Transcript Glass - Somerset Area School District

Chapter 14
Glass
“There is no den in the
wide world to hide a rogue.
Commit a crime and the
earth is made of glass.”
—Ralph Waldo Emerson
Glass Analysis
Students will learn:
 The difference between physical
and chemical properties.
 How glass can be used as
evidence.
 How individual evidence differs
from class evidence.
 The nature of glass.
 How to use the properties of
reflection, refraction, and
refractive index to classify glass
fragments.
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Glass Analysis
Students will be able to:
 Make density measurements on
very small particles.
 Use logic to reconstruct events.
 Use technology and
mathematics to improve
investigations and
communications.
 Identify questions and concepts
that guide scientific
investigations.
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Glass as Evidence
Investigators can use glass fragments
found at a crime scene to place a
suspect at the scene.
If it can be pieced back together like
a puzzle, then it can be
individualized and the source can be
considered unique.
Usually, it cannot and must be
considered class evidence
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Glass as Evidence
 Forensic scientists must use as many
physical and chemical properties of
glass as possible to characterize
fragments and link them to a crime
scene.
 Characteristics such as refractive index,
density, color, and chemical
composition can be analyzed.
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Characteristics of Glass
 Hard, amorphous solid (noncrystalline:
atoms arranged randomly)
 Usually transparent
 Primarily composed of silica (sand)
with various amounts of element
oxides
 Brittle
 Exhibits conchoidal fracture
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Common Types
 Soda-lime—used in plate and window
glass, glass containers, and light bulbs
 Soda-lead—fine table ware and art
Borosilicate—heat resistant, like Pyrex
 Silica—used in chemical ware
 Tempered—used in car side windows
 Laminated—used in the car windshields
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Composition of Glasses
 Silica
 Soda
 Lime
 Potash
 Magnesia
 Alumina
 Boric Oxide
 Lead Oxide
SiO2
Na2O
CaO
K2O
MgO
Al2O3
B2O3
PbO
Bottles/
widows
73%
16
5.2
0.6
3.6
1.0
---
Lab/
Bakware
80%
4
-0.4
-2.0
13
--
Crystal
35%
--7.2
---58
High temp
96%
----0.5
3
--
Most glass starts by melting sand (silica) and ashes (soda). Soda reduces the melting point of
the silica. Lime is added to make the glass water insoluble.
Boric oxide increases temperature stability (good for ovenware). Lead increases density and
refractive index so that the glass sparkles. Adding various elements can change color. (pg. 312)
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Physical Characteristics
 Density —mass divided by volume
 Refractive index (RI) —the measure of light
bending due to a change in velocity when
traveling from one medium to another
 Fractures
 Color
 Thickness
 Fluorescence
 Markings —striations, dimples, etc
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Density
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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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Refractive Index
When light strikes most glass, there is
a change in its velocity and therefore, a
change in direction.
This phenomenon is called
refraction.
The refractive index (RI or n)
is a comparison of the speed
of light in a vacuum to the
speed of light in another substance.
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Calculating RI
Ex: Speed of light in water is
225,000,000 meters per second. It’s
refractive index is calculated as
follows: (note: 3 X108 = speed of light in air)
3.00 X 108 = 1.33
2.25 X 108
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Immersion Method of
Determining Refractive Index
If a clear material, like glass, is
immersed in a liquid that has the same
refractive index, the glass will appear to
disappear.
Investigators use this principle when
characterizing glass.
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Immersion Method of
Determining Refractive Index
Immerse a glass fragment into a drop
of liquid with a known refractive index.
Transmit light through the sample to
the eyepiece of a microscope.
If the liquid has a higher or lower
index of refraction than the glass, the
piece of glass will be visible.
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The Becke Line
A “halo” of light, called
the Becke line will be
visible around the
perimeter of the glass.
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Match Point
If the refractive indexes are the same,
the Becke line disappears and the glass
boundaries appear to be invisible.
This is called the match point.
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If the Becke line appears on the inside
perimeter of the glass, then the
investigator will need to try another
liquid with a higher refractive index to
try to reach the match point.
 If the Becke line appears on the
outside perimeter of the glass, then a
liquid with a lower refractive index will
be required.
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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.
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Another way to determine the
refractive index using just one liquid is
to use a liquid such as silicone oil.
The refractive index of a high boiling
liquid, such as silicone oil, changes
with temperature
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Determination of
Refractive Index
 This occurs in an apparatus called a hot
stage which is attached to a microscope.
Increasing the temperature allows the
disappearance of the Becke line to be
observed
 At match point, temperature is noted and
refractive index of the liquid is read from
a calibration chart.
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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
Note: this chart is on pg. 318 not 314 as your notes say.
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Fracture Patterns
Investigators can sometimes
reconstruct a crime using glass
fracture patterns.
How cracks form, their shape, and if
the breakage came from inside or
outside can be clues as to what
happened at the crime scene.
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When a projectile such as a bullet or a
rock breaks glass, it will form two distinct
types of fractures:
Radial fractures
Concentric fractures
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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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Sequencing
 A high velocity
projectile always leaves
a hole wider at the exit
side of the glass.
 Cracks terminate at
intersections with other.
This can be used to
determine the order that
the fractures occurred.
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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.
—Forensic Science Communications
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Considerations
for Collection
 The collector must consider the
possibility that 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.
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Considerations
for Collection
Any glass samples collected should
be documented, marked (if necessary),
packaged, and labeled.
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Collecting the Sample
 The glass sample should consist of the
largest amount that can be practically
collected from each broken object and
packaged separately.
—Forensic Science Communications
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Collecting the Sample
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.
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Collecting the Sample
 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.
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Collecting the Sample
The sample should be collected with
consideration being given to the
presence of other types of evidence on
that sample (e.g., fibers, blood).
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