1. Chapter 14 Glass Evidence By the end of this chapter you will be able to:
explain how glass is formed
list some of the characteristics of glass
provide examples of different types of glass
calculate the density of glass
use the refractive index to identify different types of glass
describe how glass fractures
analyze glass fracture patterns
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Forensic Science: Fundamentals & Investigations, Chapter 14

2. Introduction and History of Glass
Analysis of glass found at a crime scene can yield trace evidence.
The earliest human-made glass objects (beads) date back to about 2500 B.C. Egypt.
Specialized glass production was an art, a science, and a state secret in the republic of Venice over a span of hundreds of years.
By the fourteenth century, the knowledge of glass production spread throughout Europe.
The industrial revolution brought the mass production of many kinds of glass.
Forensic Science: Fundamentals & Investigations, Chapter 14

3. What Is Glass?
Once it cools, glass can be polished, ground, or cut for useful or decorative purposes. Glass blowers can form glass into many different shapes.
Crystalline solids have a regular atomic structure (illus-trated above, left). But glass is an amorphous solid and so has an irregular atomic structure (illustrated above, right),
Because of this, glass breaks in a variety of fracture patterns.
Forensic Science: Fundamentals & Investigations, Chapter 14

4. Types of Glass
Glass is an amorphous solid usually made from silica, calcium oxide, and sodium oxide.
Because glass is made of a variety of compounds, there are many types of glass.
Adding different metal oxides to glass mixtures, for example, yields different colors.
And because different types of glass have different densities and refraction indexes, it is possible to compare one type of glass with another.
Forensic Science: Fundamentals & Investigations, Chapter 14

5. 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.
Forensic Science: Fundamentals & Investigations, Chapter 14

6. Refractive Index
When a beam of light moves from one medium into another, its speed changes. That change causes the beam to change direction, bend.
In the illustration above, the green line to the right of the red line shows the direction if the beam had not changed direction. But the black line shows it bent toward the red line.
Forensic Science: Fundamentals & Investigations, Chapter 14

7. Refractive Index
Refraction is the change in the direction of light as it changes speed when moving from one medium into another.
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.
Forensic Science: Fundamentals & Investigations, Chapter 14

8. Application of Refractive Index to Forensics
If light passes from water into air, will it bend toward or away from the normal?
If light passes from air into glass, will it bend toward or away from the normal?
Light passed through two fragments of glass—one found at the crime scene and the other connected to a suspect—may have the same refraction and, so, link one with the other.
Forensic Science: Fundamentals & Investigations, Chapter 14

9. Application of Refractive Index to Forensics
If the glass fragments found at the crime scene, or connected with the suspect, or both are too small to check for this consistency by passing light through them, other methods such as the submersion method can be used to provide estimates.
Use the illustration above to describe the submersion method for obtaining refraction estimates.
Forensic Science: Fundamentals & Investigations, Chapter 14

10. Fracture Patterns in Broken Glass
Glass can stretch slightly when hit.
And because it’s an amorphous solid, it will not break into regular pieces with straight line fractures.
That means fracture patterns can provide clues about the direction, rate, and sequence of the impacts.
Forensic Science: Fundamentals & Investigations, Chapter 14

11. Why Radial and Concentric Fractures Form
Impacted glass is compressed on the side it is hit.
It will stretch on the opposite side of the glass, and the tension there will radiate breaks in the glass outward from the point of impact.
Then fractures form in the shape of concentric circles on the same side of the impact.
Forensic Science: Fundamentals & Investigations, Chapter 14

12. Bullet Fractures
As a bullet passes through glass, it pushes a cone shaped piece of glass out of the glass ahead of it.
This makes the exit side of the hole larger than the entrance side of the hole.
Radiating fracture lines from a subsequent shot will stop at the edge of the fracture lines already present in the glass.
Forensic Science: Fundamentals & Investigations, Chapter 14

13. Path of a Bullet Passing through Window Glass
The entry hole will be round if the bullet was fired perpendicular to the pane.
If fired from an angle, glass pieces will be forced out to the opposite side from the shot.
The angles at which bullets enter window glass can help locate the position of the shooter.
Bits of the glass can fly backward (backscatter), possibly creating trace evidence.
Forensic Science: Fundamentals & Investigations, Chapter 14

14. Handling of Crime Scene Glass Samples
Identify and photograph any glass samples before moving them.
Collect the largest fragments that can be reasonably collected.
Identify the outside and inside surface of any glass.
If multiple panes are involved, make a diagram.
Note trace evidence such as skin, hair, blood, or fibers.
Package all materials collected to maintain the chain of custody.
Forensic Science: Fundamentals & Investigations, Chapter 14

15. Summary
Different kinds of glass can be analyzed for their density, refractiveness, and fracture patterns.
The different compounds that make up a kind of glass affect its density.
The refractiveness of glass can be measured in a number of ways.
Fracture patterns can provide information about such things as the direction, the rate, and the sequence of the impacts.
Forensic Science: Fundamentals & Investigations, Chapter 14