1. Types of Glass
Saferstein - Chapter 4

2. Glass – an amorphous solid which lacks the crystal lattice arrangement of atoms usually found in solids.
SiO2 – silicon dioxide (sand)
Na2CO3 – sodium carbonate (soda)
CaO – calcium oxide (lime)
B2O3 – boron oxide

4. Soda Lime glass “soda-lime” – sand + soda (Na2CO3) + lime (CaO)
Used for windows and bottle glass
Makes up 90% of all glass products.

5. Borosilicate glass Borosilicates – includes boron oxide
Adds heat resistance
Used for Pyrex cooking and lab glassware and for automobile headlights

6. Tempered Glass
Tempered glass – stronger than other types because of repeated stress from heating and cooling
Does not splinter
Used for side and rear windows in US-made cars; also used for windshields in some European cars

8. Laminated Glass
Laminated glass - has a layer of plastic between two pieces of ordinary window glass (windshield)

9. Matching glass fragments
Fracture Match - Individualized if 2 pieces can be matched together by shape, like puzzle pieces
Type of glass indicates class characteristics – can determine probable source of the glass, but not the exact car, window, etc.
Best matches made through refractive index and density

11. Calculating density Density = mass/volume
Can be calculated using immersion in a liquid.
Place known amount of water in a graduated cylinder, add the glass fragment, and record the difference in beginning and ending volume.
Mass fragment on a balance
Result is in grams/ml or grams/cm3

12. Refractive Index
Refractive index – indicates the extent of the bending of light by an object or medium
Measured using a hot-stage microscope
Use liquid of known RI, such as silicone-based medium
Fragment will show Becke lines – halo-like lines around the border of the fragment

13. Becke lines form a halo-like effect around the edge of the glass fragment

14. Becke lines disappear at the refractive index of the glass.

15. Glass fractures Glass fractures Radial or concentric
Formed by penetration of the glass by a bullet or other projectile.
Radial fractures form first.
Concentric fractures form second.

16. Radial – extend outward from the center of the penetration (i. e
Radial – extend outward from the center of the penetration (i.e. radius)
Concentric – form circles surrounding the point of penetration

17. The order in which the fractures occurred can be determined from the radial fractures.
A new fracture always terminates at an existing line of fracture.
Which bullet hole occurred first in the picture below?

19. Stress marks The 3 R rule –
Radial fracture form Right angles on the Reverse side of impact
Stress marks can tell you whether the impact came from the inside or outside of a window.

20. Perpendicular marks (~90 degree angles) are found on the edge opposite the way that the force was pushing

21. Point of impact can also be determined by the entrance and exit of the projectile.
Exit holes are larger than entrance holes, forming a small crater in the glass on the opposite side of impact.

22. Collection of glass evidence
Collect as many fragments as possible in case they can be reassembled.
For headlights, collect filaments to determine if lights were on or off.
Collect control samples
Package in solid containers or wrap in paper.
Identify interior/exterior if possible