All rights Reserved Cengage/NGL/South-Western © 2016

Chapter 15 Glass Evidence By the end of this chapter you will be able to: 15.1 Describe the three major components of glass. 15.2 Compare and contrast soda glass, lead glass (crystal), and heat-resistant glass. 15.3 List and describe the physical properties of glass. 15.4 Calculate the density of glass samples.

Chapter 15 Glass Evidence By the end of this chapter you will be able to: 15.5 Estimate the refractive index of glass using the submersion method and Becke lines. 15.6 Distinguish between radial and concentric fractures in terms of their appearance, how they are formed, and their location on fractured glass. 15.7 Summarize and describe the information that can be gained by analyzing bullet hole(s) in fractured glass.

Chapter 15 Glass Evidence By the end of this chapter you will be able to: 15.8 Compare and contrast laminated, tempered or safety glass, and bullet-resistant glass in terms of structure, use, and fracture pattern. 15.9 Describe how to properly collect and document glass evidence. 15.10 Summarize the ways to determine whether two glass fragments are consistent.

Chapter 15 Vocabulary amorphous backscatter bullet-resistant (“bulletproof”) glass concentric fracture density glass laminated glass lead glass (crystal) normal line radiating fracture refraction refractive index silicon dioxide tempered glass

Introduction Glass evidence can be found at many crime scenes. Automobile collisions Store break-ins Assaults Glass is typically considered class evidence.

What is Glass? Glass is a hard, amorphous, brittle material made by melting the following materials at very high temperatures: Silicon dioxide, or silica Calcium oxide Sodium oxide

Laminated Glass Two or more panes of glass bonded by a plastic middle layer are called laminated glass.

Tempered Glass Glass that has been subjected to extreme temperatures or chemical treatments to improve its strength is called tempered glass, or safety glass.

Bullet-resistant Glass Bullet-resistant (“bulletproof”) glass is a laminated and tempered glass composed of two layers, one hard and one soft.

Thickness  

Density  

Refractive Index Light changes direction as it passes from one medium, such as air, into another medium, such as water. The change in direction is called refraction.

Refractive Index (continued) The perpendicular line that forms at the interface of two media is called the normal line. In this figure, as the beam of light moves from the air into the glass, the light slows down and bends toward the normal line.

Refractive Index (continued) In this figure, light speeds up as it moves from the oil into the air. The beam of light bends away from the normal line.

Refraction  

Forensic Use of Refractive Index The submersion method is used to determine the refractive index of small glass fragments.

Becke Lines If a submerged glass fragment has a refractive index that differs from the liquid medium that it is suspended within, a halo-like ring appears around the edge of the glass. This halo-like effect is called a Becke line.

Radial and Concentric Fractures Radial fracture lines start at the point of impact and move outward.

Bullet Fractures Bullet holes in glass can be distinguished from other impacts by the greater number of concentric fractures that result from high-velocity impact.

Evidence from Bullet Fractures The angle at which a bullet enters window glass can help determine the position of the shooter.

Heat Fractures Heat fracturing produces breakage patterns on glass that are different from breakage patterns caused by an impact. Wavy fracture lines develop on glass that has been exposed to high heat. Glass tends to crack on the surface exposed to the higher temperature.

Scratches Windshield wipers may leave unique scratches on a windshield.

Scratches (continued) Any dirt or hard particles embedded in the rubber insulation of a car window may leave scratch marks when the window is opened and closed.

Collection and Documenting Glass Evidence Crime-scene investigators should initially separate collected glass by physical properties including: Size Color Texture

Forensic Glass Analysis Technology Scanning electron microscopic (SEM) X-ray fluorescence (XFR) Inductively coupled plasma-mass spectrometry (ICP-MS)

Summary Glass is an amorphous solid usually made from silica, calcium oxide, and sodium oxide. Because most glass is mass-produced, it is difficult to prove that two different glass pieces came from the same source. Glass is almost always class evidence. Laminated glass is used in windshields. Tempered glass is used in side and back windows. These types of glass protect passengers from the dangers of flying shards. Bulletproof glass is laminated and tempered. Characteristics that provide class evidence of glass include the type of glass, its density, its thickness, and its refractive index.

Summary (continued) The density of an object is calculated by dividing its mass (usually in grams) by its volume (usually in milliliters). The refractive index of a material is a measure of how much light bends, or refracts, as it travels through that material. Light will bend, or refract, when it moves between media with different refractive indexes. The submersion method can be used to estimate the refractive index of a glass sample.

Summary (continued) The position of a Becke line is another method used to estimate the refractive index of a piece of glass. When glass is hit, it first stretches, and then breaks, forming radial fracture and concentric fracture patterns. Radial fracture patterns occur on the side of the glass opposite the point of impact. Concentric fracture patterns occur on the same side as the point of impact. Fire fractures in glass appear wavy and tend to appear on the surface exposed to higher temperatures.

Summary (continued) When a bullet is shot through glass, the exit hole is larger than the entry hole. The angle of entrance of a bullet into glass can be estimated from the pattern of glass fragments left near the bullet’s exit hole. Forensic glass-analysis methods include fracture matching, scanning electron microscopy of headlight filaments after collisions, and the use of X-ray fluorescence (XRF) and inductively coupled plasma-mass spectrometry (ICP-MS), which help determine properties of glass evidence samples.