Analysis of GlassAnalysis of Glass

Glass As EvidenceGlass As Evidence

What types of information can scientists What types of information can scientists learn from broken glass evidence?learn from broken glass evidence?

Glass fragments can be identified by Glass fragments can be identified by glass type.glass type.

Glass fragments found on a victim or Glass fragments found on a victim or suspect can be matched with glass suspect can be matched with glass samples found at a crime scene.samples found at a crime scene.

The direction and sequence of The direction and sequence of projectiles can be determined from projectiles can be determined from broken glass.broken glass.

Density and Refractive IndexDensity and Refractive Index Glass Samples can be identified or Glass Samples can be identified or

matched using density and/or matched using density and/or refractive indexrefractive index

Density-Each type of glass has Density-Each type of glass has significantly different density values. significantly different density values. Even for the same type of glass, the Even for the same type of glass, the density can vary slightlydensity can vary slightly

Refractive index- varies for each type Refractive index- varies for each type of glass; FBI index of 2000 varieties of of glass; FBI index of 2000 varieties of glass.glass.

DensityDensity The density of a glass sample can be The density of a glass sample can be

measured experimentally. (water measured experimentally. (water displacement)displacement)

VolumeMassDensity

DensityDensity Two samples of Two samples of

glass can be glass can be compared by the compared by the flotation method.flotation method.

Refractive IndexRefractive Index Notice the halo of light Notice the halo of light

(Becke Line) on the (Becke Line) on the inside perimeter of the inside perimeter of the glass sample.glass sample.

When the Becke line is When the Becke line is inside the perimeter of inside the perimeter of the glass fragment, the glass fragment, the refractive index of the refractive index of the glass is higher the glass is higher than the refractive than the refractive index of the index of the surrounding medium.surrounding medium.

Refractive IndexRefractive Index Notice the halo of light Notice the halo of light

(Becke line) is outside (Becke line) is outside the perimeter of the the perimeter of the glass fragment.glass fragment.

When the Becke line is When the Becke line is outside the perimeter outside the perimeter of the glass sample, of the glass sample, the refractive index of the refractive index of the medium is higher the medium is higher than the refractive than the refractive index of the glass.index of the glass.

The glass fragment will The glass fragment will be invisible if the be invisible if the refractive indexes refractive indexes match.match.

Jigsaw EffectJigsaw Effect Suspect fragments and crime scene Suspect fragments and crime scene

fragments can be pieced togetherfragments can be pieced together Irregular edges and striations must match.Irregular edges and striations must match. This is the best method for comparing This is the best method for comparing

samples. The possibility that two pieces of samples. The possibility that two pieces of glass originating from different sources glass originating from different sources will fit together exactly is so unlikely as to will fit together exactly is so unlikely as to exclude all other sources from practical exclude all other sources from practical consideration.consideration.

Unfortunately, most glass evidence is Unfortunately, most glass evidence is either too fragmentary or too minute to either too fragmentary or too minute to permit a comparison of this typepermit a comparison of this type

Glass FracturesGlass Fractures Glass fractures can Glass fractures can

be used to be used to determine determine direction and direction and sequence of sequence of fracturing.fracturing.

Tempered glass Tempered glass can not be can not be analyzed this way.analyzed this way.

Types of fracturesTypes of fractures Radial Fractures- first fractures to Radial Fractures- first fractures to

form; extend outward from the point form; extend outward from the point of impact like spokes on a wheel; of impact like spokes on a wheel; occur on the side of glass opposite occur on the side of glass opposite the impact.the impact.

Concentric fractures-circles around Concentric fractures-circles around the point of impact; on the same side the point of impact; on the same side as the impact.as the impact.

Glass FracturesGlass Fractures

Glass FracturesGlass Fractures

3R Rule3R Rule Stress marks on a

cross section of a glass fragment are shaped like arches perpendicular to one surface and parallel to another

Radial cracks are at Right angles to the Reverse side of impact

Sequence of ImpactSequence of Impact The sequence of The sequence of

impacts can be impacts can be determined from determined from fracture patterns.fracture patterns.

A fracture always A fracture always terminates at an terminates at an existing fracture existing fracture line.line.

Which impact occurred first?Which impact occurred first?