glass analysis
TRANSCRIPT
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GLASS
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Physical Properties
the forensic scientist must constantly determine those properties that impart distinguishing characteristics to matter, giving it a unique identity.
properties are the identifying characteristics of substances
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physical properties describe a substance without reference to any other substance
example: weight, volume, color, boiling point, melting point
can be measured for a particular substance without altering the material’s composition
associated only with the physical existence of that substance
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chemical properties describe the behavior of a substance when it reacts or combines with another substance
examples: - when wood burns, it combines with oxygen in the air to form new substances
- Marquis reagent turns purple in the presence of heroin
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Physical Properties
Temperature
- measure of heat intensity
- temperatures at which a substance melts or boils are readily determinable by characteristics that will help identify a substance
Weight and Mass
- weight: force with which gravity attracts a body
W = mg
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- mass: amount of matter an object contains Density
- most important physical property of matter with respect to analysis
- mass per unit volume
- an intensive property of matter
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Refractive Index
- refraction: bending of light wave because of change in velocity
= V of light in vacuum
V of light in medium- example: at 25oC, RI of water is 1.333.
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Comparing Glass Fragments
Glass that is broken and shattered into fragments during the commission of a crime could be used to place a suspect at the CS.
Glass – a hard, brittle, amorphous substance that is composed of silicon oxides mixed with various metal oxides
= sand + metal oxides… melted at high temperature; cooled to a rigid condition without crystallization
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by adding Na2CO3 to the sand, melting point and viscosity are both lowered making it easier to work
sand + Na2CO3 + CaO = soda-lime glass
= used for manufacturing most window and bottle glass
= common metal oxides: Na, Ca, Mg, Al
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Special glass (substituting Si and other M.O.)= automobile headlights and heat resistant
glass (Pyrex) uses boron oxide [borosilicates] Tempered glass: made stronger by introducing
stress through rapid heating and cooling of the glass surfaces
= when it breaks, it does not shatter but fragments into small squares
= used in the side and rear windows (US)
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Laminated glass: sandwiched one layer of plastic between two ordinary window glass
= windshield (US)
FS problem: need to find and measure those properties that will associate one glass fragment with another while eliminating the existence of other sources
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density and refractive index are used BUT, these properties are CLASS
CHARACTERISTICS.
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Forensic analysis of glass
Physical, fracture matching Best evidence, but very rare
Density determination Float test Results subjective
Refractive index (R.I.) determination Chemical testing, destructive
Test for silicates, metal oxides, trace evidence
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Density
Can be measured
Can be “observed” by placing in a “density tower” May not be the most accurate, subjective
Dm
v
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Refractive index (R.I.) measurement
Becke - immersion method Emmons & Winchell - temp. variation
method Saylor - double diaphragm contrast method Oettle - phase contrast method
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Refractive index measurement
Refraction is the change of direction and/or speed of light as it passes from one medium to another
RI is a ratio of the speed of light between two mediums (e.g. glass & oil)
RI is dependant on: The wavelength of light The temperature of the medium
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RI measurement technique
RI of material varies with temp. The change in RI for liquids is in the order of 10-4
RI units The change in RI for solids is in the order of 10-6 RI
units What does this mean?
When the temp. of a liquid is changed, the RI changes rapidly, but the RI of an immersed solid will not
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RI measurement technique
Silicon oil usually used Oil is calibrated so RI can be determined
from its temp. Sample glass is immersed in oil Oil is heated/cooled to determine match
temp. Glass “disappears”
Oil RI = Glass RI
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RI measurement using Becke line
Bright halo observed around glass when oil has higher RI
Bright halo observed inside the edge of the glass when oil has lower RI
When oil & glass = RI, glass “disappears”
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Becke line
Glass has higher refractive index
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Becke line
Glass has lower refractive index
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Common liquid refractive indices
Liquid Refractive index
Ethyl acetate 1.373
n-butyl alcohol 1.402
Olive oil 1.467
Corn oil 1.473
Castor oil 1.482
Methyl salicylate 1.522
Clove oil 1.543
Canola oil 1.465-1.467
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Glass Fracture Patterns
Two parts: Radial Concentric
Can tell direction of force from these
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Determining direction of force
Radial edge of glass 3 R’s
Radial cracks have Right angles
on the Reverse side of applied force
Concentric edge of glass
Direction of force
Direction of force
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Procedures in Analyzing Glass
FLOTATION a solid particles will either float, sink or remain
suspended in a liquid depending on its density relative to the liquid medium
procedure: a standard/reference glass is immersed in a liquid
composition of the liquid is carefully adjusted by the addition of bromoform or bromobenzene until the glass chip remains suspended
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the standard/reference glass and liquid have the same density
glass chips of approximately the same size and shape as the SR are added to the liquid
if they both remain suspended, their densities are equal to each other and the liquid
liquid is transferred to a density meter
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IMMERSION METHOD added comparison of refractive indices procedure: glass particles are immersed in a
liquid medium whose refractive index is varied until it is equal to that of the glass particles = match point
you will note the disappearance of the Becke line
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GLASS FRACTURES
glass bends in response to any force that is exerted on any one of its surfaces
when the limit of elasticity is reached, the glass fractures
fractured window glass reveal information that can be related to the force and direction of impact = useful in crime scene investigation
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penetration of ordinary window glass by a projectile (bullet or stone) produces a familiar fracture pattern
radiating lines = radial fractures circular lines = concentric fractures
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it is difficult to determine just from size and shape of a hole in glass whether it was made by a bullet or some other projectile
high velocity projectile often leaves a round, crater-shaped hole that is surrounded by symmetrical pattern of radial and concentric cracks
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hole is wider on the exit side
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but as the velocity of the penetrating projectile decreases, the irregularity of the shape of the hole and of its surrounding cracks increases
hole shape could no longer determine the direction of impact
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when a force pushes on one side of a pane of glass, the elasticity of the glass permits it to bend in the direction of the force applied
once elastic limit is exceeded, the glass begins to crack
the first fractures form on the surface opposite to the force
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these first fractures develop into radial lines
continued motion of the force places tension on the front surface of the glass resulting in concentric cracks
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stress marks are shaped like arches that are perpendicular to one glass surface and curved nearly parallel to the opposite surface
perpendicular edge always faces the surface on which the crack originated
Radial cracks form a Right angle on the Reverse side of the force
this could not be applied to broken tempered glass
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when there have been successive penetrations, it is possible to determine the sequence of impact
A fracture always terminates at an existing line of fracture
LEFT preceded the RIGHT