Glass Analysis Forensic Science 4/7/15

Drill What are some forms of evidence that could be found at a crime scene that we have NOT yet studied? How could you determine if a clear object was made of glass, or not?

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

Forensic Glass Analysis

Unit Overview People have been producing and improving upon glass for hundreds of years. Because glass is found throughout our homes, in vehicles, and even on ourselves, crime scenes often contain glass evidence.

Unit Overview Most often the composition of glass is unique and therefore identifiable by its properties. In science what type of properties are we referring to? Let’s brainstorm examples.

Physical vs. Chemical Properties The forensic scientist must constantly determine those properties that impart distinguishing characteristics to matter, giving it a unique identity. Glass can be identified by the compounds used to make it, its density, and also by its refractive index.

The Nature of Matter What is matter? –Matter is anything that has mass and occupies space. What is an element? –An element is the simplest substance known and provides the building block from which all matter is composed.

The Nature of Matter All of the elements are listed by name and symbol in the periodic table. Two or more elements chemically combine to form a compound. An atom is the basic particle of an element and a molecule (or formula unit) is the smallest unit of a compound.

Physical vs. Chemical Properties Physical properties such as weight, volume, color, boiling point, and melting point describe a substance without reference to any other substance.

Physical vs. Chemical Properties A chemical property describes the behavior of a substance when it reacts or combines with another substance.

Physical vs. Chemical Properties Light waves travel in air at a constant velocity until they penetrate another medium, such as glass or water, at which point they are suddenly slowed, causing the rays to bend. The bending of light waves because of a change in velocity is called refraction.

Physical vs. Chemical Properties Different forms of glass have differing densities. Density = ? D = m/V Density of glass varies –Density Range: g/cm 3

Introduction and History of Glass Egypt circa 2500 B.C.—The earliest known human-made glass objects (beads) Forensic Science: Fundamentals & Investigations, Chapter 14 14

History of Glass 1st Century B.C.— glass blowing begins 13th Century— specialized glass production was an art, a science, and a state secret in the republic of Venice

Forensic Science: Fundamentals & Investigations, Chapter Introduction and History of Glass 14th 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

What is Glass? The melting together of sand and a few other compounds at very high temperatures forms most glass.

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

19 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

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 mass-produced and a particular piece of glass is not unique. Class evidence However if pieces of glass from suspect to reconstruct perfectly into a whole object, this is a match. Individual evidence

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 29

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

Closure Identify the type of breakage –Wavy breaks that show no curved pattern –Radial and concentric fractures with a small hole –Small, cube-shaped pieces of glass