1. Glass Analysis and Fractures Pg. 100-111
4.3 Notes
Glass Analysis and Fractures Pg

2. Objectives
List and explain forensic methods of comparing glass fragments
Understand how to examine glass fragments to determine the direction of impact for a projectile
Describe proper collection of glass evidence.

3. Glass Evidence
Glass evidence has its greatest value when it can be individualized such as piecing together fragments from a crime scene
matching irregular pieces and striations on the surface
Physical properties of density and refractive index are most useful for characterizing glass particles
considered to be a class characteristic

4. Immersion method
Method of comparing refractive indices after comparing density results
Glass particles immersed in a medium whose refractive index is varied until it is equal to that of the glass particles

5. Immersion cont
Match point = that point when the refractive index is equal to that of the glass particle
(disappearance of the Becke line)
Becke line = a bright halo that observed near the edge of a particle that is immersed in a liquid of different refractive index
Best varied using temperature changes from high boiling point to when the match point is reached

6. GRIM2 Special microscope or instrument used with immersion method
Glass Refractive Index Measurement
Special microscope or instrument used with immersion method
Glass fragments must have values greater than index between standard/reference and the suspect glass fragment
FBI has refractive indices data bank

7. Glass Fractures Caused by exceeding the limit of elasticity of glass
Glass naturally bends when a force is exerted
Reveals the force and direction of impact
Useful for reconstruction

8. Fractures
Radial fractures – radiating lines from a hole caused by a type of projectile
Concentric fractures – circular lines surrounding the hole created by a projectile

9. A high velocity projectile
Leaves a round crater shaped hole
Hole is surrounded by a symmetrical pattern of radial and concentric cracks
Hole is wider at the exit side than the entrance side
A decrease in the velocity of the projectile the irregularity of holes, lines and circles becomes greater

10. First fractures form on the surface opposite that of the penetrating force
Radial lines followed by concentric cracks
Stress marks are shaped like arches perpendicular to one glass surface and curved parallel to the opposite surface
Yield information about which side of the window cracked first

12. 3R rule
Radial cracks form Right angle on the Reverse side of the force
A fracture always terminates at an existing line of fracture for view
(page 109)

13. Collection and Preservation of Glass Evidence
Collect all glass found
Example: In a case of hit and run
Collect broken parts of head and tail lights
Collect filaments of headlights to determine if headlights were off or on
Must have standard/reference glass pieces from remaining broken glass
Examine suspects shoes and clothes for glass fragments

14. In-Class Assignment/Homework