Glass Analysis

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Glass Analysis
Mrs. Ashley
Glass Analysis
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How broken?
Link a suspect to a crime scene
Fingerprints
Blood
Match pieces by density, refractive index,
thickness, fracture patterns, color
Fractures
Radial
Concentric
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Glass Composition
• There are thousands of different uses for glass
• Formers-major portion of what makes up the
glass
• Fluxes- change the temperature at which the
formers melt
• Stabilizers- strengthen the glass and make it
resistant to water
Glass Ingredients
– SAND = silicon dioxide (Major Component)
– SODA = Sodium carbonate to lower the melting
point
– LIME = Calcium oxide. This is a METAL oxide and
is added to prevent the “SODA-LIME GLASS” from
dissolving in water.
6 Major Types of Commercial Glass
• Soda-lime glass 90% of all glass, container glass
and flat glass, resistance to high temperatures
and sudden changes in temperature not good
• Lead glass will not withstand high temperatures,
art glass, electrical insulating properties
• Borosilicate glass – high resistance to
temperature change; Pipelines, light bulbs,
photochromic glasses, sealed-beam headlights,
laboratory ware, and bake ware
Types of Commercial Glass
• Aluminosilicate glass used as resistors for
electronic circuitry
• Ninety six percent silica glass- resistant to
heat shock up to 900 degrees
• Fused silica glass very difficult to make and
can withstand temperatures up to 1200
degrees for short periods
Glass Uses
• Windows are made from FLOAT glass. Float
glass gets its name because the SODA-LIME
Glass is cooled on top of a bath of molten tin
• Tempered glass is used in the side and rear
windows of automobiles.
• Laminated glass is two sheets of ordinary glass
bonded together with a plastic film.
• Windshields of automobiles are made with
laminated glass.
Properties of Glass
• Mechanically Strong – Glass has great inherent strength
and is weakened only by surface imperfections, which give
everyday glass its fragile reputation. Special tempering can
minimize surface flaws.
• Hard surface – Glass resists scratches and abrasions.
(Because the composition can vary, so can the hardness.
On average the hardness of glass is about 5.5 on the Moh’s
scale)
• Elastic – Glass “gives” under stress – up to a breaking point
– but rebounds exactly to its original shape
• Chemical corrosion-resistant – Glass is affected by few
chemicals. It resists most industrial and food acids.
Properties of Glass
• Thermal shock-resistant – Glass withstands
intense heat or cold as well as sudden
temperature changes.
• Heat-absorbent – Glass retains heat, rather than
conducts it. Glass absorbs heat better than
metal.
• Optical Properties – Glass reflects, bends,
transmits and absorbs light with great accuracy.
• Electrical Insulating – Glass strongly resists
electric current. It stores electricity very
efficiently.
Additives and Color
Oxide
• Iron
• Manganese
• Cobalt
• Gold
• Antimony
• Copper
• Lead/antimony
Color
• green, brown
• Amethyst
• deep blue
• Red
• White
• light blue
• yellow
Direction
of
Penetration
A projectile hole is
inevitably wider at the
exit side
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Direction
of
Penetration
Radial
Concentric
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Comparison of Radial and Concentric
Fractures
When Formed?
On which side of the
glass
Description
Radial Fracture
Concentric Fracture
First (primary)
Opposite the side of
force or impact
Lines originate from
point of impact and
extend to edge of
glass
Second (secondary)
Same side as force
or impact
Series of circles one
inside the other
sharing the same
center
Crime Scene
• Backscatter- when window breaks, most of
the fragments will be carried forward, but
some will be projected backwards, called
backscatter
• Heat fracture-In a fire, glass may break due to
heat fracture that looks like wavy fracture
lines towards the heat source
Stress Marks
The perpendicular edge of stress
marks always face the surface on
which the crack originated
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Successive Penetrations
A fracture always terminates at an
existing line of fracture bsapp.com
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Forensic Examination of Glass
• Review definitions and map out a plan of analysis.
The FBI definition of glass is "...an inorganic product of
fusion that has cooled to a rigid condition without
crystallizing.” For the analytical methods the FBI
recommends, see below.
• Collect, handle, and identify the evidence.
• Make your initial examination. If possible, you'll want
to determine the color, fluorescence, surface features,
curvature, and thickness of the glass.
• Examine fractures. Are they radial or concentric?
Crater or hackle? A fracture match is an absolute
means of identification
Forensic Examination of Glass
• Now measure the density of the sample. Density tells you something
about the composition and thermal history of the glass.
• Measure the refractive index of the evidence. Refractive index is the most
commonly measured property in forensic glass analysis. It can give you the
same information as density, but you only need a very small piece of glass.
Testing both density and refractive index tells you more than just checking
one or the other.
• Determine major, minor, and trace elements in the glass. These methods
are destructive methods so the FBI recommends measuring density and
refractive index first (both methods are destructive).
– You can use scanning electron microscopy,
– X-ray fluorescence spectrometry, inductively coupled plasma-optimal emission
spectrophotometry,
– inductively coupled plasma-mass spectrometry,
– laser ablation-inductively coupled plasma-mass spectrometry,
– atomic absorption spectrophotometry.
What is the only way to individualize glass fragments
found at a crime scene to a single source?
• The ONLY way to individualize glass fragments
found at a crime scene to a single source is to
be able to put the pieces of crime scene glass
fragments together like a jig saw puzzle with
suspect glass fragments.
Tempered Glass
What physical properties are used most often to characterize
glass particles? What is the main drawback of using these
properties to characterize glass?
Do the glass particles from suspect and crime
scene fit together?
– Most of the time the glass evidence is too
fragmentary or too minute to permit a
comparison of this type.
- Composition of the glass
The general composition of most glass is the same.
• Density
• Refractive Index
Refractive Index
 Refractive index is represented by the letter n.
 The refractive index is a known quantity for
different types of glass.
 The FBI has a data base of over 200 refractive
indexes for different glass types and
compositions.
 The higher the refractive index, the denser the
glass.
 The higher the refractive index, the more the
light will bend toward the normal (produce a
smaller angle)
Immersion Method of Refractive Index
 The immersion method submerges the glass
fragments in a liquid with a known refractive
index.
 The temperature of the liquid is then increased
or decreased until the glass fragments disappear.
 Increasing or decreasing the temperature of the
liquid produces predictable changes in the
refractive index.
 When the glass fragments disappear- THE INDEX
of REFRACTION of the glass is the SAME as the
index of refraction of the liquid.
Becke Line
• A Becke line is a bright halo observed near the
border of a particle immersed in a liquid of a
different refractive index.
• When the Becke line is located INSIDE the glass
fragment, the fragment has a higher index of
refraction than the immersion liquid.
• When the Becke line is located OUTSIDE the glass
fragment, the fragment has a lower index of
refraction than the immersion liquid.
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