Glass Evidence Solving the Crime

Glass Properties Combinations of formers, fluxes, and stabilizers creates unique glass types with different properties:  Density  Refractive Index RI... Refractive Index• Refers to h

Glass Evidence

Solving the Crime

Ms Scribner’s Forensic Crime Science

Class Eisenhower High School

What is glass?

 Made from fused inorganic materials

 Amorphus; not crystalline (molecules not arranged)

 Varies in elemental formula

 Many shapes and colors

What is Glass?

 No specific melting point

 Softens over a temperature range.

 Uniform amorphous solid

(irregular atomic structure – middle picture).

 Because of this, glass breaks in a variety of fracture patterns

 Stabilizers (Chem./Corrosion Resistance):

CaO, MgO, Al 2 O 3 , PbO, SrO, BaO, ZnO, ZrO

How is glass involved in crime?

 Small fragments can be used as

trace evidence (link objects/people)

 Crime scene reconstruction

How is glass collected?

 For reconstruction, detailed

photographs can be taken or the entire pane of glass is collected.

 For trace evidence, usually visually

collected with forceps and packaged

in plastic bags.

How is glass collected?

Types of Glass

light bulbs, etc.

• Made by melting SiO2 and NaCO3

• CaCO3 or CaO added so NOT

water soluble

• Density is 2.44g/cm3

Soda–lime Glass

Surface of soda lime

glass

Flint Glass

• Optical glass

• High refractive index (RI)

• RI = 1.45 to 2.00 RI = 1.45 to 2.00

• Contains 4-6% iron oxide Contains 4-6% iron oxide

• Disposal poses a pollution Disposal poses a pollution

problem

• Also used to make

simulated diamonds

Flint Glass

lenses

Lead crystal Optical

instrumen

ts

CaCO3, and boron

• Higher melting point & more

break resistant

on any test tube that you are going to heat?

Answer : Pyrex or Kimex

Why is Glass important in forensics?

Trace Evidence

• CSI investigators must work

backwards from evidence at the crime scene

• Collect blood, hair, fibers,

fingerprints, & broken glass

• Used to establish a link between the suspect and the crime

• Often present in burglaries and

hit and run accidents

Properties of Glass

Glass Properties

 Combinations of formers, fluxes, and stabilizers

creates unique glass types with different properties:

 Density

 Refractive Index (RI)

Refractive Index

• Refers to how light passes

through the glass

• Variations in the composition

of the glass cause bending of light rays

• Extent to which light bends is

called refractive index

• Standards used to determine

RI

Refractive Index

 The direction of the light forms two angles with the

normal

 If the light passes into a denser medium (the gray

area), its direction will bend toward the normal

Populations of Glass RIs

 Because of all the different materials in

glass, there will be multiple RI’s  a major one will stick out, like above.

Snell’s Law of Refraction

 Can figure out the RI of materials

based on the angle the light hits

Snell’s Law Example 1

 Light traveling in air (n1=1) hits a piece of glass at an angle of 30

degrees The light refracts in the glass at an angle of 15 degrees What is the index of refraction (RI)

Snell’s Law Example 2

 Light traveling in air (n1=1) hits a piece of glass at an angle of 45

degrees If the glass has a

refractive index of 1.5, what angle should the light bend at?

Snell’s Law Practice

 1 Light traveling in air (n 1 =1) hits a piece of glass at an angle

of 37 degrees The light refracts in the glass at an angle of 30 degrees What is the index of refraction (RI) of the glass?

 2 Light traveling in air (n 1 =1) hits a piece of glass at an angle

of 60 degrees The light refracts in the glass at an angle of 15 degrees What is the index of refraction (RI) of the glass?

 3 Light traveling in air (n 1 =1) hits a piece of glass at an angle

of 25 degrees If the glass has a refractive index of 1.75, what angle should the light bend at?

 4 Light traveling in air (n 1 =1) hits a piece of glass at an angle of 55 degrees If the glass has a refractive index of 1.3, what angle

should the light bend at?

Forensic Science: Fundamentals

& Investigations, Chapter 14 27

Density

The formula for calculating density is:

D = m / V

 The mass (m) of a fragment of glass can be

found using a balance beam device

 Place the fragment of glass into a beaker filled

with water and measure the volume (V) of overflow

 Divide the mass (in grams) by the volume (in

milliliters) to find the density (D) of the glass fragment

Window glass does not have uniform

density

• Take samples from different locations

Edge of tempered glass is denser than

the interior

FBI has reported density results for 1400 glass samples recovered from 1964-

1997.

Density Practice (Left Page)

 Find the density for the following pieces of glass:

 1 Mass: 4g, Volume Displaced: 2mL

 2 Mass: 15g, Volume Displaced: 3mL

 3 Mass: 1g, Volume Displaced: 2mL

Glass Breakage

Pattern

• Glass breaks or shatters in

patterns

• Glass fragments can fit

much the glass will break

• In a court of law, which type(s) of

glass evidence do you think

convicting a suspect?

• In a court of law, which type(s) of

glass evidence do you think

convicting a suspect?

Fractured Glass

compresses the glass

• Glass begins to break

Radial Fracture

• Fractures in a radial pattern

point outward from point of impact, forming pie-shaped sections

For reconstruction:

Radial cracks

Concentric cracks

• On side of glass where

force was applied

• Similar to spider web

 When looking at a radial

section, conchoidal

fractures point back to

the origin of the break

 Core usually on opposite

side of the origin

For reconstruction:

1 Determine the direction of projectile.

2

3

For reconstruction:

2. Determine the order of projectiles when

dealing with more than one.

2

3

For reconstruction:

2. Determine the order of projectiles when

dealing with more than one.

For trace analysis:

Done with microscopic analysis

 Color, curve, thickness, etc.

 Refractive index: Determined by placing

glass in oils of different refractive indexes

Associate unknown glass found at the

crime scene with known samples to determine the source

Refractive index:

 Low relief = no outside line R.I

of glass and oil are similar

 High relief = thick outside line (Becke

line) R.I of glass and oil are different

Question: What type or types of fractures do you

see?

Other Factors in Glass

Analysis

Heat-tempered glass (safety glass)

breaks in small, cube-like pieces

• Used in car windows

Imperfections in manufacturing glass Distinct marks/scratches (i.e caused

by debris on windshield wiper

Bullet Holes

A high-speed projectile leaves exit hole larger than entrance hole

• helps determine direction of impact

Examine fracture lines to determine

order in which penetrations of the

glass occurred

• fractures from 1st shot terminate when

they meet fractures from 2nd shot

Other Properties of Glass

• Surface striations and markings

• Flameless atomic absorption

spectrophotometry (destroys sample)