Unit 4:
Forensic Glass Analysis
Introduction to Forensic Science
Forensic Glass Analysis
1. Composition of Glass
2. Types of Glass
3. Comparing Glass
A. Density
B. Refractivity
4. Glass Fracture Patterns
5. Collecting Glass As Evidence
2
1. Composition of Glass
• Glass is a hard, brittle, amorphous material
– Amorphous (without shape) because its atoms are arranged
randomly
– Due to its irregular atomic structure, it produces a variety
of fracture patterns when broken
– Brittle because it breaks easily if struck in the right
direction
• Has numerous uses and thousands of compositions
1. Composition of Glass (continued)
• Made by melting the following
ingredients at extremely high
temperatures:
– Sand also known as silica or silicon
dioxide (SiO2), is the primary ingredient
– Lime or calcium oxide (CaO) is added to
prevent the glass from becoming soluble
in water
– Sodium oxide (Na2O) is added to reduce
the melting point of silica or sand
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1. Composition of Glass (continued)
• Three categories of substances found in all glass
– Formers
• Makes up the bulk of the glass
• Examples: silicon dioxide (SiO2) in the form of sand, boron
trioxide (B2O3), and phosphorus pentoxide (P2O5)
– Fluxes
• Change formers’ melting points
• Examples: sodium carbonate (Na2CO3) and potassium
carbonate (K2CO3)
– Stabilizers
• Strengthen the glass and make it resistant to water
• Calcium carbonate (CaCO3) is the most frequently used
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1. Composition of Glass (continued)
• The raw materials for making glass are all oxides
(contain oxygen)
– The composition of a sample can be expressed in
percentage of different oxides
– Example: the approximate composition of window or
bottle glass is
•
•
•
•
•
•
Silica (SiO2) – 73.6 %
Soda (Na2O) – 16.0 %
Lime (CaO) – 5.2 %
Potash (K2O) – 0.6 %
Magnesia (MgO) – 3.6 %
Alumina (Al2O3) – 1.0
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Video Clip – How Glass is Made
• 6:37 min
• https://www.youtube.com/watch?v=IjNusHQO
hTM
• 5:04 min Plate Glass
• https://www.youtube.com/watch?v=PSurxsGQ
L90
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Glass Analysis
1. Composition of Glass
2. Types of Glass
3. Comparing Glass
A. Density
B. Refractivity
4. Glass Fracture Patterns
5. Collecting Glass As Evidence
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2. Types of Glass
There are many different types of glass. We
will discuss the following five types:
A. Obsidian
B. Soda Lime
C. Leaded
D. Tempered
E. Laminated
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2. Types of Glass
A. Obsidian is a natural form of glass that is
created by volcanoes
B. Soda-lime glass
– The most basic, common, inexpensive glass –
also the easiest to make
– Used for manufacturing windows and bottle glass
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2. Types of Glass
C. Borosilicate Glass
- Contains boron oxide
- heat-resistant glass: can be heated and cooled
rapidly without breaking
- Car headlights, Pyrex dishes, lab beakers and
test tubes.
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2. Types of Glass
D. Leaded glass
– Contains lead oxide which makes it denser
– Sparkles as light passes through it (light waves
are bent)
– Used for manufacturing fine glassware and art
glass (stained-glass windows)
– Is commonly called crystal
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2. Types of Glass
E. Tempered glass
– Stronger than ordinary glass
– Strengthened by introducing stress through a cycle
of rapid heating and cooling of its surface
– When broken, this glass does not break into large
shards, but fragments or breaks into small squares
– Used in the side and rear windows of automobiles
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2. Types of Glass
F. Laminated glass
– Constructed by bonding two ordinary sheets of
glass together with a plastic film
– Prevents shattering
– Also used by automobile manufactures for
windshields
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Video Clip
• 5:34 min – Making Laminated Glass
• https://www.youtube.com/watch?v=fg3moEI9
V5g
• 0:54 min – Laminated Glass Safety Test
• https://www.youtube.com/watch?v=RVRAA4
VoAk0
• 5:34 min – Making Technical Glass
• https://www.youtube.com/watch?v=ixkZMyQ
3Gac
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Glass Analysis
1. Composition of Glass
2. Types of Glass
3. Comparing Glass
A. Density
B. Refractivity
4. Glass Fracture Patterns
5. Collecting Glass As Evidence
16
3. Comparing Glass
• Investigation/Analysis includes
– Finding
– Measuring
– Comparing
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3. Comparing Glass
Individualized Characteristics
– Only occurs if the suspect and crime scene
fragments are fit together exactly, like a puzzle
– This would require matching broken edges AND
irregularities and striations on the broken surfaces
– Most glass evidence is either too fragmented or
too small to permit a comparison of this type
So, what
type of
evidence is
glass?
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3. Comparing Glass (continued)
Class Characteristics
• (Density and Refractive Index)
– General composition of glass is a class characteristic
– Trace elements in glass may prove to be distinctive
and measureable characteristics, but still class
– The physical properties of density and refractive
index are used most successfully for characterizing
glass particles, but only as a class characteristic
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Glass Analysis
1. Composition of Glass
2. Types of Glass
3. Comparing Glass
A. Density
B. Refractivity
4. Glass Fracture Patterns
5. Collecting Glass As Evidence
20
3A. Methods of Comparison:
Density
• Density is defined as the mass per unit volume.
(D = M/V)
– Density is an intensive property of matter, meaning
it remains the same regardless of sample size.
– Density is considered a characteristic property of a
substance and can be used as an aid in
identification
3A. Methods of Comparison:
Density
M
D
V
Density comparison
– Density (D) is calculated by dividing the mass (M)
of a substance by its volume (V)
D=M÷V
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3A. Density
Density comparison (continued)
• Step 1: Find the sample’s volume by Archimedes’
Principal
–
–
–
–
Fill a beaker half way, and put it on the scale
Record the initial mass
Gently lower the glass into the water attached to a string
Record the final mass
– 𝐺𝑙𝑎𝑠𝑠 𝑉𝑜𝑙𝑢𝑚𝑒 =
𝐹𝑖𝑛𝑎𝑙 𝑚𝑎𝑠𝑠 𝑔 − 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑚𝑎𝑠𝑠(𝑔)
1𝑔
𝑐𝑚3
The density of water
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3A. Density
Density comparison (continued)
• Step 2: Find the sample’s mass in grams usingRead
a the bottom of
the meniscus, and
balance or scale
• Density can now be calculated from
the equation in units of grams per cm3
estimate between
marks. This is 52.5
milliliters (mL)
D=M÷V
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Glass Analysis
1. Composition of Glass
2. Types of Glass
3. Comparing Glass
A. Density
B. Refractivity
4. Glass Fracture Patterns
5. Collecting Glass As Evidence
25
3B. Refractivity
Let’s talk about light.
What do you know?
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3B. Refractivity
• Light waves travel in air at a constant velocity until
they enter another medium (such as glass or water).
• Upon entering a new medium the light waves are
suddenly slowed, which causes the light rays to bend.
3B. Refractivity
• The bending of light waves because of a change in
velocity is called refraction.
• The Refractive Index for a medium is a measure of
how much light slows (or speeds up) upon entering
that 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.
3B. Methods of Comparison:
Refractivity
Refractive Index
– A measure of how much an object slows light*
• Light slows down when it passes through any medium
(the denser the medium, the slower the light travels)
• Any object that transmits light has a refractive index
• Intensive property. Can be used to identify a substance.
Refractive index = velocity of light in a vacuum ÷ velocity of
light in a medium
*compared to light in a vacuum
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3B. Methods of Comparison:
Refractivity (continued)
When light passes through media
(substances) with different refractive
indexes, refraction (bending of light)
occurs
– Refraction is why objects appear bent or
distorted underwater
– Every liquid has its own refractive index
– If a piece of glass is placed in a liquid with a
different refractive index an outline of the
glass is clearly visible
• This line is known as a Becke Line
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3B. Methods of Comparison:
Refractivity (continued)
When light passes through a piece of glass placed
in a liquid with the same refractive index:
– The glass bends light at the same angle as the liquid
– The Becke Lines disappear
– The glass seems to disappear
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Video Clip
•
•
•
•
•
•
4:14 Becke Line Demo
https://www.youtube.com/watch?v=qH1S83Bkttw
1:34 min Beck Line Test
https://www.youtube.com/watch?v=B6Gis-wloSI
5:04 The Becke Line Test
https://www.youtube.com/watch?v=1QA1EhsfTQ
Y
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Lab 4.2: Identifying Unknown
Glass by Density and Refractive
Index
Glass Analysis
1. Composition of Glass
2. Types of Glass
3. Comparing Glass
A. Density
B. Refractivity
4. Glass Fracture Patterns
5. Collecting Glass As Evidence
34
4. Glass Fracture Patterns
Glass has a degree (amount) of elasticity
(flexible-ness)
– It breaks when its elastic limit is exceeded
– The elasticity produces fractures when it is
penetrated by a projectile (i.e. a bullet)
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4. Glass Fracture Patterns
When the elasticity of glass is exceeded, it
fractures (breaks).
There are several distinct types of glass fracture
patterns:
A. Radial
B. Concentric
C. Conchoidal
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4. Glass Fracture Patterns (continued)
A. Radial
– Produced first
– Form on the side of the
glass opposite to where the
impact originated (on the back)
– Look like spider webs that spread outward from
the impact hole
– Always terminate (stop) at an existing fracture
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A. Radial Fractures
4. Glass Fracture Patterns (continued)
B. Concentric (rings)
– Form second
– Encircle (surround)
the bullet hole
– Start on the same side as that of the destructive force
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B. Concentric Fractures
. Glass Fracture Patterns (continued)
C. Conchoidal Lines
(conchoidal = smooth and rounded, like a conch shell)
– Amorphous solids do not break along any natural
plane of separation as a crystalline solid would.
– The side of broken glass will show curved, rippling
conchoidal lines that can be used to determine
direction of impact.
• 4 R Rule: “On Radial cracks, Ridges make Right angles to
the Rear.”
This is
opposite for
sides of
concentric
fractures!
rear
Video Clip
• 0:54 min – High Speed Glass Fracture
• https://www.youtube.com/watch?v=dwnBGnK
SG2o
• 3:57 min – Tempered vs. Plate Glass Fracture
• https://www.youtube.com/watch?v=CiGHW9rDbA
• 2:43 min – Bullet patterns in glass
• https://www.youtube.com/watch?v=QA1PG_J
hkWM
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4. Glass Fracture Patterns (continued)
I. Determining the sequence of multiple bullet
holes
• The radial fractures from the second bullet hole
always terminate into the fractures from the first
bullet hole, and so forth
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4. Glass Fracture Patterns (continued)
II. Determining
bullet path
direction
• Compare the size of the
entrance hole to the size
of the exit hole
• Exit holes are larger.
More fragmented glass is
knocked out of the
surface around the hole
where the bullet is
leaving because glass is
elastic and bows outward
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4. Glass Fracture Patterns (continued)
II. Determining bullet path
direction
• Entrance holes
– The bullet makes a very small
hole when it enters
– The glass blows back in the
direction of the impact because of
its elasticity
– The glass snaps back violently
after being stressed and can blow
shattered glass back several
meters
– Most of the shattered glass lands
on the impacted side (front) of
the glass, instead of by the exit
hole
Shot fired into Obama campaign
office in Denver, CO (Oct. 2012)
Source: telegraph.co.uk
Lab 4.1: Glass Fracture Patterns
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Glass Analysis
1. Composition of Glass
2. Types of Glass
3. Comparing Glass
A. Density
B. Refractivity
4. Glass Fracture Patterns
5. Collecting Glass As Evidence
47
5. Collecting Glass as Evidence
• Avoid the loss or contamination of
any evidence samples
• Identify and photograph all glass
samples before moving them
• Collect the largest fragments
• Identify the outside and inside
surfaces of any glass
• Indicate the relative position of
multiple window panes in a diagram
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5. Collecting Glass as Evidence (continued)
• Note any other trace evidence found on or embedded
in the glass, such as skin, hair, blood, or fibers
• Package all of the collected materials properly in
order to maintain the chain of custody
• Separate the glass by physical properties, such as
size, color, and texture
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5. Collecting Glass as Evidence (continued)
• Catalog the samples and keep them separated in
order to avoid contamination between two
different sources
• Separate the glass fragments from any other trace
evidence (e.g., hair, blood, fibers) once in the lab
• Examine any clothing (or other objects that may
have been used to break the glass) related to the
crime scene for glass fragments and other trace
evidence
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Video Clips
• 6:26 min – Bullet Proof Glass Vandalism Test
• https://www.youtube.com/watch?v=EibjrMy1
TtI
• 6:41 min – Bulletproof vests
• https://www.youtube.com/watch?v=eRabvfq8
TdA
• 10:06 min – The Bulletproof Tailor
• https://www.youtube.com/watch?v=nQM6zLi
Sn1E
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Resources
•
•
•
•
•
•
•
•
Texas Education Agency, Forensic Certification Training, Sam Houston State University
Forensic Science: Fundamentals & Investigation (1st Edition), Anthony Bertino
Forensic Science: From the Crime Scene to the Crime Lab (1st Edition), Richard Saferstein
ChemMatters, “More Than Meets The Eye” Brian Rohrig
The Science Spot – Forensic Science
– http://www.sciencespot.net/Pages/classforsci.html
Investigator/Officer’s Personal Experience
Corning Museum of Glass site
– http://www.cmog.org/default.asp
Federal Bureau of Investigation: Laboratory Services
–
–
Forensic Glass Comparison: Background Information Used in Data Interpretation
http://www.fbi.gov/about-us/lab/forensic-science-communications/fsc/april2009/review
Introduction to Forensic Glass Examination
http://www.fbi.gov/about-us/lab/forensic-sciencecommunications/fsc/jan2005/standards/2005standards4.htm/
–
–
Collection, Handling, and Identification of Glass http://www.fbi.gov/about-us/lab/forensic-sciencecommunications/fsc/jan2005/standards/2005standards5.htm/
Glass Density Determination
http://www.fbi.gov/about-us/lab/forensic-sciencecommunications/fsc/jan2005/standards/2005standards8.htm/
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