1. Unit 4: Forensic Glass Analysis
Introduction to Forensic Science

2. Forensic Glass Analysis
Composition of Glass
Types of Glass
Comparing Glass
Density
Refractivity
Glass Fracture Patterns
Collecting Glass As Evidence

3. 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

4. 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

5. 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

6. 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

7. Video Clip – How Glass is Made
6:37 min
5:04 min Plate Glass

8. Glass Analysis Composition of Glass Types of Glass Comparing Glass
Density
Refractivity
Glass Fracture Patterns
Collecting Glass As Evidence

9. 2. Types of Glass
There are many different types of glass. We will discuss the following five types:
Obsidian
Soda Lime
Leaded
Tempered
Laminated

10. 2. Types of Glass
Obsidian is a natural form of glass that is created by volcanoes
Soda-lime glass
The most basic, common, inexpensive glass – also the easiest to make
Used for manufacturing windows and bottle glass

11. 2. Types of Glass C. Borosilicate Glass Contains boron oxide
heat-resistant glass: can be heated and cooled rapidly without breaking
Car headlights, Pyrex dishes, lab beakers and test tubes.

12. 2. Types of Glass D. 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
Sparkles

13. 2. Types of Glass E. 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

14. 2. Types of Glass F. Laminated glass
Constructed by bonding two ordinary sheets of glass together with a plastic film
Prevents shattering
Also used by automobile manufactures for windshields

15. Video Clip 5:34 min – Making Laminated Glass
0:54 min – Laminated Glass Safety Test
5:34 min – Making Technical Glass

16. Glass Analysis Composition of Glass Types of Glass Comparing Glass
Density
Refractivity
Glass Fracture Patterns
Collecting Glass As Evidence

17. 3. Comparing Glass Investigation/Analysis includes Finding Measuring

18. 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
So, what type of evidence is glass?

19. 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

20. Glass Analysis Composition of Glass Types of Glass Comparing Glass
Density
Refractivity
Glass Fracture Patterns
Collecting Glass As Evidence

21. 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

22. 3A. Methods of Comparison: Density
D V
Density comparison
Density (D) is calculated by dividing the mass (M) of a substance by its volume (V)
D = M ÷ V

23. 3A. Density Density comparison (continued)
Step 1: Find the sample’s volume by Archimedes’ Principal
Fill a beaker half way, and put it on the scale
Record the initial mass
Gently lower the glass into the water attached to a string
Record the final mass
𝐺𝑙𝑎𝑠𝑠 𝑉𝑜𝑙𝑢𝑚𝑒 = 𝐹𝑖𝑛𝑎𝑙 𝑚𝑎𝑠𝑠 𝑔 − 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑚𝑎𝑠𝑠(𝑔) 1𝑔 𝑐𝑚 3
The density of water

24. 3A. Density D = M ÷ V Density comparison (continued)
Step 2: Find the sample’s mass in grams using a balance or scale
Density can now be calculated from the equation in units of grams per cm3
D = M ÷ V
Read the bottom of the meniscus, and estimate between marks. This is 52.5 milliliters (mL)

25. Glass Analysis Composition of Glass Types of Glass Comparing Glass
Density
Refractivity
Glass Fracture Patterns
Collecting Glass As Evidence

26. 3B. Refractivity
Let’s talk about light.
What do you know?

27. 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 the light waves are suddenly slowed, which causes the light rays to bend.

28. 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
This means that light travels times faster in a vacuum than it does in water.

29. 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 vacuum

30. 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

31. 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

32. Video Clip 4:14 Becke Line Demo
1:34 min Beck Line Test
5:04 The Becke Line Test

33. Lab 4.2: Identifying Unknown Glass by Density and Refractive Index

34. Glass Analysis Composition of Glass Types of Glass Comparing Glass
Density
Refractivity
Glass Fracture Patterns
Collecting Glass As Evidence

35. 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)

36. 4. Glass Fracture Patterns
When the elasticity of glass is exceeded, it fractures (breaks).
There are several distinct types of glass fracture patterns:
Radial
Concentric
Conchoidal

37. 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

38. A. Radial Fractures

39. 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

40. B. Concentric Fractures

41. . 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

42. Video Clip 0:54 min – High Speed Glass Fracture
3:57 min – Tempered vs. Plate Glass Fracture
2:43 min – Bullet patterns in glass

43. 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

44. 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

45. 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

46. Lab 4.1: Glass Fracture Patterns

47. Glass Analysis Composition of Glass Types of Glass Comparing Glass
Density
Refractivity
Glass Fracture Patterns
Collecting Glass As Evidence

48. 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

49. 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

50. 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

51. Video Clips 6:26 min – Bullet Proof Glass Vandalism Test
6:41 min – Bulletproof vests
10:06 min – The Bulletproof Tailor

52. 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
Investigator/Officer’s Personal Experience
Corning Museum of Glass site
Federal Bureau of Investigation: Laboratory Services
Forensic Glass Comparison: Background Information Used in Data Interpretation
Introduction to Forensic Glass Examination
Collection, Handling, and Identification of Glass
Glass Density Determination