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

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.

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

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

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

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 .0002 index between standard/reference and the suspect glass fragment FBI has refractive indices data bank

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

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

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

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

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)

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

In-Class Assignment/Homework 4.3 Review Questions