Transcript csi_glass_evidence_1

Glass Evidence
Solving the Crime
Ms. Scribner’s Forensic Crime Science Class
Eisenhower High School
What is glass?
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Made from fused inorganic materials
Amorphus; not crystalline (molecules not arranged)
Varies in elemental formula
Many shapes and colors
What is Glass?
 No specific melting point
 Softens over a temperature range.
 Uniform amorphous solid (irregular
atomic structure – middle picture).
 Because of this, glass breaks in a
variety of fracture patterns.
What’s in Glass?
 Formers (form glassy, non-crystalline structure):
SiO2, B2O3, P2O5, GeO2, V2O5, As2O3, Sb2O5
 Fluxes (lowers melting point):
Na2O, K2O, LiO, Al2O3, B2O3, Cs2O
 Stabilizers (Chem./Corrosion Resistance):
CaO, MgO, Al2O3, PbO, SrO, BaO, ZnO, ZrO
How is glass involved in crime?
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Small fragments can be used as trace
evidence (link objects/people)
Crime scene reconstruction
How is glass collected?
For reconstruction, detailed
photographs can be taken or the entire
pane of glass is collected.
How is glass collected?
For trace evidence, usually visually
collected with forceps and packaged in
plastic bags.
Types of Glass
Soda–lime Glass
• Most prevalent type of glass
• Inexpensive to process
• Used for window panes,
bottles, mirrors, data storage
disks, light bulbs, etc.
• Made by melting SiO2 and
NaCO3
• CaCO3 or CaO added so NOT
water soluble
• Density is 2.44g/cm3
Soda–lime Glass
Surface of soda lime
glass
Flint Glass
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Optical glass
High refractive index (RI)
RI = 1.45 to 2.00
Contains 4-6% iron oxide
Disposal poses a pollution
problem
• Also used to make simulated
diamonds
Flint Glass
Lead crystal
Optical
instruments
lenses
Borosilicate Glass
• Heat-resistant glass
• First sold under brand name
“Duran”
• Called Pyrex by 1915
• Made of quartz, NaCO3, CaCO3,
and boron
• Higher melting point & more
break resistant
• Density 2.23 g/cm3
Borosilicate Glass
Question:
In lab, what
label(s) should
be on any
test tube
that you are
going to
heat?
Answer: Pyrex or Kimex
Why is Glass
important in
forensics?
Trace Evidence
• CSI investigators must work
backwards from evidence at
the crime scene
• Collect blood, hair, fibers,
fingerprints, & broken glass
• Used to establish a link
between the suspect and the
crime
• Often present in burglaries
and hit and run accidents
Properties of Glass
Glass Properties
 Combinations of formers, fluxes, and stabilizers
creates unique glass types with different properties:
 Density
 Refractive Index (RI)
Refractive Index
• Refers to how light passes
through the glass
• Variations in the composition
of the glass cause bending of
light rays
• Extent to which light bends
is called refractive index
• Standards used to determine
RI
Refraction Index
https://www.youtube.com/watch?v=eX4JqYHjthE
https://www.youtube.com/watch?v=s3EK1lGkf2s
Disappearing glass video:
https://www.youtube.com/watch?v=qH1S83Bkttw
Refractive Index
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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.
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Populations of Glass RIs
 Because of all the different materials in glass,
there will be multiple RI’s  a major one will
stick out, like above.
Snell’s Law of Refraction
 Can figure out the RI of materials
based on the angle the light hits
and bends.
 n1sin(θ1) = n2sin(θ2)
 n1 = RI of incident light
 θ1= angle of incident light
 n2 = RI of refracted light
 θ2= angle of refracted light
Snell’s Law Example 1
 Light traveling in air (n1=1) hits
a piece of glass at an angle of
30 degrees. The light refracts in
the glass at an angle of 15
degrees. What is the index of
refraction (RI) of the glass?
 n1=1, θ1= 30, θ2= 15, n2=?
 1sin(30)=n2sin(15)
 0.5=0.2588n2
 n2 = 1.93
Snell’s Law Example 2
 Light traveling in air (n1=1) hits
a piece of glass at an angle of
45 degrees. If the glass has a
refractive index of 1.5, what
angle should the light bend at?
 n1=1, θ1= 45, θ2= ?, n2=1.5
 1sin(45)=1.5sin(θ2)
 0.7071=1.5sin(θ2)
 .4714= sin(θ2)
 θ2= 27 degrees
Snell’s Law Practice
 1. Light traveling in air (n1=1) hits a piece of glass at
an angle of 37 degrees. The light refracts in the glass
at an angle of 30 degrees. What is the index of
refraction (RI) of the glass?
 2. Light traveling in air (n1=1) hits a piece of glass at
an angle of 60 degrees. The light refracts in the glass
at an angle of 15 degrees. What is the index of
refraction (RI) of the glass?
 3. Light traveling in air (n1=1) hits a piece of glass at
an angle of 25 degrees. If the glass has a refractive
index of 1.75, what angle should the light bend at?
 4. Light traveling in air (n1=1) hits a piece of glass at an
angle of 55 degrees. If the glass has a refractive index of
1.3, what angle should the light bend at?
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.
Forensic Science:
Fundamentals &
Investigations, Chapter 14
27
Density
Window glass does not have uniform
density
• Take samples from different locations
Edge of tempered glass is denser than
the interior
FBI has reported density results for 1400
glass samples recovered from 19641997.
Density Practice (Left Page)
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Find the density for the following pieces of glass:
1. Mass: 4g, Volume Displaced: 2mL
2. Mass: 15g, Volume Displaced: 3mL
3. Mass: 1g, Volume Displaced: 2mL
Glass Breakage Pattern
• Glass breaks or shatters in
patterns
• Less quantitative (numerical
data) and more qualitative
• Glass fragments can fit
together like a jigsaw puzzle
• Composition determines how
much the glass will break
Question:
• In a court of law, which type(s)
of glass evidence do you think
would be the most helpful in
convicting a suspect?
• In a court of law, which type(s)
of glass evidence do you think
would be the least helpful in
convicting a suspect?
Fractured Glass
Shattered Glass
• When glass is
struck, it breaks in
a specific manner
• Applied force
compresses the
glass
• Glass begins to
break OPPOSITE
the applied force
• Forms 2 types of
fractures – radial &
concentric
Radial Fracture
• Fractures in a radial pattern
point outward from point of
impact, forming pie-shaped
sections
For reconstruction:
Radial cracks
Concentric cracks
Concentric Fractures
• Secondary breaks/fractures
• Occur between the radial
fractures
• On side of glass where force
was applied
• Similar to spider web
For reconstruction:
1.
Determine the direction of projectile.
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When looking at a radial
section, conchoidal
fractures point back to
the origin of the break
Core usually on opposite
side of the origin
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For reconstruction:
2.
Determine the order of projectiles
when
dealing with more than
one.
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For reconstruction:
2.
Determine the order of projectiles
when
dealing with more than
one.
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3
For trace analysis:
Associate unknown glass found at the crime
scene with known samples to determine
the source.
Done with microscopic analysis
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Color, curve, thickness, etc.
Refractive index: Determined by placing
glass in oils of different refractive indexes
Refractive index:
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Low relief = no outside line
R.I. of glass and oil are similar
High relief = thick outside line (Becke
line) R.I. of glass and oil are different
Question: What type or types
of fractures do you see?
Other Factors in Glass Analysis
Heat-tempered glass (safety glass)
breaks in small, cube-like pieces
• Used in car windows
Imperfections in manufacturing
glass
Distinct marks/scratches (i.e.
caused by debris on windshield
wiper
Bullet Holes
A high-speed projectile leaves exit hole
larger than entrance hole
• helps determine direction of impact
Examine fracture lines to determine
order in which penetrations of the
glass occurred
• fractures from 1st shot terminate when
they meet fractures from 2nd shot
Other Properties of Glass
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Surface striations and markings
Surface contaminants
Thickness
Hardness
Color
Elemental analysis
• scanning electron microscope
• X-ray analysis
• Flameless atomic absorption
spectrophotometry (destroys sample)