Analysis of Glass- Glass Evidence

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Analysis of Glass- Glass Evidence

• Chapter 4 Properties of Matter and the Analysis
  of Glass

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Glass evidence can be found at many crime scenes.

• Automobile accident sites may be littered with
  broken headlight or windshield glass.
• The site of a store break-in may contain shards
  of window glass with fibers or blood on them.
• If shots are fired into a window, the sequence
  and direction of the bullets can often be
  determined by examining the glass.
• Minute particles of glass may be transferred to a
  suspects shoes or clothing and can provide a
  source of trace evidence linking a suspect to a
  crime.

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How is glass formed?

• Long before humans began making glass, glass
  formed naturally.
• When certain types of rock are exposed to
  extremely high temperatures, such as lightning
  strikes or erupting volcanoes, glass can form.
• Obsidian is a type of glass formed by volcanoes.

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Timeline of Events

• Prehistoric humans used obsidian as a cutting
  tool.
• The earliest man-made glass objects (glass beads)
  were found in Egypt dating back to 2500 BC.
• Glass blowing began sometime during the first
  century BC.
• By the 14th century, knowledge of glass making
  spread throughout Europe.
• The Industrial Revolution brought the mass
  production of many kinds of glass.

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How is Glass Formed?

• Glass is a hard, brittle, amorphous material made
  by melting sand (aka silica,silicon dioxide,
  SiO2) lime (aka calcium oxide CaO) and soda,
  sodium carbonate (Na2CO3) at very high
  temperatures.
• The lime (CaO) is added to prevent the glass from
  being soluble in water.
• The soda (Na2CO3) is added to lower the melting
  point of silica (sand) and make it easier to work
  with.

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Soda-lime Glass (amorphous solid)The atoms are
arranged in a random fashion
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Types of Glass

• Soda-lime glass Mostly sand, sodium carbonate
  and calcium oxide
• Used for manufacturing most window and bottle
  glass

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Float Glass

• Flat glass typically used for windows.
• Soda-lime glass that has been cooled on top of a
  bath of molten tin.

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Borosilicates

• The common metal-oxides found in soda-lime glass
  are sodium, calcium, magnesium and aluminum.
• In addition, a wide variety of special glasses
  can be made by substituting in whole or in part
  other metal oxides for the silica, sodium and
  calcium oxides.
• Automobile headlights, heat-resistant glass such
  as Pyrex are manufactured by adding Boron oxide
  to the oxide mix
• Lab glassware, thermometers, cookware.

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Leaded Glass

• Fine glassware and decorative art glass, called
  crystal or leaded glass substitutes lead oxide
  for calcium oxide (lime).
• The addition of lead oxide makes the glass
  denser. As light passes through the more-dense
  glass, the light waves are bent, giving the glass
  a sparkling effect.

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Tempered Glass

• This glass is made stronger than ordinary window
  glass by introducing stress through rapid heating
  and cooling of the glass surfaces.
• When tempered glass breaks, it does not shatter
  but rather fragments or dices into small
  squares with litter splintering.
• Used for side and rear windows of automobiles
  sold in the United States.

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Laminated Glass

• This glass derives its strength by sandwiching
  one layer of plastic between two pieces of
  ordinary window glass.
• The windshields of all cars manufactured in the
  United States are constructed from laminated
  glass.

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Bulletproof Glass

• Bulletproof glass is a combination of two or more
  types of glass, one hard and one soft.
• The softer layer makes the glass more elastic so
  it can flex instead of shatter.
• The index of refraction for both of the glasses
  used in the bulletproof layers must be almost the
  same to keep the glass transparent and allow a
  clear view through the glass.
• Bulletproof glass varies in thickness from
  three-quarter inch to three inches.

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Properties of Glass and Comparing Glass Fragments

• For the forensic scientist, comparing glass
  consists of finding and measuring the properties
  that will associate one glass fragment with
  another while minimizing or eliminating the
  possible existence of other sources.
• Considering the prevalence of glass in our
  society, it is easy to appreciate the magnitude
  of this analytical problem.
• Obviously, glass possesses its greatest
  evidential value when it can be individualized to
  one source.

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Jigsaw Effect Most Beneficial

• When the suspect and crime-scene fragments are
  assembled and physically fitted together.
• Comparisons of this type require piecing together
  irregular edges of broken glass as well as
  matching all irregularities and striations on the
  broken surfaces. The possibility that two pieces
  of glass originating from different sources will
  fit together exactly is so unlikely as to exclude
  all other sources from practical consideration.
• Unfortunately, most glass evidence is either too
  fragmentary or too minute to permit a comparison
  of this type

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Density and Refractive Index

• The physical properties of density and refractive
  index are used most successfully for
  characterizing glass particles.
• These properties are class characteristics which
  can not provide the sole criteria for
  individualizing glass to a common source.
• These properties do give the analyst sufficient
  data to evaluate the significance of a glass
  comparison, and the absence of comparable density
  and refractive index values will certainly
  exclude glass fragments that originate from
  different sources.

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Measuring and Comparing Density

• Each type of glass has a density that is specific
  to that glass. One method of matching glass
  fragments is by density comparison.
• Density (D) is calculated by dividing the mass
  (m) of a substance by its volume (V). The
  formula for calculating density can be written as
  D m
• V

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Comparing Densities Flotation

• A solid particle will either float, sink, or
  remain suspended in a liquid, depending upon its
  density relative to the liquid medium.
• Flotation a standard / reference glass particle
  is immersed in a liquid a mixture of bromoform
  or bromobenzene may be used. The composition of
  the liquid is carefully adjusted by adding small
  amounts of bromoform or bromobenzene until the
  glass chip remains suspended in the liquid
  medium. At this point, the standard / reference
  glass sample and the liquid each have the same
  density. Glass chips (same size and shape as
  reference sample) are added to the liquid for
  comparison. If both the unknown and standard /
  reference samples remain suspended, they have the
  same density.

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Flotation
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Refractive Index

• Refractive Index
• When light travels from one medium to another its
  speed changes relative to the density of the
  medium. This can be observed as the light bends
  when traveling from one medium to another.

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Refractive Index
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Index of Refraction (Refractive Index)

• The speed of light in a vacuum is always the
  same,
• but when light moves through any other medium it
  travels more slowly since it is constantly being
  absorbed and reemitted by the atoms in the
  material.
• The ratio of the speed of light in a vacuum to
  the speed of light in another substance is
  defined as the index of refraction (aka
  refractive index or n) for the substance.

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Methods for Determining Refractive Index

• The FBI has a database off over 2000 refractive
  indexes of different types of glass which shows
  that glass is very distinctive and helps assign
  an appropriate statistical probability that the
  two pieces of glass share a common source.

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Snells Law

• Snells Law describes the behavior of light as it
  travels from one medium into a different medium.
  Snells law can be written as
• n1 (sine angle 1) n2 (sine angle 2)
• n1 is the refractive index of medium 1 and n2 is
  the refractive index of medium 2. Angle 1 is the
  angle of incidence and angle 2 is the angle of
  refraction. BOTH angles are measured relative to
  the normal or line drawn perpendicular to the
  surfaces where the two medium meet.

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Determining and Comparing Refractive Index

• Submersion method Place glass fragment into
  different liquids of known refractive indexes.
  If a piece of glass and a liquid have the same
  refractive index, the glass fragment will seem to
  disappear when placed in the liquid.
• Submersion and Low Power of Microscope. Submerge
  fragment of glass in a liquid and then view it
  under low power using a compound microscope. If
  the refractive index (n) of the liquid medium is
  different from the refractive index of the piece
  of glass, a halo-like ring appears around the
  edge of the glass. This halo-like effect is
  called a Becke line.
• The Becke line appears because the refracted
  light becomes concentrated around the edges of
  the glass fragment A Becke line is visible under
  a microscope when the glass and liquid have
  different refractive indexes.

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Becke line
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Becke line

• If the Becke line is located inside the perimeter
  of the glass fragment, than the refractive index
  of the glass is higher than the refractive index
  of the surrounding liquid.
• If the Becke line is located on the outside
  perimeter of the glass fragment, than the
  refractive index of the surrounding medium is
  higher than the refractive index of the glass.

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Becke line Indication of Refractive Index

• Notice the halo of light on the inside perimeter
  of the glass sample.
• When the Becke line is inside the perimeter of
  the glass fragment, the refractive index of the
  glass is higher than the refractive index of the
  surrounding medium.

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Becke line Indication of Refractive Index

• Notice the halo of light (Becke line) is outside
  the perimeter of the glass fragment.
• When the Becke line is outside the perimeter of
  the glass sample, the refractive index of the
  medium is higher than the refractive index of the
  glass.

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Annealing

• When trying to make a distinction between
  tempered glass and nontempered glass particles a
  process known as annealing is used.
• Annealing- slowly heating and then cooling the
  glass. A heat treatment that alters the
  microstructure of a material causing changes in
  properties such as strength and hardness ...
• The change in the refractive index value for
  tempered glass upon annealing is significantly
  greater when compared to nontempered glass and
  thus serves as a point of distinction.

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Fracture Patterns in Broken Glass

• Glass has some flexibility. When glass is hit,
  it can stretch slightly.
• When glass is forced to stretch too far, fracture
  lines appear and the glass may break.
• The fracture patterns on broken glass can provide
  clues about the direction and rate of impact.

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Primary Radial Fractures

• When glass breaks, fracture patterns form on the
  surface.
• Breaks, called primary radial fractures, are
  produced.
• These fractures start at the point of impact and
  radiate (like spokes on a wheel) outward from
  there.
• Radial fractures form on the side opposite the
  point of impact.

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Secondary Fractures

• Secondary fractures may also form.
• These fractures take the form of concentric
  circles around the point of impact.
• Concentric circles are circles that have the same
  center.
• Concentric circles form on the same side of the
  glass as the point of impact.

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Analyzing Glass Fracture Patterns
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3 R Rule- Determining Side of Impact

• Radial Cracks form a Right Angle on the Reverse
  side of the force.

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3R Rule

• Radial cracks are at Right angles to the Rear
  (side opposite theimpact)
• Exceptions
• tempered glassdices without forming
  ridgesvery small windows held tightly in
  framecant bend or bulge appreciably
• windows broken by heat or explosionno point
  of impact

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Successive Penetrations of Glass

• When there have been successive penetrations of
  glass, it is frequently possible to determine the
  sequence of impact by observing the existing
  fracture lines and their points of termination.
• A fracture always terminates at an existing line
  of fracture.

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Breakage of Glass from a Fire

• During a fire, glass may break as a result of
  heat fracturing.
• Heat fracturing produces breakage patterns on
  glass that are different from breakage patterns
  caused by impact.
• Wavy fracture lines develop in glass that has
  been exposed to high heat.
• Glass will tend to break toward the region of
  higher temperature.
• If the glass was not broken before the fire,
  there will be no radial or concentric circle
  fracture patterns in glass that is broken by high
  heat.

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Proper Collection of Glass Evidence

• Standard reference glass should be taken from the
  crime scene (1 in2)
• Package in solid containers to prevent breakage
• Preserve garment (shoe, pants, shirt) with glass
  on it
• All broken glass must be recovered and submitted
  for analysis when direction of impact is desired.
• Whenever possible, the exterior and interior
  surfaces of the glass must be indicated. The
  presence of dirt, paint, grease or putty may
  indicate the exterior surface of the glass.