1. Properties and Analysis
Glass
Properties and Analysis

2. Glass Fragments
Glass: hard, brittle, amorphous substance that is composed of silicon oxide (sand) and other various metal oxides
Refraction—Bending of a light wave as it passes through one medium to another

3. Amorphous Solids
Amorphous solids: atoms or molecules are arranged RANDOMLY
NO regular order to the atoms
Glass

4. Typical Types of Glass
Pyrex: A borosilicate— boron oxide is added to the glass to increase resistance to heat
Tempered Glass: glass to which strength is added by introducing stress through rapid heating and cooling of the glass surfaces
When tempered glass breaks, it does not shatter
Usually found in car windows because of safety issues
Laminated Glass: two sheets of ordinary glass bonded together with plastic film
Usually used in auto windshields in the United States

5. Glass Comparison
Comparison of glass is possible with broken or fractured glass through:
1. Glass Properties:
Density
Refractive Index (Becke Line)
2. Physical Match
3. Probability of common origin
4. Direction of Impact
5. Sequence of Impact
Ballistics

6. Physical Property: Density
A physical property of matter that is equivalent to the mass-per-unit volume of a substance
Density: D= m/v
(mass= grams, volume=mL)

8. Density
Density is an INTENSIVE PROPERTY of matter, meaning it is the same regardless of the size of a substance and can be used as an aid in identification.
Solids tend to be more dense than liquids, and liquids more dense than gases.

9. Density
It is important to keep controlled temperatures when calculating Density. The volumes of gases and liquids vary considerably with temperatures. When the temperature of water increases it will expand, causing a difference in Density.

10. Density Example: Water at 4℃ Density = 1.0 g/mL

11. Refraction
Refraction: The bending of a light wave as it passes from one medium to another.

12. Refraction

13. Refraction

14. Refraction

15. Refraction

16. Refraction

17. Refraction
Glass that is broken and shattered into fragments during a crime can be used to place a suspect at the crime scene.
Example: During a burglary, glass can be lodged in a suspects shoe or garment. Also, headlight glass found at a hit-and-run scene may offer clues that can confirm the identity of the suspect vehicle.

18. Becke Line
A bright halo that is observed near the border of a particle immersed in a liquid of a different refractive index.

19. Becke Line

20. Becke Line

21. Becke line In other words:
You will see the edges of the particle (glass) underneath a microscope.

22. Becke Line

23. Becke Line
Match Point- the point where the Becke Line is no longer observable. At this point you have reached the particles Refractive Index. You are also able to compare an unknown to a known.

24. Becke Line

25. Refraction
To determine the Refractive index of a piece of glass, one utilizes the Immersion Method. For this, glass particles are immersed in a liquid medium whose refractive index is varied until it is equal to that of the glass particles. At this point, known as the Match Point, the observer will note the disappearance of the Becke Line and minimum contrast between the glass and liquid medium.

26. Glass Comparison
Difficult because of prevalence of glass in our society
Often matched by piecing fragments together like a puzzle