Glass EvidenceSolving 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.

What’s in Glass? Formers (form glassy, non-crystalline

structure):

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

Fluxes (lowers melting point):Na2O, K2O, LiO, Al2O3, B2O3, Cs2O

Stabilizers (Chem./Corrosion Resistance):CaO, MgO, Al2O3, 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 GlassTypes of Glass

Soda–lime Glass

•Most prevalent type of glass• Inexpensive to process•Used for window panes, bottles,

mirrors, data storage disks, 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 GlassFlint Glass• OpticalOptical glass glass• HighHigh refractive index (RI) refractive index (RI)• RI = RI = 1.45 to 2.001.45 to 2.00• Contains Contains 4-6% iron oxide4-6% iron oxide• Disposal poses a Disposal poses a pollution pollution

problemproblem• Also used to make Also used to make

simulated diamondssimulated diamonds

Flint GlassFlint Glass

lenses

Lead crystal

Optical instrumen

ts

Borosilicate GlassBorosilicate Glass

•Heat-resistant glass•First sold under brand name

“Duran”•Called Pyrex by 1915•Made of quartz, NaCO3,

CaCO3, and boron•Higher melting point &

more break resistant•Density 2.23 g/cm3

Borosilicate GlassBorosilicate Glass

Question:

In lab, what label(s)

should be on any test tube that you are going to

heat?Answer: Pyrex or Kimex

Why is Glass important in forensics?

Trace EvidenceTrace 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 GlassProperties of Glass

Glass Properties Combinations of formers, fluxes, and stabilizers

creates unique glass types with different properties: Density Refractive Index (RI)

Refractive IndexRefractive 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

Refraction Index

https://www.youtube.com/watch?v=eX4JqYHjthE

https://www.youtube.com/watch?v=s3EK1lGkf2s

Disappearing glass video: https://www.youtube.com/watch?v=qH1S83Bkttw

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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 Because of all the different materials in glass, there will be multiple RI’s glass, there will be multiple RI’s a major a major one will stick out, like above.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 and bends.

n1sin(θ1) = n2sin(θ2)

n1 = RI of incident light

θ1= angle of incident light

n2 = RI of refracted light

θ2= angle of refracted light

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) of the glass?

n1=1, θ1= 30, θ2= 15, n2=?

1sin(30)=n2sin(15)

0.5=0.2588n2

n2 = 1.93

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?

n1=1, θ1= 45, θ2= ?, n2=1.5

1sin(45)=1.5sin(θ2)

0.7071=1.5sin(θ2)

.4714= sin(θ2)

θ2= 27 degrees

Snell’s Law Practice 1. Light traveling in air (n1=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 (n1=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 (n1=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 (n1=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

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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.

DensityWindow 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 Glass Breakage PatternPattern

•Glass breaks or shatters in patterns

•Less quantitative (numerical data) and more qualitative

•Glass fragments can fit together like a jigsaw puzzle

•Composition determines how much the glass will break

Question:Question:

• In a court of law, which type(s) of glass evidence do you think would be the most helpful in convicting a suspect?

• In a court of law, which type(s) of glass evidence do you think would be the least helpful in convicting a suspect?

Fractured GlassFractured Glass

Shattered GlassShattered Glass•When glass is

struck, it breaks in a specific manner

•Applied force compresses the glass

•Glass begins to break OPPOSITE the applied force

•Forms 2 types of fractures – radial & concentric

Radial FractureRadial Fracture

•Fractures in a radial pattern point outward from point of impact, forming pie-shaped sections

For reconstruction:

Radial cracks

Concentric cracks

Concentric FracturesConcentric Fractures

•Secondary breaks/fractures

•Occur between the radial fractures

•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.

1

2

3

For reconstruction:

2. Determine the order of projectiles when dealing with more than one.

1

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 Question: What type or types of fractures do you types of fractures do you

see?see?

Other Factors in Glass AnalysisHeat-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 HolesA 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

• Surface contaminants

• Thickness

• Hardness

• Color

• Elemental analysis• scanning electron microscope• X-ray analysis• Flameless atomic absorption

spectrophotometry (destroys sample)