More about Matter and Glass Analysis 11/14/13 Forensic Science

Drill Describe some of the chemical and physical properties of glass. HW: Finish Chapter 4 Questions

Objectives IWBAT: –Define the physical and chemical properties of glass. –Distinguish between crystalline solids and amorphous solids. –Explain how to determine the order of impacts in a window. –Describe different types of glass breakage.

Agenda Take notes at computers on glass and glass breakage patterns. Start “Glass is Breaking up” dry lab.

5 What 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

What is Glass? What state of matter is glass?

The States of Matter Matter can be classified according to the physical form it takes. Solid-definite shape and volume Liquid-specific volume, takes the shape of its container Gas/vapor-neither a definite shape nor volume

The States of Matter Substances can change from one phase to another without forming a new chemical species, matter is simply being changed from physical state to another. Whenever a situation exists in which a substance can be distinguished by a visible boundary, different phases exist.

Types of Glass 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.

Types of Glass Tempered glass- safety glass, auto glass, shower doors and plate glass windows in store Same chemical makeup as soda lime glass, but top layer is cooled first causing internal stresses. When broken this glass fractures into small blunt pieces.

Types of Glass Obsidian – volcanic glass Leaded glass – glass containing lead oxide

Different glasses: domestic windows, car headlamps, light bulb glass All are produced differently with different chemistries.

Forensic Science: Fundamentals & Investigations, Chapter Types of Glass Many types of glass Adding metal oxides yields different colors Different densities Refraction indexes These characteristics allow comparisons

How is glass used by Forensic Scientist? 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? 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 into a whole object, this is a match.

Forensic Science: Fundamentals & Investigations, Chapter Fracture Patterns in Broken Glass 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

Microscopic characteristics can assist in making decision about suspected match Random stress patterns and breaks are unique

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 18

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

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

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 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 Path of a Bullet Passing through Window Glass 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 perpendicular to the glass shot from the left shot from the right

Sequencing Fractures Radial lines always end in existing radial lines

Blunt Object Fractures 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 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

Glass is Breaking Up Each group will work on the lab independently, turning in one packet.