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1 Chapter 14 Glass Evidence Saferstein Shattering glass in slo mo Bullet slo mo
Forensic Science: Fundamentals & Investigations, Chapter 14 2 Introduction and History of Glass Analysis of glass found at a crime scene can yield trace evidence. The earliest human-made glass objects (beads) date back to about 2500 B.C. Egypt. Specialized glass production was an art, a science, and a state secret in the republic of Venice over a span of hundreds of years. By the fourteenth century, the knowledge of glass production spread throughout Europe. The industrial revolution brought the mass production of many kinds of glass.
Forensic Science: Fundamentals & Investigations, Chapter 14 3 What Is Glass? Once it cools, glass can be polished, ground, or cut for useful or decorative purposes. Glass blowers can form glass into many different shapes. Crystalline solids have a regular atomic structure (illus- trated above, left). But glass is an amorphous solid and so has an irregular atomic structure (illustrated above, right), Because of this, glass breaks in a variety of fracture patterns.
Forensic Science: Fundamentals & Investigations, Chapter 14 4 Types of Glass Glass is an amorphous solid usually made from silica (SiO 2 ), calcium oxide, and sodium oxide. Because glass is made of a variety of compounds, there are many types of glass. Adding different metal oxides to glass mixtures, for example, yields different colors. And because different types of glass have different densities and refraction indexes, it is possible to compare one type of glass with another.
Forensic Science: Fundamentals & Investigations, Chapter 14 5 Fracture Patterns in Broken Glass Glass can stretch slightly when hit. And because it’s an amorphous solid, it will not break into regular pieces with straight line fractures. That means fracture patterns can provide clues about the direction, rate, and sequence of the impacts.
7 Why Radial and Concentric Fractures Form 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 8 Radial Fracture Primary Fracture Crack that extends out like spoke of wheel from the point at which glass was struck Show up on surface opposite of where pressure was
Forensic Science: Fundamentals & Investigations, Chapter 14 9 Concentric Fracture Secondary Fracture Crack that forms rough circles around point of impact Crack is on entrance side of glass
Forensic Science: Fundamentals & Investigations, Chapter Bullet Fractures As a bullet passes through glass, it pushes a cone shaped piece of glass out of the glass ahead of it. Forming a saucer shaped depression on exit side. This makes the exit side of the hole larger than the entrance side of the hole.
Forensic Science: Fundamentals & Investigations, Chapter Path of a Bullet Passing through Window Glass The entry hole will be round if the bullet was fired perpendicular to the pane. If fired from an angle, glass pieces will be forced out to the opposite side from the shot. The angles at which bullets enter window glass can help locate the position of the shooter. Bits of the glass can fly backward (backscatter), possibly creating trace evidence.
Forensic Science: Fundamentals & Investigations, Chapter Radial Cracks Edges of broken glass contain ridge lines (curved) 3 R rule – radial cracks (factures) form at right angles on the reverse side of the force
Forensic Science: Fundamentals & Investigations, Chapter Comparison of Glass Fragments 1. Match fractured pieces (puzzles) – don’t touch actual pieces (causes flaking) (Indvl) 2. Match wavy lines that appear when you shine light through it. (Indvl) 3. Density (class) 4. Index of Refraction (class)
Forensic Science: Fundamentals & Investigations, Chapter Density The formula for calculating density is: D = m / V The mass (m) of a fragment of glass can be found using a balance. Displacement Method -Place the fragment of glass into a graduated cylinder filled with water and measure the volume (V) of overflow. Floatation Method – float solid in a liquid of the same density. Archimedes Principle
Density Lab Obtain a balance, piece of fine string, and a small beaker find the mass of a piece of glass to the nearest.01 g Tie the string around the piece of glass Fill the beaker nearly full of water. Zero out the balance Place the piece of glass in the beaker, adjusting the height so that it is not touching the sides or bottom of the beaker. Record the new mass (use units of ml) This is the volume of the glass (because the density of water is 1 g/mL). Repeat this process for all the known samples of glass and the unknown. Calculate the density of each piece of glass. Identify your unknown. Forensic Science: Fundamentals & Investigations, Chapter 14 21
Forensic Science: Fundamentals & Investigations, Chapter Refractive Index When a beam of light moves from one medium into another, its speed changes. That change causes the beam to change direction, bend. In the illustration above, the green line to the right of the red line shows the direction if the beam had not changed direction. But the black line shows it bent toward the red line.
Forensic Science: Fundamentals & Investigations, Chapter Refractive Index Refraction is the change in the direction of light as it changes speed when moving from one medium into another. The direction of the light forms two angles with the normal. If the light passes into a denser medium (the gray area), its direction will bend toward the normal.
Forensic Science: Fundamentals & Investigations, Chapter Application of Refractive Index to Forensics Water Index = 1.333, (travels times faster in vacuum than in water) Light passed through two fragments of glass— one found at the crime scene and the other connected to a suspect—may have the same refraction and, so, link one with the other. Becke Line- bright halo observed near the border of a particle that is immersed in a liquid that has a different Index of Refrac.
Forensic Science: Fundamentals & Investigations, Chapter Application of Refractive Index to Forensics If the glass fragments found at the crime scene, or connected with the suspect, or both are too small to check for this consistency by passing light through them, other methods such as the submersion method can be used to provide estimates. Use the illustration above to describe the submersion method for obtaining refraction estimates.
Forensic Science: Fundamentals & Investigations, Chapter Handling of Crime Scene Glass Samples 1. Identify and photograph any glass samples before moving them. 2. Collect the largest fragments that can be reasonably collected. 3. Identify the outside and inside surface of any glass. 4. If multiple panes are involved, make a diagram. 5. Note trace evidence such as skin, hair, blood, or fibers. 6. Package all materials collected to maintain the chain of custody.
Forensic Science: Fundamentals & Investigations, Chapter Summary Summary Different kinds of glass can be analyzed for their density, refractiveness, and fracture patterns. The different compounds that make up a kind of glass affect its density. The refractiveness of glass can be measured in a number of ways. Fracture patterns can provide information about such things as the direction, the rate, and the sequence of the impacts.