Transcript 1 - Southgate Schools

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 © 2009
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Forensic Science: Fundamentals & Investigations, Chapter 14
Introduction and History of Glass
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Analysis of glass found at a crime scene can yield
trace evidence.
The earliest human-made glass objects (beads) date
back to about 2500 B.C. Egypt.
Specialized glass production was an art, a science,
and a state secret in the republic of Venice over a
span of hundreds of years.
By the fourteenth century, the knowledge of glass
production spread throughout Europe.
The industrial revolution brought the mass production
of many kinds of glass.
Forensic Science: Fundamentals & Investigations, Chapter 14
What Is Glass?
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Once it cools, glass can be polished, ground, or cut for
useful or decorative purposes. Glass blowers can form
glass into many different shapes.
Crystalline solids have a regular atomic structure (illustrated above, left). But glass is an amorphous solid and
so has an irregular atomic structure (illustrated above,
right),
Because of this, glass breaks in a variety of fracture
patterns.
Forensic Science: Fundamentals & Investigations, Chapter 14
Types of Glass
 Glass is an amorphous solid usually made
from silica, calcium oxide, and sodium oxide.
 Because glass is made of a variety of
compounds, there are many types of glass.
 Adding different metal oxides to glass
mixtures, for example, yields different colors.
 And because different types of glass have
different densities and refraction indexes, it is
possible to compare one type of glass with
another.
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Forensic Science: Fundamentals & Investigations, Chapter 14
Density
The formula for calculating density is:
D=m/V
 The mass (m) of a fragment of glass can be
found using a balance beam device.
 Place the fragment of glass into a beaker filled
with water and measure the volume (V) of
overflow.
 Divide the mass (in grams) by the volume (in
milliliters) to find the density (D) of the glass
fragment.
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Forensic Science: Fundamentals & Investigations, Chapter 14
Refractive Index
 When a beam of light moves from one medium
into another, its speed changes. That change
causes the beam to change direction, bend.
 In the illustration above, the green line to the
right of the red line shows the direction if the
beam had not changed direction. But the black
line shows it bent toward the red line.
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Forensic Science: Fundamentals & Investigations, Chapter 14
Refractive Index
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Refraction is the change in the direction of light
as it changes speed when moving from one
medium into another.
The direction of the light forms two angles with
the normal .
If the light passes into a denser medium (the gray
area), its direction will bend toward the normal.
Forensic Science: Fundamentals & Investigations, Chapter 14
Application of Refractive Index
to Forensics
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If light passes from water into air, will it bend
toward or away from the normal?
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If light passes from air into glass, will it bend
toward or away from the normal?
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Light passed through two fragments of glass—
one found at the crime scene and the other
connected to a suspect—may have the same
refraction and, so, link one with the other.
Forensic Science: Fundamentals & Investigations, Chapter 14
Application of Refractive Index
to Forensics
 If the glass fragments found at the crime scene, or
connected with the suspect, or both are too small to
check for this consistency by passing light through
them, other methods such as the submersion method
can be used to provide estimates.
 Use the illustration above to describe the submersion
method for obtaining refraction estimates.
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Forensic Science: Fundamentals & Investigations, Chapter 14
Fracture Patterns in Broken
Glass
 Glass can stretch slightly when hit.
 And because it’s an amorphous solid, it will
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not break into regular pieces with straight line
fractures.
 That means fracture patterns can provide
clues about the direction, rate, and sequence
of the impacts.
Forensic Science: Fundamentals & Investigations, Chapter 14
Why Radial and Concentric
Fractures Form
 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.
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Forensic Science: Fundamentals & Investigations, Chapter 14
Bullet Fractures
 As a bullet passes through glass, it pushes a cone
shaped piece of glass out of the glass ahead of it.
 This makes the exit side of the hole 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.
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Forensic Science: Fundamentals & Investigations, Chapter 14
Path of a Bullet Passing through
Window Glass
 The entry hole will be round if the bullet was fired
perpendicular to the pane.
 If fired from an angle, glass pieces will be forced out
to the opposite side from the shot.
 The angles at which bullets enter window glass can
help locate the position of the shooter.
 Bits of the glass can fly backward (backscatter),
possibly creating trace evidence.
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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
 Different kinds of glass can be analyzed for their
density, refractiveness, and fracture patterns.
 The different compounds that make up a kind of glass
affect its density.
 The refractiveness of glass can be measured in a
number of ways.
 Fracture patterns can provide information about such
things as the direction, the rate, and the sequence of
the impacts.
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Forensic Science: Fundamentals & Investigations, Chapter 14