Physical Properties:
Glass & Soil
Chapter 4
Physical & Chemical
Properties
 Physical properties describe a substance without
reference to any other substance.
 Ex. Weight, volume, color, boiling point, & melting point
 Chemical property describes the behavior of a
substance when it reacts or combines with another
substance.
 Ex. Wood burning or to test for heroin chemical reagent
Marquis reagent turns purple when positive
Comparing Glass Fragments
•Glass that can not be physically  Glass is hard, brittle,
pieced together glass is compared amorphous that is made sand,
silicon oxides & metal oxides
with refractive index and
 Borosilicates use boron oxides
density
to increase heat resistance ex)
pyrex, headlights
 Tempered Glass- increased
strength by adding stress
through rapid heating &
cooling; it doesn’t shatter, it
dices into small squares ex)
side and rear windows of cars
 Laminated glass- 2 sheet of
glass sandwiching plastic
Methods for Distinguishing Glass
 Density
Densities of Some Common
Materials
Methods for Distinguishing Glass
 Refractive Index
 When light travels
from one medium to
another its speed
changes relative to the
density of the medium.
This can be observed
as the light bends
when traveling from
one medium to
another.
Refractive Index
Refractive Index Tutorial
Methods for Determining
Refractive Index
 The FBI has a database
off over 2000 refractive
indexes of different types
of glass which shows that
glass is very distinctive
and helps assign an
appropriate statistical
probability that the two
pieces of glass share a
common source.
Methods for Determining
Refractive Index
 Becke Line
Methods for Determining
Refractive Index
 Refractive Index changes with temperature
Glass Fracture Patterns
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
Glass Fractures
 Fracture pattern of glass is radial (extends outward like spoke o a
wheel) & concentric lines (rough circle around point of impact)
 Crater-shaped hole in glass made by penetration indicates the exit
side of glass
 Stress marks on the edge of a radial crack near the point of impact
are parallel to the side on which the force was applied
Glass Fracture Patterns
Glass Fracture Patterns
Glass Fracture Patterns
3R rule – Radial cracks form a Right angle on the Reverse side
of the force
Stress Marks
 Stress marks on the edge
of a radial crack near the
point of impact are
parallel to the side on
which the force was
applied
Three ‘R’ Rule
 Radial cracks are at Right angles to
the Rear (side opposite the
impact)
 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”

Scenario: A storefront window is broken and a
robbery committed. A suspect is later found
running from the scene. Examination of his
shoes reveals glass particles embedded in a
heel. Describe the proper collection and
preservation of glass evidence for laboratory
examination.
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The entire shoe is to be
submitted for lab
examination. The evidence
collector is to avoid removing
glass evidence from the shoe
unless there is a distinct
possibility that the glass will
be lost in transit.
Shoes are to be packaged in
a box or paper bag and sent
to the lab.
A control glass from the
broken window is to be
submitted for laboratory
examination. Normally a
square-inch piece of glass
will suffice.
A pillbox, druggist fold or vial
are convenient containers for
small glass fragments.
All packages are to be
properly labeled for id.
Minimal info should contain
evidence collector’s name,
date and sampling location.
All evidence are to be
described in field notes.
Collection of Glass Evidence
 Standard reference glass should be taken
from the crime scene (1 in2)
 Package in solid containers to prevent
breakage
 Preserve garment (shoe, pants, shirt) with
glass on it
Comparing Soils
 Mineral: 2,200 naturally occurring crystal
 Physical properties: color, geometric shape,
density, refractive index (birefringence)
 The difference between the two indices of refraction
for calcite: 1.486 & 1.658
birefringence for calcite is 0.172
 Density-gradient tube: a glass tube filled
from bottom to top with liquids of
successively lighter densities; used to
determine the density destruction of soil
Soil Characteristics
 Mixture of:
 Weathered rock and minerals (quartz, mica,
feldspar)
 Humus (decomposed organic material)
 Other vegetable matter (leaves, pine needles,
pollen, etc)
 Manufactured materials (asphalt, paint, glass)
Soil Analysis
 1st – Gross Analysis
 Color and Texture
Analysis
 1,100 distinguishable
colors of soil
 Darker when wet
Soil Analysis
 2nd – Microscopic
Analysis
 Rock and Mineral
Analysis
 Minerals have physical
properties like color,
crystal shape, density,
and refractive index
 2,200 minerals
(40 common)
Soil Analysis
 Microscopic Analysis
con’t
Humus
Pollen
Collection of Soil Evidence
 Collect samples at various intervals in a
100 yard radius of crime scene
 Top surface of soil is usually enough
 Preserve garment (shoe, pants) with soil
on it
 Note layering of soil (as on car tires) when
collecting soil evidence
Scenario: A suspect has been found
miles from the scene of a murder. Soil
is found adhering to the suspect’s
shoe. Describe the proper collection
and preservation of soil evidence that
will permit a thorough comparison of
the soil on the shoe to soil at the crime
site.
 Do not remove soil from shoes. Each object that
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has soil on it is to be packaged in a leak proof
container
Collect representative control soils at the crime
scene and at various locations within a 100 yard
radius of the scene. Package in solid containers
or druggist folds
Collect soil at all possible alibi locations the
suspect may claim.
Collect approximately 1-2 tablespoons of soil,
top layer only.
Label all specimens collected. Evidence
collector’s name or initials, the date and
sampling location are to be shown. All items
collected are to be described in the evidence
collector’s field notes.
The End