Test Friday!!!
FORENSIC ANALYSIS OF GLASS
GLASS…AN AMORPHOUS SOLID
• Physical Properties: hard, elastic, brittle, nonconductor of electricity, density, refractive index.
• Main component is silicon dioxide (SiO 2)
• Also called silica or sand
• Different types of glass are made by adding
different metals such as (Na, Ca, Mg, & Al) and
other compounds
TYPES OF GLASS
• Soda lime glass—glass bottles & windows
• Sodium carbonate (Na2CO3)—lowers
viscosity and melting point
• Calcium oxide (CaO)
TYPES OF GLASS
• Pyrex—aka Borosilicate Glass
• Boron oxide (B2O3) is added to resist
breaking when heated and cooled
(expands and contracts)
TYPES OF GLASS
• Colored glass
• Add metal oxides or colloidal iron (Fe) &
sulfur (S) to change color
• Eyeglasses
• Add potassium oxide (K2O)
• Adds hardness to the glass
TYPES OF GLASS
• Flat glass—made by a “float glass process”
• Molten glass is floated on a pool of tin while cooling
• Commonly found in doors and windows
TYPES OF GLASS
• Laminated glass—made by placing a sheet of
plastic in between two sheets of glass
• Found in windshields—holds together when broken
TYPES OF GLASS
• Tempered glass—made by using a rapid heating
and cooling process
• Found in car side windows, breaks into tiny pieces
TYPES OF GLASS
• Bullet proof glass—made from a combination of
two or more types of glass (one hard, one soft)
• Softer layer makes it more elastic so it stretches
instead of shattering
HOW DOES GLASS BREAK?
• Each force causes a deformation
that may leave a visible mark or
fracture the glass. These markings
can be used to determine
• Direction of the force, amount of
force applied, sequence of impacts
• Glass acts as an elastic surface
and bends away when an initial
force is applied. When the force
increases beyond its tensile
strength, it CRACKS
FRACTURE PATTERNS OF GLASS
• There are two types of
fracture patterns:
• Radial cracks—form FIRST
and span out in short
(spider-like) segments on
the OPPOSITE side of the
force
• Concentric cracks—come
SECOND on the SAME
side as the force applied
CLICK PICTURE FOR VIDEO OF GLASS
BREAKING
ANOTHER VIDEO CLIP…
HEAT FRACTURES
• No point of impact
• Curved or wavy fractures
• Smooth edges at break points
BULLET FRACTURES
• Bullets are a projectile force
that can pass through glass
• EXIT side of a bullet fracture is
WIDER than then entry side
• As bullet velocity increases,
central hole becomes smaller
& cracking patterns become
simpler
WHICH BULLET HOLE CAME FIRST?
• The sequence of bullet holes can be determined
by looking a the radial fractures
• Radial cracks end/are stopped by earlier
cracks
B was shot
first because
the radial
lines of A
terminate into
the radial
lines of B
WHICH HOLE WAS SHOT FIRST?
A
B
WHICH HOLE WAS SHOT FIRST?
COLLECTION OF GLASS SAMPLES
1. Identify and photograph any glass samples before
2.
3.
4.
5.
6.
moving them.
Collect the largest fragments that can be
reasonably collected.
Identify the outside and inside surface of any glass.
If multiple panes are involved, make a diagram.
Note trace evidence such as skin, hair, blood, or
fibers.
Package all materials collected to maintain the
chain of custody.
COLLECTION OF GLASS SAMPLES
7. The glass fragments should be packaged in boxes to
avoid further breakage.
8. If evidence is to be examined for glass fragments, it
should be taken whole and each item individually
wrapped in paper and boxed.
9. If even the remotest possibility exists that glass
fragments may be pieced together, every effort must be
made to collect all glass fragments.
10. Submit glass evidence along with a representative
sample of each type of glass from the crime scene.