Glass and Soil
Trace Evidence
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Properties of Matter
• The distinguishing characteristics of a
substance used in its identification &
description
– characteristics by which people are
recognized
• hair color, tone of voice, walk, shape of nose
– chemical substances are recognized by how
they look & behave
• each chemical substance has a unique set of
properties that distinguish it from all other
substances
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Properties of Matter
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Physical and chemical properties.
Metric and British systems.
Celsius (Centigrade) vs. Fahrenheit.
Mass vs. weight.
Density
Refractive Index
Crystalline vs amorphous solids.
Double refraction and birefringes.
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Metric System
Based on the units being divisions of 10
• Gram (g) – mass
• Liter (L) volume
• Meter (m) - length
Uses prefixes to establish unit relationships
• Deci (d) – 10-1 1/10
Deca (D) - 101 10
• Centi (c) -10-2 1/100 Hecta (H) -102 100
• Milli (m) – 10-3 1/1000 Kilo (k) - 103 1000
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• Micro () – 10 1/1,000,000
Properties of Matter
• Chemical Properties
– a characteristic of a substance that
describes the way the substance undergoes
or resists change to form a new substance
• Physical Properties
– a characteristic of a substance that can be
observed without changing the substance
into another substance
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Properties of Matter
• Physical property: describes the
behavior of a substance without
having to alter the substance’s
composition through a chemical
reaction
• Chemical property: describes the
behavior of a substance when it
reacts or combines with another
substance
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Physical Properties
• Extensive Properties
– depend on the amount of sample
• volume, mass, weight
• Intensive Properties
– do not depend on the amount of
sample
• melting point, density, specific heat
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• Fahrenheit scale: the temperature
scale using the melting point of ice as
320 and the boiling point of water as
2120, with 180 equal divisions or
degrees between them.
• Celsius scale: the temperature scale
using the melting point of ice as 00
and the boiling point of water as
1000, with 100 equal divisions or
degrees between
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Comparison of
the Celsius and
Fahrenheit
temperature
scales.
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• Weight: a property of matter that
depends on the mass of a substance and
the effects of gravity on that mass
• Mass: a constant property of matter
that reflects the amount of material
present
• Density: a physical property of matter
that is equivalent to the mass-per-unit
volume of a substance
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Density
• The ratio of the mass of an object to
the volume occupied by that object
– g/cm3 (solids); g/mL (liquids)
d = m/V
• Densities of solids & liquids are often
compared to the density of water
– sink or float
• Varies with temperature
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Refraction
• The bending that occurs when a light
wave passes at an angle from one
medium to another (air to glass)
– bending occurs because the velocity of
the wave decreases
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Light is
refracted
when it
travels
obliquely
from one
medium to
another.
The bending of light waves because of a
change in velocity is called refraction.
Refractive index is the ratio of the velocity
of light in a vacuum to that in the medium
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under examination.
Refractive Index (ND)
• The ratio of the velocity of light in a
vacuum to the velocity of light in a
given medium
– ND (water) = 1.333
• light travels 1.333 time faster in
vacuum than in water
• An intensive property
• Varies with temperature and the
light frequency
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Representation of the
Dispersion of Light
by a glass prism.
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Double Refraction
• Crystals refract a beam of light into
two different light-ray components
– extraordinary ray
• refracted (bent)
– ordinary ray
• path unchanged
• Causes a double image to be seen
• No double refraction with isometric
crystals
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What is Glass?
• Glass is formed upon the cooling of
a molten liquid in such a manner
that the ordering of atoms into a
crystalline formation is prevented
• materials which form glasses are
relatively rare
– SiO2 (silica) is the most common
example
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What is Glass?
• One of the oldest of all
manufactured materials
• A simple fusion of sand, soda & lime
(all opaque)
– produces a transparent “solid” when
cooled
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What is Glass?
• An extended, 3D network of atoms
which lacks the repeated, orderly
arrangement typical of crystalline
materials
• The viscosity is such a high value
that the amorphous material acts
like a solid
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Structure of Glass
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Physical Properties
• Common Properties
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hard
perfectly elastic
brittle
non-conductors of electricity
chemically stable
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Types of Glasses
• ~a thousand chemical formulations
• each has its own combination of
properties
• more than 700 compositions in
commercial use
• Most common type encountered by the
forensic scientist is “flat” glass
– glass used in windows & doors
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Borosilicate Glass
• Over 5% B2O3 added to the silica
– a heat resistant glass that expands only
~1/3 as much as silicate glass
– more resistant to breaking on rapid heating
& cooling
• Pyrex ™
– Uses
• laboratory ware & thermometers
• household glassware
• sealed-beam headlights
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Impact Fractures
• Impact causes a pane
of glass to bulge
– Side opposite the
impact will stretch
more & rupture
first
• Radial cracks are
rapidly propagated in
short segments from
the point of impact
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Impact Fractures
• Ridges will be seen as irregularities on the
broken edge of a radial crack
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Impact Fractures
• If the pane is held firmly on both sides
– a circular pattern of cracks
(concentric) will form around point
of impact
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Two bullet holes in a piece of glass.
The left hole preceded the right hole.
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Analyzing Cracks
• The penetration of window glass by a
projectile, whether it is a bullet or a
stone, produces cracks which radiate
outward (radial fractures) and encircle
the hole (concentric fractures).
• By analyzing the radial and concentric
fracture patterns in glass, the forensic
scientist can determine the direction of
impact.
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Analyzing Cracks
• A high-velocity projectile such as a bullet
often leaves a hole that is wider at the exit
side, and hence its examination is important
in determining the direction of impact.
• The direction of impact can also be
accomplished by applying the 3R Rule: Radial
cracks form a Right angle on the Reverse side
of the force.
• The sequence of impacts when there have
been successive penetrations of glass is
frequently possible to determine because a
fracture always terminates at an existing line
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of fracture.
Forensic Examination
• To compare glass fragments, a
forensic scientist evaluates two
important physical properties:
– Density
– refractive index
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Forensic Examination
• The FLOTATION METHOD is a precise and rapid
method for comparing glass densities.
• In the flotation method, a glass particle is
immersed in a liquid.
• The density of the liquid is carefully adjusted by
the addition of small amounts of an appropriate
liquid until the glass chip remains suspended in
the liquid medium.
• At this point, the glass will have the same
density as the liquid medium and can be
compared to other relevant pieces of glass
which will remain suspended, sink, or float.
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Forensic Examination
• Crystalline solids have definite geometric
forms because of the orderly arrangement of
their atoms.
• These solids refract a beam of light in two
different light-ray components.
• This results in double refraction.
• BIREFRINGENCE is the numerical difference
between these two refractive indices.
– Not all solids are crystalline in nature. For
example, glass has a random arrangement
of atoms to form an amorphous or
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noncrystalline solid.
Forensic Examination
• Ridges on radial cracks can be used to
determine on which side of pane
impact occurred
• To perform examination
– identify one or more pieces which have
cracks terminating at a point of impact
– fit these pieces onto one or more pieces
marked “inside” or “outside”
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Four R Rule
Ridges on
Radial cracks are at
Right angles to the
Rear (side opposite
the impact)
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Four R Rule
• Exceptions
– tempered glass
• “dices” without forming ridges
– very small windows held tightly in frame
• can’t bend or bulge appreciably
– windows broken by heat or explosion
• no “point of impact”
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Heat Fractures
– typical heat crack is curved
– has a smooth edge (“mirror edge”)
– no indication of point of impact
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Glass
• Fragments of glass can be compared
to determine if they originated from
the same source.
• When victim of a hit and run hit the
windshield of a car they will likely
have traces of glass fragments on
their clothing and perhaps in their
hair.
• Burglars who break glass when
entering a job also run the risk of
evidence being left on their clothing
and any tools they use in their heist.
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Glass
• Glass particles found by examining the
clothing can be compared to particles
collected from the crime scene to
determine if they have a common
origin.
• Some properties of glass include tint,
thickness, UV fluorescence, density,
and refractive index. All of these
properties must all be similar for the
questioned and known samples to
have originated from the same source.
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Glass
• Glass that is broken or shattered
into fragments during the
commission of a crime can be used
to place the suspect at the crime
scene. For example chips of
broken glass from a window can
lodge into a suspects shoes or
garments during a burglary.
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Types of Glass
• Float glass – flat glass typically used for
windows
• Tempered glass – made stronger than float
glass by introducing stress through rapid
heating and cooling. When this type of glass
breaks, it does not shatter, but fragments
into small squares with little splintering
• Laminated glass – derives its strength by
sandwiching one layer of plastic between
two pieces of ordinary window glass
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Ways They Can Be Fractured
• Glass bends in response to any force
exerted on its surface.
• When the limit of the elasticity of the
glass is reached the glass fractures. This
reveals information that can be related
to the force and direction of an impact,
and may be useful for reconstructing
events of a crime scene.
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Ways They Can Be Fractured
• Penetration of ordinary window glass by
a projectile, such as a bullet or a stone
produces a pattern in which cracks
radiate outward and encircle the hole.
These are known as radial fractures.
• Circular lines are concentric fractures.
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Collection and Preservation of
Glass Evidence
• The gathering of glass evidence at the crime
scene, and from the suspect must be thorough if
the examiner is to have any chance to
individualize the fragments to a common source.
• Every effort must be made to collect all the glass
found
• The standard reference glass should always be
taken from any remaining glass in the window or
door frames as close as possible to the point of
breakage.
• About 1 square inch of sample is adequate.
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Collection and Preservation of
Glass Evidence
• The glass fragments should be packaged in solid
containers to avoid further breakage
• If the suspects shoes or clothing are to be
examined for presence of glass, they should be
individually wrapped in paper and transmitted
to the lab.
• When a determination of the direction of impact
is in question, all broken glass must be
recovered and submitted for analysis. Where
ever possible the exterior and interior surfaces
of the glass must be indicated.
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Soil
• For forensic purposes, soil is thought
of as any broken down surface
material, natural or artificial, that
lies on or near the earth’s surface
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Examination of Soil
• Soil is not only concerned with the
analysis of naturally occurring
rocks, minerals, vegetation, and
animal matter. It also includes the
detection of manufactured
objects.(Glass, paint chips, asphalt,
brick fragments, and cinders.)
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Distinguishing Types of Soil
• Most soil can be differentiated and
distinguished by their gross appearance.
• A side by side visual comparison of color
and texture is easy to perform, and may
provide a sensitive property for
distinguishing soils that originated from
different locations
• Soil is darker when it is wet, therefore
color comparisons must be made when all
the samples are dried under identical
conditions.
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Distinguishing Types of Soil
• It is estimated that there are nearly
1,100 distinguishable soil colors
• Low power microscopic examination
reveal the presence of plant and
animal materials as well as artificial
debris
• High power microscopic examination
will help characterize minerals and
rocks present in earth materials
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Value of Soil
• The value of soil as evidence rests
with its prevalence at a crime
scene and its transfer ability
between the scene and the
criminal.
• The ultimate forensic value of soil
evidence depends on its variation
at the crime scene
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Value of Soil
• Considering the vast variety of
minerals and rocks, and the possible
presence of artificial debris in the soil,
the forensic geologist is presented with
many points of comparison between 2
or more specimens.
• The number of comparative points and
the frequency of occurrence must all
be considered before similarity
between specimens can be concluded
and the probability of common origin
can be judged.
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Collection and Preservation of
Soil
• Standard reference soils are to be
collected at various intervals within
a 100 yard radius of the crime
scene.
• Soil specimens should also be
collected at all possible alibi
locations that the suspect may claim
• A quantity of soil equal to
approximately a tablespoon or two is
all the lab needs for analysis
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Collection and Preservation of
Soil
• All specimens collected should
be packaged in individual
containers, such as plastic vials
• Each vial should be marked to
indicate location at which the
sample was taken.
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Collection and Preservation of
Soil
• Soil found on a suspect must be
carefully preserved for analysis.
If it is found adhering to an
object (soil on a shoe) the
investigator must remove it
• Each item must be individually
wrapped in paper with the soil
intact and transmitted to the lab.
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TRACE EVIDENCE can be any number of things, from a
paint chip, to a piece of glass, to plant debris. There are
numerous methods used for analysis. Some objects, have
large databases available for comparisons, while the
science of others has not advanced that far.
TRACE EVIDENCE is therefore, a foreign object or piece
of material present at a crime scene and tracing its origin
can assist in the arrest and conviction of an individual.
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