There is no den in the wide world to hide a rogue' Commit a crime and the earth is made of glass'

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• There is no den in the wide world to hide a
  rogue. Commit a crime and the earth is made of
  glass.
• Ralph Waldo Emerson

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Glass Analysis
Objectives

• explain how glass is formed
• learn the characteristics of different types of
  glass
• calculate the refractive index and density of
  glass
• describe how glass fractures

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What are some characteristics of glass?

• Hard, amorphous solid
• Usually transparent
• Primarily composed of silica with various amounts
  of elemental oxides
• Brittle
• Exhibits conchoidal fracture

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What Is Glass?

• 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.

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Common Types

• Soda-limeused in plate and window glass, glass
  containers, and electric light bulbs
• Leadedfine table ware and art objects
• Heat resistantoven and lab glass (Pyrex)
• Tempered / Laminated used in car windows

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Physical Characteristicsthat can be used in
forensics

• Densitymass divided by volume
• Refractive index (RI)the measure of light
  bending due to a change in velocity when
  traveling from one medium to another
• Fractures
• Color
• Thickness
• Markingsstriations, dimples, etc

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Density M / V
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Determination ofRefractive Index

• Immersion methodlower fragments into liquids
  whose refractive index is different.
• Match pointwhen the refractive index of the
  glass is equal to that of the liquid
• Becke linea halo-like shadow that appears around
  an object immersed in a liquid. It disappears
  when the refractive index of the liquid matches
  the refractive index of the glass fragment (the
  match point)

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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.

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Refractive Index

• Refraction is the change in the direction of
  light as it changes speed when moving from one
  medium into another.
• If the light passes into a denser medium (the
  gray area), its direction will bend toward the
  normal.

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Refractive Index

• If light passes from water into air, will it bend
  toward or away from the normal?
• If light passes from air into glass, will it bend
  toward or away from the normal?

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• The submersion method can be used to obtain
  refraction estimates.

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The Becke Line

• The Becke line is a halo that can be seen on
  the inside of the glass on the left, indicating
  that the glass has a higher refractive index than
  the liquid medium. The Becke line as seen on the
  right is outside of the glass, indicating just
  the opposite.

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Refractive Index
15
Fracture Patterns

• Radial fracture lines radiate out from the origin
  of the impact they begin on the opposite side of
  the force
• Concentric fracture lines are circular lines
  around the point of impact they begin on the
  same side as the force
• 3R ruleradial cracks form a right angle on the
  reverse side of the force.

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Fracture Patterns

• Glass can stretch slightly when hit.
• Does not break into regular pieces with straight
  line fractures (amorphous solid)
• That means fracture patterns can provide clues
  about the direction, rate, and sequence of the
  impacts.

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Sequencing

• A high velocity projectile always leaves a hole
  wider at the exit side of the glass.
• Cracks terminate at intersections with others.
  This can be used to determine the order that the
  fractures occurred.

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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.

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Glass as Evidence

• Class characteristics physical and chemical
  properties such as refractive index, density,
  color, chemical composition
• Individual characteristics if the fragments can
  fit together like pieces of a puzzle, the source
  can be considered unique

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Handling of Crime Scene Glass Samples

• Identify and photograph any glass samples 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.