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FORENSIC ANALYSIS OF GLASS

GLASS…AN AMORPHOUS SOLID• Physical Properties: hard, elastic, brittle, non-

conductor of electricity, density, refractive index.

• Main component is silicon dioxide (SiO2)

• 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

• 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

TYPES OF GLASS

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?

1. Identify and photograph any glass samples before moving them.

2. Collect the largest fragments that can be reasonably collected.

3. Identify the outside and inside surface of any glass. 4. If multiple panes are involved, make a diagram. 5. Note trace evidence such as skin, hair, blood, or

fibers. 6. Package all materials collected to maintain the

chain of custody.

COLLECTION OF GLASS SAMPLES

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.