Glass Analysis and FracturesPg. 100-111

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 has its greatest value when it can be individualized such as piecing together fragments from a crime scenematching 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

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

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

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

Caused by exceeding the limit of elasticity of glassGlass naturally bends when a force is

exerted Reveals the force and direction

of impact Useful for reconstruction

Radial fractures – radiating lines from a hole caused by a type of projectile

Concentric fractures – circular lines surrounding the hole created by a 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

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)

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