Analysis of Glass

Glass As Evidence What types of information can scientists learn from broken glass evidence? Glass fragments can be identified by glass type. Glass fragments can be identified by glass type. Glass fragments found on a victim or suspect can be matched with glass samples found at a crime scene. Glass fragments found on a victim or suspect can be matched with glass samples found at a crime scene. The direction and sequence of projectiles can be determined from broken glass. The direction and sequence of projectiles can be determined from broken glass.

Density and Refractive Index Glass Samples can be identified or matched using density and/or refractive index Glass Samples can be identified or matched using density and/or refractive index Density-Each type of glass has significantly different density values. Even for the same type of glass, the density can vary slightly Density-Each type of glass has significantly different density values. Even for the same type of glass, the density can vary slightly Refractive index- varies for each type of glass; FBI index of 2000 varieties of glass. Refractive index- varies for each type of glass; FBI index of 2000 varieties of glass.

Density The density of a glass sample can be measured experimentally. (water displacement) The density of a glass sample can be measured experimentally. (water displacement)

Density Two samples of glass can be compared by the flotation method. Two samples of glass can be compared by the flotation method.

Refractive Index Notice the halo of light (Becke Line) on the inside perimeter of the glass sample. Notice the halo of light (Becke Line) on the inside perimeter of the glass sample. When the Becke line is inside the perimeter of the glass fragment, the refractive index of the glass is higher than the refractive index of the surrounding medium. When the Becke line is inside the perimeter of the glass fragment, the refractive index of the glass is higher than the refractive index of the surrounding medium.

Refractive Index Notice the halo of light (Becke line) is outside the perimeter of the glass fragment. Notice the halo of light (Becke line) is outside the perimeter of the glass fragment. When the Becke line is outside the perimeter of the glass sample, the refractive index of the medium is higher than the refractive index of the glass. When the Becke line is outside the perimeter of the glass sample, the refractive index of the medium is higher than the refractive index of the glass. The glass fragment will be invisible if the refractive indexes match. The glass fragment will be invisible if the refractive indexes match.

Jigsaw Effect Suspect fragments and crime scene fragments can be pieced together Suspect fragments and crime scene fragments can be pieced together Irregular edges and striations must match. Irregular edges and striations must match. This is the best method for comparing samples. The possibility that two pieces of glass originating from different sources will fit together exactly is so unlikely as to exclude all other sources from practical consideration. This is the best method for comparing samples. The possibility that two pieces of glass originating from different sources will fit together exactly is so unlikely as to exclude all other sources from practical consideration. Unfortunately, most glass evidence is either too fragmentary or too minute to permit a comparison of this type Unfortunately, most glass evidence is either too fragmentary or too minute to permit a comparison of this type

Glass Fractures Glass fractures can be used to determine direction and sequence of fracturing. Glass fractures can be used to determine direction and sequence of fracturing. Tempered glass can not be analyzed this way. Tempered glass can not be analyzed this way.

Types of fractures Radial Fractures- first fractures to form; extend outward from the point of impact like spokes on a wheel; occur on the side of glass opposite the impact. Radial Fractures- first fractures to form; extend outward from the point of impact like spokes on a wheel; occur on the side of glass opposite the impact. Concentric fractures-circles around the point of impact; on the same side as the impact. Concentric fractures-circles around the point of impact; on the same side as the impact.

Glass Fractures

3R 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 Impact The sequence of impacts can be determined from fracture patterns. The sequence of impacts can be determined from fracture patterns. A fracture always terminates at an existing fracture line. A fracture always terminates at an existing fracture line.

Which impact occurred first?