Forensic Glass Analysis Introduction and History of Glass

Forensic Glass Analysis

Introduction and History of Glass n n n Egypt circa 2500 B. C. —The earliest known human-made glass objects (beads) 1 st Century B. C. —glass blowing begins 13 th Century—specialized glass production was an art, a science, and a state secret in the republic of Venice Forensic Science: Fundamentals & Investigations, Chapter 14 2

Introduction and History of Glass n n n 14 th Century—glass-making spreads through Europe The industrial revolution applies mass production to many types of glass Analysis of glass found at a crime scene can yield trace evidence Forensic Science: Fundamentals & Investigations, Chapter 14 3

What Is Glass? n n What State of matter is glass Crystalline solids have a regular atomic structure Glass is an amorphous solid and so has an irregular atomic structure Therefore, glass breaks in a variety of fracture patterns 4

What is Glass? n n Glass is an amorphous solid usually made from silica, calcium oxide, and sodium oxide. Silica’s chemical name is silicon dioxide

How is glass made? n The melting together of sand a few other compounds at very high temperatures forms most glass.

What is Glass? n n One of the earliest forms of man-made glass was accidentally produced in Syria during food preparation. Metal oxides added to glass produce glass of different colors.

Types of Glass n n n Some types of glass are made with specific characteristics, such as glass for cooking and laboratory use, that make the glass withstand a wide range of temperatures. Glass used in vehicle wind shields have been created to not break apart when hit. Safety glass is made so it will not crack and not break apart upon impact

Types of Glass n n Adding metal oxides yields different colors Each type has a different density Each type has different refractive indexes These characteristics allow comparisons Forensic Science: Fundamentals & Investigations, Chapter 14 9

How is glass used by Forensic Scientist? n By analyzing the fracture patterns that form when glass is hit, forensic investigators can determine what object hit the glass and from which direction the object came.

How is glass used by Forensic Scientist? n n Glass evidence is considered class evidence, because it is mass-produced and a particular piece of glass is not unique. However if pieces of glass from suspect to reconstruct perfectly fit into a whole object, this is a match.

Refractive Index n When a beam of light moves from one medium into another: n The speed changes n The direction bends n n Refractive Index—a tool used to study how light bends as it passes from one substance to another Normal line is perpendicular to the glass surface Forensic Science: Fundamentals & Investigations, Chapter 14 12

Refractive Index When a beam of light moves from less dense medium (air) into a more dense medium (water): n Its speed slows, and n Bends light toward the normal line Forensic Science: Fundamentals & Investigations, Chapter 14 13

Snell’s Law N=1. 52 N=1. 33 The higher the n, the more the light bends

Refractive Index When a beam of light moves from a more dense medium (glass) into a less denser medium (air): n Its speed increases n And bends light away from the normal line Forensic Science: Fundamentals & Investigations, Chapter 14 15

Application of Refractive Index to Forensics Submersion method—used when glass fragments found at the crime scene are small Forensic Science: Fundamentals & Investigations, Chapter 14 16

Application of Refractive Index to Forensics n n Place the glass fragment into different liquids of known refractive indexes The glass fragment will seem to disappear when placed in a liquid of the same refractive index Forensic Science: Fundamentals & Investigations, Chapter 14 17

Application of Refractive Index to Forensics n Becke Line—a halo-like effect appearing at the edges of a glass fragment when the reflective index of the glass and liquid are different n If the line is inside the glass perimeter, the glass index is higher than the index of the liquid n If the line is outside the glass perimeter, the glass index is lower Forensic Science: Fundamentals & Investigations, Chapter 14 18

Becke Lines Glass has higher refractive index-note white line inside • Glass has lower refractive index-note white line outside

Fracture Patterns in Broken Glass n n Being an amorphous solid, glass will not break into regular pieces with straight line fractures Fracture patterns provide clues about the direction, rate, and sequence of the impacts Forensic Science: Fundamentals & Investigations, Chapter 14 20

Why Radial and Concentric Fractures Form n n n Impacted glass is compressed on the side it is hit. It will stretch on the opposite side of the glass, and the tension there will radiate breaks in the glass outward from the point of impact. Then fractures form in the shape of concentric circles on the same side of the impact. Forensic Science: Fundamentals & Investigations, Chapter 14 21

Why Radial and Concentric Fractures Form Glass after an impact shows radial fractures (red) and concentric circle fractures (blue) Forensic Science: Fundamentals & Investigations, Chapter 14 22

Why Radial and Concentric Fractures Form Forensic Science: Fundamentals & Investigations, Chapter 14 23

Bullet Fractures n n n As a bullet passes through glass, it pushes a cone shaped piece of glass out of the glass ahead of it The exit side of the hole is larger than the entrance side of the hole Radiating fracture lines from a subsequent shot will stop at the edge of the fracture lines already present in the glass Forensic Science: Fundamentals & Investigations, Chapter 14 24

Path of a Bullet Passing through Window Glass perpendicular to the glass n n shot from the left shot from the right The angles at which bullets enter window glass help locate the position of the shooter Bits of the glass can fly backward (backscatter), creating trace evidence Forensic Science: Fundamentals & Investigations, Chapter 14 25

Blunt Object Fractures n n Glass fractures caused by a blunt object will show a pattern of fractures but not as regular of a pattern as the fractures from a bullet The difference is due to the impacting force being dispersed over a greater area.

Blunt Object Fractures

Heat Fractures n n Wave shaped No regular pattern of radial or concentric lines Show little if any curve patterns along the edges Expansion of glass occurs first on the side exposed to the heat

Heat Fractures

Handling of Crime Scene Glass Samples 1. Identify and photograph any glass samples 2. 3. 4. 5. 6. before moving them. Collect the largest fragments that can be reasonably collected. Identify the outside and inside surface of any glass. If multiple panes are involved, make a diagram. Note trace evidence such as skin, hair, blood, or fibers. Package all materials collected to maintain the chain of custody. Forensic Science: Fundamentals & Investigations, Chapter 14 30