Glass Analysis

GLASS

Physical Properties

the forensic scientist must constantly determine those properties that impart distinguishing characteristics to matter, giving it a unique identity.

properties are the identifying characteristics of substances

physical properties describe a substance without reference to any other substance

example: weight, volume, color, boiling point, melting point

can be measured for a particular substance without altering the material’s composition

associated only with the physical existence of that substance

chemical properties describe the behavior of a substance when it reacts or combines with another substance

examples: - when wood burns, it combines with oxygen in the air to form new substances

- Marquis reagent turns purple in the presence of heroin

Physical Properties

Temperature

- measure of heat intensity

- temperatures at which a substance melts or boils are readily determinable by characteristics that will help identify a substance

Weight and Mass

- weight: force with which gravity attracts a body

W = mg

- mass: amount of matter an object contains Density

- most important physical property of matter with respect to analysis

- mass per unit volume

- an intensive property of matter

Refractive Index

- refraction: bending of light wave because of change in velocity

= V of light in vacuum

V of light in medium- example: at 25oC, RI of water is 1.333.

Comparing Glass Fragments

Glass that is broken and shattered into fragments during the commission of a crime could be used to place a suspect at the CS.

Glass – a hard, brittle, amorphous substance that is composed of silicon oxides mixed with various metal oxides

= sand + metal oxides… melted at high temperature; cooled to a rigid condition without crystallization

by adding Na2CO3 to the sand, melting point and viscosity are both lowered making it easier to work

sand + Na2CO3 + CaO = soda-lime glass

= used for manufacturing most window and bottle glass

= common metal oxides: Na, Ca, Mg, Al

Special glass (substituting Si and other M.O.)= automobile headlights and heat resistant

glass (Pyrex) uses boron oxide [borosilicates] Tempered glass: made stronger by introducing

stress through rapid heating and cooling of the glass surfaces

= when it breaks, it does not shatter but fragments into small squares

= used in the side and rear windows (US)

Laminated glass: sandwiched one layer of plastic between two ordinary window glass

= windshield (US)

FS problem: need to find and measure those properties that will associate one glass fragment with another while eliminating the existence of other sources

density and refractive index are used BUT, these properties are CLASS

CHARACTERISTICS.

Forensic analysis of glass

Physical, fracture matching Best evidence, but very rare

Density determination Float test Results subjective

Refractive index (R.I.) determination Chemical testing, destructive

Test for silicates, metal oxides, trace evidence

Density

Can be measured

Can be “observed” by placing in a “density tower” May not be the most accurate, subjective

Dm

v

Refractive index (R.I.) measurement

Becke - immersion method Emmons & Winchell - temp. variation

method Saylor - double diaphragm contrast method Oettle - phase contrast method

Refractive index measurement

Refraction is the change of direction and/or speed of light as it passes from one medium to another

RI is a ratio of the speed of light between two mediums (e.g. glass & oil)

RI is dependant on: The wavelength of light The temperature of the medium

RI measurement technique

RI of material varies with temp. The change in RI for liquids is in the order of 10-4

RI units The change in RI for solids is in the order of 10-6 RI

units What does this mean?

When the temp. of a liquid is changed, the RI changes rapidly, but the RI of an immersed solid will not

RI measurement technique

Silicon oil usually used Oil is calibrated so RI can be determined

from its temp. Sample glass is immersed in oil Oil is heated/cooled to determine match

temp. Glass “disappears”

Oil RI = Glass RI

RI measurement using Becke line

Bright halo observed around glass when oil has higher RI

Bright halo observed inside the edge of the glass when oil has lower RI

When oil & glass = RI, glass “disappears”

Becke line

Glass has higher refractive index

Becke line

Glass has lower refractive index

Common liquid refractive indices

Liquid Refractive index

Ethyl acetate 1.373

n-butyl alcohol 1.402

Olive oil 1.467

Corn oil 1.473

Castor oil 1.482

Methyl salicylate 1.522

Clove oil 1.543

Canola oil 1.465-1.467

Glass Fracture Patterns

Two parts: Radial Concentric

Can tell direction of force from these

Determining direction of force

Radial edge of glass 3 R’s

Radial cracks have Right angles

on the Reverse side of applied force

Concentric edge of glass

Direction of force

Direction of force

Procedures in Analyzing Glass

FLOTATION a solid particles will either float, sink or remain

suspended in a liquid depending on its density relative to the liquid medium

procedure: a standard/reference glass is immersed in a liquid

composition of the liquid is carefully adjusted by the addition of bromoform or bromobenzene until the glass chip remains suspended

the standard/reference glass and liquid have the same density

glass chips of approximately the same size and shape as the SR are added to the liquid

if they both remain suspended, their densities are equal to each other and the liquid

liquid is transferred to a density meter

IMMERSION METHOD added comparison of refractive indices procedure: glass particles are immersed in a

liquid medium whose refractive index is varied until it is equal to that of the glass particles = match point

you will note the disappearance of the Becke line

GLASS FRACTURES

glass bends in response to any force that is exerted on any one of its surfaces

when the limit of elasticity is reached, the glass fractures

fractured window glass reveal information that can be related to the force and direction of impact = useful in crime scene investigation

penetration of ordinary window glass by a projectile (bullet or stone) produces a familiar fracture pattern

radiating lines = radial fractures circular lines = concentric fractures

it is difficult to determine just from size and shape of a hole in glass whether it was made by a bullet or some other projectile

high velocity projectile often leaves a round, crater-shaped hole that is surrounded by symmetrical pattern of radial and concentric cracks

hole is wider on the exit side

but as the velocity of the penetrating projectile decreases, the irregularity of the shape of the hole and of its surrounding cracks increases

hole shape could no longer determine the direction of impact

when a force pushes on one side of a pane of glass, the elasticity of the glass permits it to bend in the direction of the force applied

once elastic limit is exceeded, the glass begins to crack

the first fractures form on the surface opposite to the force

these first fractures develop into radial lines

continued motion of the force places tension on the front surface of the glass resulting in concentric cracks

stress marks are shaped like arches that are perpendicular to one glass surface and curved nearly parallel to the opposite surface

perpendicular edge always faces the surface on which the crack originated

Radial cracks form a Right angle on the Reverse side of the force

this could not be applied to broken tempered glass

when there have been successive penetrations, it is possible to determine the sequence of impact

A fracture always terminates at an existing line of fracture

LEFT preceded the RIGHT