Unit 4:Forensic Glass AnalysisIntroduction to Forensic Science

Forensic Glass Analysis

1. Composition of Glass

2. Types of Glass

3. Comparing GlassA. Density

B. Refractivity

4. Glass Fracture Patterns

5. Collecting Glass As Evidence

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1. Composition of Glass• Glass is a hard, brittle, amorphous material

– Amorphous (without shape) because its atoms are arranged randomly

– Due to its irregular atomic structure, it produces a variety of fracture patterns when broken

– Brittle because it breaks easily if struck in the right direction

• Has numerous uses and thousands of compositions

1. Composition of Glass (continued)

• Made by melting the following ingredients at extremely high temperatures:

– Sand also known as silica or silicon dioxide (SiO2), is the primary ingredient

– Lime or calcium oxide (CaO) is added to prevent the glass from becoming soluble in water

– Sodium oxide (Na2O) is added to reduce the melting point of silica or sand

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1. Composition of Glass (continued)

• Three categories of substances found in all glass– Formers

• Makes up the bulk of the glass• Examples: silicon dioxide (SiO2) in the form of sand, boron

trioxide (B2O3), and phosphorus pentoxide (P2O5)– Fluxes

• Change formers’ melting points• Examples: sodium carbonate (Na2CO3) and potassium

carbonate (K2CO3)– Stabilizers

• Strengthen the glass and make it resistant to water• Calcium carbonate (CaCO3) is the most frequently used

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1. Composition of Glass (continued)

• The raw materials for making glass are all oxides (contain oxygen)– The composition of a sample can be expressed in

percentage of different oxides– Example: the approximate composition of window or

bottle glass is• Silica (SiO2) – 73.6 %

• Soda (Na2O) – 16.0 %

• Lime (CaO) – 5.2 %

• Potash (K2O) – 0.6 %

• Magnesia (MgO) – 3.6 %

• Alumina (Al2O3) – 1.0 6

Video Clip – How Glass is Made

• 6:37 min• https://

www.youtube.com/watch?v=IjNusHQOhTM • 5:04 min Plate Glass • https://

www.youtube.com/watch?v=PSurxsGQL90

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Glass Analysis

1. Composition of Glass

2. Types of Glass

3. Comparing GlassA. Density

B. Refractivity

4. Glass Fracture Patterns

5. Collecting Glass As Evidence

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2. Types of Glass

There are many different types of glass. We will discuss the following five types:

A. Obsidian

B. Soda Lime

C. Leaded

D. Tempered

E. Laminated

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2. Types of GlassA. Obsidian is a natural form of glass that is

created by volcanoes

B. Soda-lime glass– The most basic, common, inexpensive glass –

also the easiest to make– Used for manufacturing windows and bottle glass

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C. Leaded glass– Contains lead oxide which makes it denser– Sparkles as light passes through it (light waves

are bent) – Used for manufacturing fine glassware and art

glass (stained-glass windows)– Is commonly called crystal

2. Types of Glass

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Sparkles

2. Types of GlassD. Tempered glass

– Stronger than ordinary glass – Strengthened by introducing stress through a cycle

of rapid heating and cooling of its surface– When broken, this glass does not break into large

shards, but fragments or breaks into small squares– Used in the side and rear windows of automobiles

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2. Types of GlassE. Laminated glass

– Constructed by bonding two ordinary sheets of glass together with a plastic film

– Prevents shattering– Also used by automobile manufactures

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Video Clip

• 5:34 min – Making Laminated Glass• https://

www.youtube.com/watch?v=fg3moEI9V5g • 0:54 min – Laminated Glass Safety Test• https://

www.youtube.com/watch?v=RVRAA4VoAk0 • 5:34 min – Making Technical Glass • https://

www.youtube.com/watch?v=ixkZMyQ3Gac 14

Glass Analysis

1. Composition of Glass

2. Types of Glass

3. Comparing GlassA. Density

B. Refractivity

4. Glass Fracture Patterns

5. Collecting Glass As Evidence

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3. Comparing Glass

• Investigation/Analysis includes– Finding– Measuring– Comparing

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3. Comparing Glass Individualized Characteristics

– Only occurs if the suspect and crime scene fragments are fit together exactly, like a puzzle

– This would require matching broken edges AND irregularities and striations on the broken surfaces

– Most glass evidence is either too fragmented or too small to permit a comparison of this type

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So, what type of evidence is glass?

3. Comparing Glass (continued)

Class Characteristics • (Density and Refractive Index)

– General composition of glass is a class characteristic

– Trace elements in glass may prove to be distinctive and measureable characteristics, but still class

– The physical properties of density and refractive index are used most successfully for characterizing glass particles, but only as a class characteristic

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Glass Analysis

1. Composition of Glass

2. Types of Glass

3. Comparing GlassA. Density

B. Refractivity

4. Glass Fracture Patterns

5. Collecting Glass As Evidence

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3A. Methods of Comparison:Density

• Density is defined as the mass per unit volume. (D = M/V)– Density is an intensive property of matter,

meaning it remains the same regardless of sample size.

– Density is considered a characteristic property of a substance and can be used as an aid in identification

3A. Methods of Comparison:Density

Density comparison– Density (D) is calculated by dividing the mass (M)

of a substance by its volume (V)

D = M ÷ V

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M

D V

3A. DensityDensity comparison (continued)• Step 1: Find the sample’s mass in grams using a

balance or scale

• Step 2: Find the sample’s volume by water displacement in a graduated cylinder– Record the initial volume, then add the sample– Record the final volume– Sample volume = final – initial

• Density can now be calculated from the equation in units of

grams per milliliter 22

Read the bottom of the meniscus, and estimate between marks. This is 52.5 milliliters (mL)

Lab 4.1: Comparing Density of Glass

Glass Analysis

1. Composition of Glass

2. Types of Glass

3. Comparing GlassA. Density

B. Refractivity

4. Glass Fracture Patterns

5. Collecting Glass As Evidence

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3B. Refractivity

Let’s talk about light. What do you know?

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3B. Refractivity • Light waves travel in air at a constant velocity until

they enter another medium (such as glass or water). • Upon entering a new medium they are suddenly

slowed, which causes the light rays to bend.

3B. Refractivity

• The bending of light waves because of a change in velocity is called refraction.

• The Refractive Index for a medium is a measure of how much light slows (or speeds up) upon entering that medium.

• For example:– At 25oC the refractive index of water is 1.333. – This means that light travels 1.333 times faster in a

vacuum than it does in water.

3B. Methods of Comparison:Refractivity

Refractive Index – A measure of how much an object slows light*

• Light slows down when it passes through any medium (the denser the medium, the slower the light travels)

• Any object that transmits light has a refractive index• Intensive property. Can be used to identify a substance.

Refractive index = velocity of light in a vacuum ÷ velocity of light in a medium

*compared to light in a vacuum28

3B. Methods of Comparison:Refractivity (continued)

When light passes through media (substances) with different refractive indexes, refraction (bending of light) occurs– Refraction is why objects appear bent or

distorted underwater– Every liquid has its own refractive index– If a piece of glass is placed in a liquid with a

different refractive index an outline of the glass is clearly visible• This line is known as a Becke Line

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3B. Methods of Comparison:Refractivity (continued)

When light passes through a piece of glass placed in a liquid with the same refractive index:– The glass bends light at the same angle as the liquid– The Becke Lines disappear– The glass seems to disappear

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Video Clip

• 4:14 Becke Line Demo• https://www.youtube.com/watch?v=qH1S83Bkttw • 1:34 min Beck Line Test • https://www.youtube.com/watch?v=B6Gis-wloSI • 5:04 The Becke Line Test • https://

www.youtube.com/watch?v=1QA1EhsfTQY

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Lab 4.2: Comparing Refractive Indexes of Glass

Glass Analysis

1. Composition of Glass

2. Types of Glass

3. Comparing GlassA. Density

B. Refractivity

4. Glass Fracture Patterns

5. Collecting Glass As Evidence

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4. Glass Fracture Patterns

Glass has a degree (amount) of elasticity (flexible-ness)– It breaks when its elastic limit is exceeded– The elasticity produces fractures when it is

penetrated by a projectile (i.e. a bullet)

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4. Glass Fracture Patterns

When the elasticity of glass is exceeded, it fractures (breaks).

There are several distinct types of glass fracture patterns:

A. Radial

B. Concentric

C. Conchoidal

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4. Glass Fracture Patterns (continued)

A. Radial– Produced first– Form on the side of the

glass opposite to where the

impact originated (on the back)– Look like spider webs that spread outward from

the impact hole– Always terminate (stop) at an existing fracture

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A. Radial Fractures

4. Glass Fracture Patterns (continued)

B. Concentric (rings)– Form second– Encircle (surround)

the bullet hole – Start on the same side as that of the destructive force

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B. Concentric Fractures

. Glass Fracture Patterns (continued)

C. Conchoidal Lines (conchoidal = smooth and rounded, like a conch shell)

– Amorphous solids do not break along any natural plane of separation as a crystalline solid would.

– The side of broken glass will show curved, rippling conchoidal lines that can be used to determine direction of impact.

• 4 R Rule: “On Radial cracks, Ridges make Right angles to the Rear.”

This is opposite for

sides of concentric fractures! rear

Video Clip

• 0:54 min – High Speed Glass Fracture• https://

www.youtube.com/watch?v=dwnBGnKSG2o • 3:57 min – Tempered vs. Plate Glass Fracture• https://

www.youtube.com/watch?v=CiGH-W9rDbA • 2:43 min – Bullet patterns in glass• https://

www.youtube.com/watch?v=QA1PG_JhkWM 41

4. Glass Fracture Patterns (continued)

I. Determining the sequence of multiple bullet holes

• The radial fractures from the second bullet hole always terminate into the fractures from the first bullet hole, and so forth

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4. Glass Fracture Patterns (continued)

II. Determining bullet path direction

• Compare the size of the entrance hole to the size of the exit hole

• Exit holes are larger. More fragmented glass is knocked out of the surface around the hole where the bullet is leaving because glass is elastic and bows outward when struck

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4. Glass Fracture Patterns (continued)

II. Determining bullet path direction

• Entrance holes – The bullet makes a very small hole

when it enters– The glass blows back in the

direction of the impact because of its elasticity

– The glass snaps back violently after being stressed and can blow shattered glass back several meters

– Most of the shattered glass lands on the impacted side (front) of the glass, instead of by the exit hole Shot fired into Obama campaign

office in Denver, CO (Oct. 2012)Source: telegraph.co.uk

Lab 4.3: Glass Fracture Patterns

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Glass Analysis

1. Composition of Glass

2. Types of Glass

3. Comparing GlassA. Density

B. Refractivity

4. Glass Fracture Patterns

5. Collecting Glass As Evidence

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5. Collecting Glass as Evidence

• Avoid the loss or contamination of any evidence samples

• Identify and photograph all glass samples before moving them

• Collect the largest fragments • Identify the outside and inside surfaces

of any glass• Indicate the relative position of

multiple window panes in a diagram 47

5. Collecting Glass as Evidence (continued)• Note any other trace evidence found on or embedded

in the glass, such as skin, hair, blood, or fibers• Package all of the collected materials properly in

order to maintain the chain of custody• Separate the glass by physical properties, such as

size, color, and texture

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5. Collecting Glass as Evidence (continued)• Catalog the samples and keep them separated in

order to avoid contamination between two different sources

• Separate the glass fragments from any other trace evidence (e.g., hair, blood, fibers) once in the lab

• Examine any clothing (or other objects that may have been used to break the glass) related to the crime scene for glass fragments and other trace evidence

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Video Clips

• 6:26 min – Bullet Proof Glass Vandalism Test• https://www.youtube.com/watch?v=EibjrMy1

TtI

• 6:41 min – Bulletproof vests• https://

www.youtube.com/watch?v=eRabvfq8TdA • 10:06 min – The Bulletproof Tailor• https://

www.youtube.com/watch?v=nQM6zLiSn1E 50

Resources• Texas Education Agency, Forensic Certification Training, Sam Houston State University• Forensic Science: Fundamentals & Investigation (1st Edition), Anthony Bertino• Forensic Science: From the Crime Scene to the Crime Lab (1st Edition), Richard Saferstein• ChemMatters, “More Than Meets The Eye” Brian Rohrig• The Science Spot – Forensic Science

– http://www.sciencespot.net/Pages/classforsci.html• Investigator/Officer’s Personal Experience• Corning Museum of Glass site

– http://www.cmog.org/default.asp• Federal Bureau of Investigation: Laboratory Services

– Forensic Glass Comparison: Background Information Used in Data Interpretation http://www.fbi.gov/about-us/lab/forensic-science-communications/fsc/april2009/review

– Introduction to Forensic Glass Examination

http://www.fbi.gov/about-us/lab/forensic-science-communications/fsc/jan2005/standards/2005standards4.htm/

– Collection, Handling, and Identification of Glass http://www.fbi.gov/about-us/lab/forensic-science-communications/fsc/jan2005/standards/2005standards5.htm/

– Glass Density Determination

http://www.fbi.gov/about-us/lab/forensic-science-communications/fsc/jan2005/standards/2005standards8.htm/ 51