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Shattered: Forensic Glass

Analysis

What is Glass? “An inorganic product of fusion which has

cooled to a rigid condition without crystallizing”

Uniform amorphous solid

No specific m.p.

Softens over a temperature

range

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'Ordinary' sheet glass

-made by passing the molten glass through rollers; process gives an

almost flat finish but with distortions.

-glass is used in glazing greenhouses and garden sheds where the visual

distortions do not matter.

Float glass (plate)

Float glass gets its name from the method of production used to

manufacture it.

-molten glass is 'floated' onto a bed of molten tin

-produces a glass which is flat and distortion free

-process imparts a fluorescence to the glass [UV light]

Rolled Glass Float Glass

Float Glass Process

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Toughened (Safety glass)

-glass is produced by applying a special treatment to ordinary float

glass after it has been cut to size and finished.

-treatment involves heating the glass so that it begins to soften (about

620 C) and then rapidly cooling it.

-produces a glass which, if broken, breaks into small pieces without

sharp edges.

Laminated glass

-glass is made up of a sandwich of two or more sheets of glass (or

plastic), bonded together by a flexible, normally transparent

material.

-if cracked or broken, the flexible material is designed to hold the

glass fragments in place.

What’s in Glass?

Network Components-Formers:

SiO2, B2O3, P2O5, GeO2, V2O5, As2O3, Sb2O5

Fluxes–Softeners [lowers melting point]:

Na2O, K2O, LiO, Al2O3, B2O3, Cs2O

Stabilizers–Chemical/Corrosion Resistance:

CaO, MgO, Al2O3, PbO, SrO, BaO, ZnO, ZrO

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Major Types and Uses

Soda lime silicate glass

SiO2 + Na2O / K2O + CaO / Al2O3 / MgO

Flat glass, container glass, electric light

bulbs

Borosilicate glass

> 5% B2O3 (replaces Na2O)

Lab glassware, thermometers, cookware,

sealed-beam headlights

Aluminosilicate glass

Higher percentage of aluminum

higher temperatures than borosilicate

Aluminoborosilicate – 50:50 Al/B

Labware, cookware, and glass fibers

Lead alkali silicate (leaded glass)

Up to 80% PbO

high refractive index

“Crystal” tableware,

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Different glasses-domestic windows, car headlamps,

light bulb glass-are produced differently from different

chemistries.

Forensic Individualization of Glass

Flat Glass–Soda lime silicate

-Rolling or Floating

Tempered: Rapid Cooling

Adds Strength

Dices when broken

Automotive windows & security windows

Float Glass floresces when excited at 254 nm.

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Coated: Surface modification

Mirrors

Laminated: Sandwiched around plastic

Automotive windshields

Headlights: often borosilicate

Light bulbs: soda lime glass

Heat absorbing/ UV filtering

tinting

Eyeglasses: prescription

lenses/photosensitive

Container Glass

Lower magnesium, higher sodium

Clear vs. greenish (window)

Glass Fibers

Fiberglass insulation

Alumino-borosilicate

Binder (red or yellow) to hold fibers in

bundles

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Forensic Analysis of Glass

Fundamental Question: Is it Glass?

Isotropic vs. Anisotropic

Glass: Isotropic dark when rotated under crossed polarizers

Crystalline solids and plastics: Anisotropic Change retardation when rotated under crossed

polarizers

Hardness

Solubility

Comparison of Physical

Characteristics Color

Flatness

Thickness

Fluorescence–Float Glass

254 nm

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Physical Properties Density

Sink-Float Method

CHBr3, C2H2Br4, Sodium Polytungstate

Density Range:

2.465-2.540 g/cm3

When 2 substances have different RI, light passing through them produces

a Becke Line

A Becke line is a band or rim of light visible along a grain/crystal boundary

in plane-polarized light.

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Glass analysis is often

performed using our old

friend the PLM

-the goal of glass analysis

by PLM is to individualize an

evidence glass fragment to an

exemplar from a crime scene

[primary or secondary]

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Populations of Glass RIs

GRIM II System -automated system for matching glass RI

-uses a step-heating stage and a RI liquid that varies

with temperature

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GRIM II identifies the RI match by monitoring a

video image of the glass fragment in the liquid

-as it is heated/cooled the contrast is measured until

a minimum is reached-the match point

Chemical Composition –

Inductively Coupled Plasma (ICP)

SEM/EDS

Neutron Activation Analysis

X-Ray Fluorescence

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How does Glass break? Forensic Fracture Analysis

Radial

Begin at a point and radiate

outward from point of impact

-initial crack is on the side

opposite the applied force

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Concentric

Motion of the projectile through the glass

puts tension on the front surface of the glass,

causing concentric fractures.

-high-speed projectile [a rock] penetrates the

glass, it leaves an exit hole larger than it's

entrance hole.

-helps to determine the direction on impact.

A determination of the direction of

force in breaking a window pane:

-direction of the rib marks

[stress marks on broken edges of glass

that are perpendicular to one side of

glass]

For radial fractures (radiating from the

center):

- the direction of the force is on the

same side as the tangential parts of

the rib marks.

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Bullets and Glass

It’s possible to determine

the bullet's direction by

noting the side of the cone-

shaped hole left by the

bullet.

-small opening is on the

entrance

side

Counting Bullet Holes

Determination of the sequence of

bullet holes can be made by noting

the radial fractures.

Radial fractures caused by the

passage of a bullet stop at pre-

existing fracture.

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