1. Warm Up
Objective: Scientists will describe glass evidence by taking notes and analyzing the activity.
What is the topic?
What will you be doing?
Why is this important?
How will you know if you have done well?

2. Warm Up
Objective: Scientists will describe glass evidence by taking notes and analyzing the activity.
What is the topic? Glass evidence
What will you be doing? Taking notes; analyzing activity
Why is this important? Evidence used for convictions; future career choice
How will you know if you have done well? Able to describe glass evidence; took notes and analyzed the activity

3. Glass Evidence

4. CHARACTERISTICS OF GLASS
Hard, amorphous solid
Usually transparent
Primarily composed of silica, with various amounts of elemental oxides
Brittle
Exhibits conchoidal fracture
Curved breaks that look like concentric ripples

5. COMMON TYPES OF GLASS
Soda-lime—used in plate and window glass, glass containers, and electric lightbulbs
Soda-lead—fine tableware and art objects
Borosilicate —heat-resistant, like Pyrex
Silica —used in chemical ware
Tempered —used in side windows of cars
Laminated —used in the windshield of most cars

7. PHYSICAL CHARACTERISTICS
Density—mass divided by volume
Refractive index (RI)—the measure of light bending due to a change in velocity when traveling from one medium to another
Fractures
Color
Thickness
Fluorescence
Markings—striations, dimples, etc.

8. FORENSIC USE OF REFRACTIVE INDEX
The submersion method is used to determine the refractive index of small glass fragments.
The FBI has a database with over 2,000 types of glass to assist in matching glass found at a crime scene with known sources

9. GLASS AS EVIDENCE
Class characteristics: chemical and physical properties such as refractive index, density, chemical composition, color
Individual characteristics: the source can be considered unique if the piece can fit together to complete the puzzle.

10. GLASS AND CRIME SCENES For glass comparison at a crime scene:
Find and measure properties
Associate one glass fragment with another
Minimize or eliminate other sources
Collection
Collect all glass found at the scene for piecing
Submit all glass evidence found in the possession of the suspect
Packaged in solid containers to avoid further breakage
Suspect’s shoes and/or clothing are to be examined
Individually wrapped in paper
Transmitted to laboratory

11. ANALYZING CRACKS CRACKS Radial fractures: radiate outward
Concentric fractures: encircle hole

14. ANALYZING CRACKS Sequence of impacts: Successive penetrations of glass
Fracture always terminates at existing line of fracture
Figure 4–20  Two bullet holes in a piece of glass.
Which bullet hole came first?
The left hole preceded the right hole.

15. EVIDENCE FROM BULLET FRACTURES
The angle at which a bullet enters window glass can help determine the position of the shooter.

16. Glass Activity Work in groups of 2-3.
Get two microscope slides for your group.
Tape both sides of the microscope slide as demonstrated.
Put on your goggles.
Using the hammer, hit the glass slide on the left side and then the right side. DO NOT BREAK THROUGH! BE CAREFUL!
Using the screwdriver, hit the glass slide on the left side and then the right side. DO NOT BREAK THROUGH! BE CAREFUL!
Record your observations as indicated and answer the questions.

17. Glass Activity Observations and Analysis
For both the hammer slide and screwdriver slide:
Draw the fracture patterns that you see in the space provided.
Label the radial and concentric fractures on your drawing.
Label the order in which the impacts to the glass occurred. Explain your reasoning for labeling them this way.
Answer the analysis questions:
How could investigators use glass as evidence?
Look at sample A. What tool created these markings? How do you know?
Look at sample B. What tool created these markings? How do you know?
How might the fracture pattern be different if a different tool or a human hand was used? Explain.