Forensic Glass Analysis

1
Forensic Glass Analysis
2
Pre- assessment

• When you pick up a clear object, how do you
  determine if the object is glass?

3
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.

4
Unit Overview

• Most often the composition of glass is unique and
  therefore identifiable.
• In science what type of properties are we
  referring and give two examples?

5
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.

6
The Nature of Matter

• What is the nature of matter?
• What is matter?
• What is an element?

7
The Nature of Matter

• Matter is anything that has a mass and occupies
  space.
• An element is the simplest substance known and
  provides the building block from which all matter
  is composed.

8
The Nature of Matter

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

9
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.
• A chemical property describes the behavior of a
  substance when it reacts or combines with another
  substance.

10
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.

11
Particle Nature of Light

• Light also behaves as a particle
• In 1900, the German physicist Max Planck
  (18581947) began searching for an explanation.
• That is, while light has wavelike
  characteristics, it also can be a stream of tiny
  particles, or bundles of energy, called photons

12
Electrons Light

• Light (electromagnetic radiation) is a form of
  energy - it acts like a wave and a particle
• Caused by exciting electrons jumping energy
  levels and then returning to original energy
  level
• We see different colors of light (wavelengths)
  depending on how many energy levels electrons
  move and the amount of energy they possess

13
Electromagnetic Spectrum
14
Physical vs. Chemical Properties

• The bending of light waves because of a change in
  velocity is called refraction.
• Refractive index is the ratio of the velocity of
  light in a vacuum to that in the medium under
  examination.

15
Physical Properties

• Density of glass varies
• Sink-Float Method
• CHBr3, C2H2Br4, Sodium Polytungstate
• Density Range
• 2.465-2.540 g/cm3

16
Introduction and History of Glass

• Egypt circa 2500 B.C.The earliest known
  human-made glass objects (beads)
• 1st Century B.C.glass blowing begins
• 13th Centuryspecialized glass production was an
  art, a science, and a state secret in the
  republic of Venice

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Introduction and History of Glass

• 14th Centuryglass-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

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What is Glass?

• The melting together of sand and a few other
  compounds at very high temperatures forms most
  glass.

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What is Glass?

• Glass is an amorphous solid usually made from
  silica, calcium oxide, and sodium oxide.
• Silicas chemical name is silicon dioxide

20
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

21
What is Glass?

• 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.

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What is Glass?

• What state of matter is glass?

23
What is glass?

• Some people say that glass is a liquid because it
  flows over time. They note that glass in very
  old church windows is thicker at the bottom of
  the pane than at the top, claiming that the glass
  has flowed to the bottom. In actuality, when
  placed into windows, the panes were installed
  with the heavier, thicker side at the bottom.

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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

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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.

26
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.
• Glass used in vehicle wind shields have been
  created to not break apart when hit.

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Types of Glass

• Obsidian volcanic glass
• Leaded glass glass containing lead oxide
• There are mainly six basic types of glass. See
  Types of Glass handout

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Types of Glass

• Many types of glass
• Adding metal oxides yields different colors
• Different densities
• Refraction indexes
• These characteristics allow comparisons

29
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.

30
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.

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Different glasses-domestic windows, car
headlamps, light bulb glass-are produced
differently from different chemistries.
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• Safety glass is made so it will not crack and not
  break apart into pieces upon impact.

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Refractive Index

• When a beam of light moves from one medium into
  another
• The speed changes
• The direction bends
• Refractive Indexa tool used to study how light
  bends as it passes from one substance to another
• Normal line is perpendicular to the glass surface

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Refractive Index

• When a beam of light moves from less dense
  medium (air) into a more dense medium (water)
• Its speed slows, and
• Bends light toward the normal line

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Snells Law
N1.52
N1.33
The higher the n, the more the light bends
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Refractive Index

• When a beam of light moves from a more dense
  medium (glass) into a less denser medium (air)
• Its speed increases
• And bends light away from the normal line

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Application of Refractive Index to Forensics

• Submersion methodused when glass fragments
  found at the crime scene are small

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Application of Refractive Index to Forensics

• 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

39
Application of Refractive Index to Forensics

• Becke Linea halo-like effect appearing at the
  edges of a glass fragment when the reflective
  index of the glass and liquid are different
• If the line is inside the glass perimeter, the
  glass index is higher than the index of the
  liquid
• If the line is outside the glass perimeter, the
  glass index is lower

40
Becke Lines
Glass has higher refractive index-note white line
inside

• Glass has lower refractive index-note white line
  outside

41
Fracture Patterns inBroken 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

42
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.

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Why Radial and Concentric Fractures Form

• Glass after an impact shows radial fractures
  (red) and concentric circle fractures (blue)

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Why Radial and Concentric Fractures Form
45
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

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

• 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

47
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.

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Blunt Object Fractures
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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

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Heat Fractures
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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.