic Science: Fundamentals & Investigations, Chapter 14
What Is Glass?
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Materials used to create (soda-lime) glass:
• Silicon dioxide (SiO2) – silica found in sand
• Sodium oxide (Na20) – soda
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Calcium oxide (CaO) – lime
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Reduces melting point of silica
Prevents glass from being soluble in water
Once cooled, glass can be polished, ground,
or cut
Forensic Science: Fundamentals & Investigations, Chapter 14
What Is Glass?
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Crystalline solids have a regular atomic structure
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Glass is an amorphous solid and so has an
irregular atomic structure
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Therefore, glass breaks in a variety of fracture
patterns
Forensic Science: Fundamentals & Investigations, Chapter 14
Types of Glass
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Many types of glass
Soda-lime glass most common
Leaded glass (crystal) uses lead oxide instead of
calcium oxide
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Denser glass, sparkles as light waves bend
Pyrex contains compounds that allows it to withstand
a wide range of temperatures
Adding metal oxides yields different colors
Different densities
Refractive indexes
These characteristics allow comparisons
Forensic Science: Fundamentals & Investigations, Chapter 14
Density
The formula for calculating density is:
D=m/V
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m = mass, measured by a balance beam
device
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V = volume, place the glass fragment into a
beaker filled with water and measure the
overflow
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D = density, divide the mass (in grams) by
the volume (in milliliters)
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Forensic Science: Fundamentals & Investigations, Chapter 14
Density—Common Examples
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Type of Glass
Density (g/ml)
Bottle glass
2.50
Window glass
2.53
Lead crystal
2.98-3.01
Pyrex
2.27
Tempered (auto)
2.98
Flint
3.70
Crown
2.50
Forensic Science: Fundamentals & Investigations, Chapter 14
Refractive Index
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When a beam of light moves from one
medium into another:
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The speed changes
The direction bends
Refractive Index—a tool used to study how
light bends as it passes from one substance
to another
Normal line is perpendicular to the glass
surface
Forensic Science: Fundamentals & Investigations, Chapter 14
Refractive Index
When a beam of light
moves from less dense
medium (air) into a more
dense medium (water):
• Its speed slows, and
• Bends light toward
the normal line
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Forensic Science: Fundamentals & Investigations, Chapter 14
Refractive Index
When a beam of light
moves from a more dense
medium (glass) into a less
denser medium (air):
• Its speed increases
• And bends light away
from the normal line
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Forensic Science: Fundamentals & Investigations, Chapter 14
Application of Refractive Index
to Forensics
Submersion
method—used
when glass
fragments found
at the crime
scene are small
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Forensic Science: Fundamentals & Investigations, Chapter 14
Application of Refractive Index
to Forensics
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Place the glass
fragment into different
liquids of known
refractive indexes
The glass fragment will
seem to disappear
when placed in a liquid
of the same refractive
index
Forensic Science: Fundamentals & Investigations, Chapter 14
Using refractive index as evidence
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Car dealers and glass manufacturers have
databases containing the refractive index of
their products.
Forensic Science: Fundamentals & Investigations, Chapter 14
Becke Line
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halo-like effect appearing at the edges of a glass
fragment when the refractive index of the glass
and liquid are different
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Put glass fragment on a slide in a drop of liquid with
known RI
Lower microscope stage
• If the line is inside the glass perimeter, the glass index
is higher than the index of the liquid
• If the line is outside the glass perimeter, the glass
index is lower
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Forensic Science: Fundamentals & Investigations, Chapter 14
Fracture Patterns in
Broken Glass
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Being an amorphous solid, glass will not
break into regular pieces with straight line
fractures
Fracture patterns provide clues about the
direction, rate, and sequence of the impacts
Forensic Science: Fundamentals & Investigations, Chapter 14
Why Radial and Concentric
Fractures Form
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Impacted glass is compressed on the side it is hit.
It will stretch on the opposite side of the glass, and
the tension there will radiate breaks in the glass
outward from the point of impact.
Then fractures form in the shape of concentric
circles on the same side of the impact.
Slower objects will make more concentric circles
than faster objects
Forensic Science: Fundamentals & Investigations, Chapter 14
Why Radial and Concentric
Fractures Form
Glass after an impact
shows radial fractures (red)
and concentric circle
fractures (blue)
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Forensic Science: Fundamentals & Investigations, Chapter 14
Why Radial and Concentric
Fractures Form
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Forensic Science: Fundamentals & Investigations, Chapter 14
Bullet Fractures
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As a bullet passes through glass, it pushes a cone
shaped piece of glass out of the glass ahead of it
The exit side of the hole is larger than the entrance
side of the hole
Radiating fracture lines from a subsequent shot will
stop at the edge of the fracture lines already present
in the glass
Forensic Science: Fundamentals & Investigations, Chapter 14
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Forensic Science: Fundamentals & Investigations, Chapter 14
Path of a Bullet Passing
through Window Glass
perpendicular
to the glass
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shot from
the left
shot from
the right
The angles at which bullets enter window glass
help locate the position of the shooter
Bits of the glass can fly backward (backscatter),
creating trace evidence
Forensic Science: Fundamentals & Investigations, Chapter 14
Path of a Bullet
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If shot is perpendicular to window, entry hole will
be round
If shot is from the left, glass will be forced to the
right, exit hole will be an irregular oval to the right
If shot is from right, glass will be forced to the left
exit hole will be an irregular oval to the left.
Ammunition type and distance can be
determined by pattern
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But, high speed bullet from a distance will make a
similar pattern to slower speed bullet up close.
Forensic Science: Fundamentals & Investigations, Chapter 14
Bulletproof glass
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Combo of two or more types of glass
Softer layer makes glass more elastic
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Index of refraction must be similar for both
pieces of glass
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transparent
Varies in thickness
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Won’t shatter
¾ inch – 3 inches
Forensic Science: Fundamentals & Investigations, Chapter 14
Tempered glass
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Safety glass used in windshields
Two layers of glass bonded with plastic in middle
Breaks into small, cubic pieces that don’t cut
Makes fewer concentric circle patterns
Hard to tell point of impact
Forensic Science: Fundamentals & Investigations, Chapter 14
Backscatter
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When a window is broken, most glass flies
forward
Some will be projected backwards
Can land on person who broke window
Trace evidence
Forensic Science: Fundamentals & Investigations, Chapter 14
Heat fractures
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During fire glass may fracture
Wavy lines
Will break toward region of high heat
No radial or concentric fracture patterns
Forensic Science: Fundamentals & Investigations, Chapter 14
Handling of Crime Scene
Glass Samples
1. Identify and photograph any glass samples before
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moving them.
Collect the largest fragments that can be reasonably
collected.
Identify the outside and inside surface of any glass.
If multiple panes are involved, make a diagram.
Note trace evidence such as skin, hair, blood, or
fibers.
Package all materials collected to maintain the chain
of custody.
Forensic Science: Fundamentals & Investigations, Chapter 14
. . . . . . . . . . . . . Summary . . . .
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Glass is an amorphous solid
Glass can be analyzed for its density,
refractiveness, and fracture patterns
Density of glass = Mass (grams) divided by
Volume (milliliters)
Refractive index is a measurement of how light
bends, or refracts, as it travels through a
material
Forensic Science: Fundamentals & Investigations, Chapter 14
. . . . . . . . . . . . . . . . . Summary
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Methods for measuring the refractive index
include:
• Snell’s law
• Submersion method
• Becke line method
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Fracture patterns provide information about
such things as the direction, the rate, and the
sequence of the impacts
Forensic Science: Fundamentals & Investigations, Chapter 14