Physical Properties:
Glass and Soil
Chapter 4
Properties
• Physical - describes a substance without
reference to any other substance.
– Ie. Mass, volume, color, BP, MP
– Intensive - retains property regardless of how
much material is there
• ie. Density, MP, BP
– Extensive - dependent upon the amount of
material
• ie. Mass, length
Properties
• Chemical - describes the behavior of a
substance when it reacts or combines with
another substance; a new substance formed
– ie. Wood burning, digestion,
– Heroin test - the presence of heroin is tested by
reacting the specimen with a chemical reagent
called Marquis reagent. If heroin in present the
reagent turns purple; hence the color change
indicates a chemical property of heroin
Properties and Forensics
• Which physical and chemical properties the
forensic scientist chooses to observe and
measure will depend on the type of material
that is being examined.
• Logic requires the fact that if the property
can be assigned a numerical value, it must
be one that relates to a standard system of
measurement accepted by the scientific
community.
The Metric System
• 1791 - The French Academy of Science
• Base units for length, mass, and volume are
meter, gram, and liter respectively.
• All other subunits are multiples of 10 for
the base unit.
• Subunits are represented by a prefix.
Common Conversions
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1 mL = 1 cm3
1 inch = 2.54 cm
1 meter = 39.37 inches
1 m = 453.6 grams
1 liter = 1.06 quarts
1 kilogram = 2.2 pounds
Temperature
• Fahrenheit and Celsius Scales
– C = (1.8*F) + 32
– F = (C/1.8) - 32
• 0 C = 32 F
• 100 C = 212 F
Weight and Mass
• Weight - the force with which gravity
attracts a body; dependent upon its location
• Mass - the amount of matter an object
contains; independent of its location on
earth
• W = m*g
• Weight and mass are used interchangeably
Density
• The most important physical property of
matter with respect to the analysis of certain
kinds of physical evidence.
• Density = mass/volume
• Intensive property
• Solids - volume measured by submersion;
independent of temperature
Density
• Liquids and gases - very dependent on
temperature so temperature must be
recorded at time of analysis
• Observation of whether objects sink, float,
or suspend is an indication of density.
Refractive Index
• Refraction - the bending of a light wave as
it passes from one medium to another.
• The refraction is caused by the change in
velocity of the wave.
• The ratio of the velocity of light in a
vacuum to its speed in a given substance
determines the refractive index of that
substance.
Refractive Index
• Refractive Index = velocity of light in vacuum
velocity of light in medium
ie. 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 at 25 degree Celsius.
• Refractive index is an intensive physical
property.
Refractive Index
• Procedures used to determine a substance’s
refractive index must be performed under
carefully controlled temperature and lighting
conditions, because the index of a substance varies
with its temperature and the frequency wavelength
of light passing through it.
• Nearly all indices are determined at the
wavelength commonly emitted by sodium light;
589.3 nm; known as Sodium D light
Crystalline Solids
• Crystalline solids have definite geometric
forms because of the orderly arrangement of
the fundamental particle of a solid; the
atom.
• Atom - the smallest unit of an element; not
divisible by ordinary chemical means.
• In any type of crystal, the relative location
and distance between its atoms are
repetitive throughout the solid.
Amorphous Solids
• Solids in which the constituent atoms or
molecules are arranged in random or
disordered positions. There is no regular
order in amorphous solids.
• ie. glass, plastic, gel
Double Refraction
• Crystals that refract a light beam into two different
light-ray components.
• The two light rays are refracted at different angles
which in turn gives two different refractive index
values.
• The difference of the two indices is known as the
birefringence.
• Thus, the optical properties of crystals provide
points of identification that will help characterize a
crystal.
Dispersion
• The separation of light into its component
wavelengths.
• The ability of a prism to disperse light into its
component colors is explained by the property of
refraction.
• Each color component of light, on passing through
the glass, will be slowed to a speed slightly
different from the others, thus causing each
component to bend at a different angle as it
emerges from the prism.
Comparing Glass Fragments
• Glass that is broken and shattered into
fragments and minute particles during the
commission of a crime can be used to place
a suspect at the crime scene.
• Chips of glass may be found imbedded in
the shoe or clinging to the clothing;
headlight glass can be matched to the
suspect vehicle.
Glass
• Defined: a hard, brittle, amorphous
substance that is composed of silicon oxides
mixed with various metal oxides.
– Main ingredient - silicon dioxide (SiO2)
– Adding Na2CO3 lowers the melting point and
viscosity making the substance easier to work.
– Common metal oxides: sodium, calcium,
magnesium, and aluminum
Specialty Glass
• Pyrex - Replacing some of the silica, sodium or calcium
oxides with boron oxide allows for a more heat resistant
glass.
• A.k.a. - borosilicates
• Tempered glass - made stronger by introducing stress
through rapid heating and cooling of glass surfaces.
Prevents glass from shattering, instead it breaks into small
square fragments. ie…back and side car windows.
• Laminated glass - gains strength by sandwiching a
layer of plastic between two layers of glass. Windshields.
Evidential Value - Glass
• Greatest value when it can be individualized
to one source.
• To do this the glass from the suspect and the
crime scene must be assembled and
physically fitted together.
• Not only do the irregular edges match but
also the glass ream patterns.
Characterizing Glass
• Physical properties of density and refractive
index are used.
• Density - flotation method; a
standard/reference sample is suspended in a
liquid mixture of known density, then a
similar piece of suspect glass is immersed in
the same liquid. If it behaves the same then
the densities of the two pieces are similar.
Characterizing Glass
• Similar density results also required additional comparison
of refractive indices.
• Refractive Index - Immersion method; glass particles are
immersed in liquid medium whose refractive index is
varied until it is equal to that of the glass particles.
• When the indices are equal the glass particle will not be
visible and the Becke line will also disappear.
• Becke line - a bright halo that is observed near the border
of a particle immersed in a liquid of a different refractive
index.
• Match Point - the point at which the refractive indices are
equal.
Probabilities
• The FBI has started to collect density and
refractive indices values for glass submitted
to the lab for examination.
• This means that a criminalist may correlate
the density and refractive index values to
their frequency of occurrence and can make
a meaningful assessment as to the
probability that the fragments were at one
time from the same source.
Glass Fractures
• Fracture patterns tell whether the breakage
was due to a high velocity bullet, low
velocity bullet, or another type of projectile
or object.
• The direction of impact may also be
determined from the close examination of
the glass breakage.
• It is also possible to determine the sequence
of fractures when more than one is present.
Fracture Patterns
• Conchoidal - shell-like fracture patterns in
glass are a result of stress on the glass and
breakage due to application of a force.
– Radial - extend outward in a line from the point
on the glass where the force originated.
• Radial cracks form a Right angle on the Reverse
side of the force (3R rule).
– Concentric - make circular patterns around the
point of origin of the force.
• Concentric cracks form right angle on the same side
of the force.
Fracture Patterns
• An object traveling at high velocity will
cause a small hole in the impact side and a
larger hole on the exit side.
• The slower an object is traveling the less
symetrical the breakage and the less likely
to contain usable data to determine direction
of impact.
• More than one impact - the fracture always
terminates at an existing line of fracture.
Collection and Preservation
• If there is even the remotest possibility that
the fragments may be pieced together, every
effort must be made to collect ALL glass.
• Improbable for putting together, then all the
pieces from the suspect plus
standard/reference samples must be
collected from the scene for comparison.
Soil Characteristics
• Soil - including any disintegrated surface material,
both natural and artificial, that lies on or near the
earth’s surface.
• Forensic examination includes not onlynaturally
occurring rocks, minerals, vegetation, and animal
matter, but also the detection of such
manufactured objects as glass, paint chips, asphalt,
brick fragments, and cinders.
• The presence of the above items may impart soil
with characteristics that will make it unique to a
particular location.
Soil
• Most soils can be differentiated and distinguished
by their gross appearance.
• Side-by-side visual comparison of the color and
texture of soil specimens is easy to perform and
provides a sensitive property for distinguishing
soils that originate from different locations.
• All soils must be dried under the same laboratory
conditions to assure color comparison.
• Approximately 1,100 distinguishable soil colors,
hence, color comparison is a logical first step.
Soil
• A mineral is a naturally occurring crystal that
is commonly found in soils.
• More than 2,200 minerals are known, however,
only approx. 40 are common to soils.
• Rocks are characterized by their mineral
content and grain size.
• Rocks and minerals are also used to
manufacture a large variety of industrial and
commercial products.
Soil Analyses
• Density-gradient tube is used to compare soil samples.
– A glass tube filled with liquids of varying densities; soil
separates out so that similar soil particles suspend in the
liquid with the similar density.
• Because many soils may have similar density gradients the
density-gradient test results must be used in combination
with other tests.
• Forensic geologists try to individualize soil to any one
location by determining the variation in the soil at the
crime scene.
• The more the soil varies the more individual the
characteristics become.
Collection and Preservation
• Standard/reference samples must be taken at
various intervals within a 100-yard radius of
the crime scene, as well as the crime site.
• Soil samples are also taken at all possible
alibi locations that the suspect claims.
• Generally speaking, only the top-surface
soil would be picked up by the suspect,
therefore that is the only type of soil that
should be collected for the comparison.
Collection and Preservation
• Soil found on different clothing or locations
on the suspect must be packed separately.
• Lumps of soil are to be kept intact. The
lumps tend to gather soil layers from
numerous locations over the period of time,
this could enable the scientist to
individualize the soil and in turn give the
soil more evidential value.
Vocabulary
Vocabulary
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Amorphous solid
atom
Becke line
birefringence
Celsius scale
chemical property
concentric fracture
crystalline solid
density
density-gradient tube
dispersion
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Fahrenheit scale
intensive property
extensive property
laminated glass
mass
mineral
physical property
radial fracture
refraction
refractive index
tempered glass
weight