1. What’s the Dirt on Glass?

2. What is Glass?

3. What is Glass?
Hard
Brittle
Amorphous substance
Made primarily of sand

4. Glass Manufacture
Melt
Form
Slow Cool
Ingredients Mixed

5. Glass Ingredients Window +Soda Bottle Glass: Sand
Soda (Na2CO3)-lowers melting point
Lime (CaO)-makes it water insoluble
Combination of metal oxides:
Ex. Sodium, calcium, magnesium, aluminum

6. Glass Ingredients
Pyrex+Car Headlight Glass
Add boron oxide
Vids

7. Tempered or Safety Glass
Car glass
Dicing and little splintering
Stress glass by rapid heating and cooling
=annealing
Glass also strong from lamination
Thin piece of plastic between glass layers

8. Glass and Crime Hit and Run Breaking and Entering Bullet Hole

9. How Can We Compare Glass?
Physical Properties:
Weight
Volume
Density (d=m/v)
Color
Boiling Point
Melting Point

10. How Can We Compare Glass?
Density:
Mass/volume
Density Gradient Tube
Or weigh and calculate
volume displacement in
graduated cylinder

11. How Can We Compare Glass?
Refractive Index:
=light is bent as it
travels from one
medium to another
Refractive index=velocity of light in vacuum/ velocity of light in medium

12. Becke Line=Bright Halo that disappears when medium and fragment have the same refractive indexes
Medium index can be varied by adjusting temperature of stage

13. Other Methods of Comparison
Physical Fit of Edges
Look at Glass Ream Patterns

14. Comparative Studies If R.I.=1.5290 then found in 1/2000 specimens
FBI has a data base of from glass manufacturers of densities and refractive indexes
=Used for frequency of occurrence issues
If R.I.= then found in 1/2000 specimens
If R.I.= then found in 22/2000 specimens

15. Problem!
Most glass is uniform in its making and show no individualistic traits!

16. Trace Chemical Analysis of Glass

17. Glass Fractures
What Can We Learn?

18. Glass Fractures Crime Scene Reconstruction:
-Force and Direction of Impact
-Type of projectile (bullet or rock)
Projectiles:
Concentric Fractures
Radial Fractures

19. Projectiles+Direction of Force
Exit side of bullet is a deep crater
3R Rule
=Radial Cracks form a Right angle on the Reverse side of force
Concentric fractures originate on the force side of glass

20. Multiple Bullet Holes A B
Which Bullet Came First?