Fragile Evidence

1
Fragile Evidence

• A look at the formation, characteristics, history
  and types of glass.

2
Vocabulary

• Amorphous
• Becke Line
• Density
• Glass
• Leaded Glass
• Normal Line
• Obsidian
• Refraction
• Refractive Index
• Silicon Dioxide

3
History

• Glass a hard, amorphous, transparent material
  made by heating a mixture of sand and other
  additives.
• Rock exposed to high temperatures
  (lightning/volcanoes) forms glass
• Glass formed by volcanoes is called obsidian
  glasswas the 1st glass used by humans.
• Glass beads were found in Egypt and glass making
  used to take place on secluded islands due to the
  danger of fires and to keep it a secret.
• The Industrial Revolution brought about the mass
  production of glass.

4
How is glass formed?

• Hard, amorphous (without shape and random atom
  pattern) material made by melting sand, lime
  (calcium oxide), and sodium oxide at a high
  temperature.
• The main ingredient in glass is silicon dioxide
  (chemical name for silica).
• Soda-Lime Glass sodium oxide decreases the
  melting point of silica (melts easier) and
  calcium oxide prevents glass from being soluble
  in water.
• Glass is formed through heating, cooled,
  polished, ground, cut for decoration, and glass
  blowers form shapes by blowing hot air into the
  semi-liquid glass stage.
• Atoms of glass are arranged in a random pattern
  and this is why when it is broken, various
  patterns are formed.

5
Types of Glass

• Soda-Lime is the most common/inexpensive.
• Crystal/Leaded glass substitutes lead oxide for
  calcium oxidemakes glass more densesparkling
  effect as light passes through and light waves
  are bent in several directions.
• Pyrex/Limaxwithstands wide range of
  temperaturesmetal oxides are added to make
  different colorsNickel oxides make
  yellows/purplescobalt oxide makes purple-blue
  and selenium oxides make red.

6
Density

• Each type of glass has a different density
• Density is mass/volume
• Water displacement is used to measure the volume
  of a glass fragment
• Mass is found using a balance
• Bottled Glass2.5g/mL Window Glass2.53g/mL
  Lead Crystal2.98-3.01g/mL Pyrex2.27g/mLTempere
  d2.98g/mL Flint3.70g/mL Crown2.50g/mL

7
Refractive Index

• Refraction is the change in direction of light as
  it speeds up/slows down when moving from one
  medium to another.
• Direction/amount of bending, depends on the
  mediums density.
• Refractive index is a tool used to study how
  light bends as it passes through one substance
  into another.
• Each type of substance has its own refractive
  index.
• As density of a medium goes up, the speed of
  light passing through it goes down.
• RIspeed of light in air/speed of light in a
  substance
• RI of air and vacuum 1

8
RI Calculations

• A normal line is the line perpendicular to the
  surface where 2 mediums meet.
• Incident Rayincoming ray of light through first
  medium
• Refracted Rayincoming ray of light through
  second medium
• Angle of IncidenceAngle that the incident ray in
  first medium forms with the normal.
• Angle of RefractionAngle of refracted ray in
  second medium forms with the normal.
• Snells Law---n1(sine of angle )n2(sine angle 2)

9
Submersion Method

• Dropping a piece of glass in 2 different types of
  liquid will help determine which type of glass
  you have. If the glass disappears when dropped,
  it has the same RI as the liquid.
• RIs of different liquids Methanol/water1.33
  Isopropyl alcohol1.37 Olive oil/glycerin1.47
  Castor oil1.48 Clove oil1.54 Cinnamon
  oil1.62

10
Becke Lines

• Submerging glass in liquid (on a microscope
  slide) and viewing under a compound
  microscopehalo-like ring forms around glass if 2
  RIs are different.halo is called a Becke line
• When glass/liquid have different RIs if the
  Becke line is on the inside perimeter of the
  glass fragment, the RI of the glass is higher
  than the RI of the liquid.and vice versa.
• GRIM 2 instruments do this for you, but are too
  expensive for most crime labs to buy.

11
Thickness/Patterns of Glass

• Picture frame glass1/8 in thick window
  glass3/32-1/8 in thick door glass3/16-1/4 in.
• Fracture patterns help identify glass type
  primary radial fracturesbreaks start at point
  of impact and move outward Concentric circles
  form around the point of impact Radial fractures
  form on the opposite side of the point of impact.
  Concentric circles form after radial fractures.

12
Fracture Patterns

• If shooter fired from left angle, glass is forced
  out to the right exit hole forms an irregular
  oval.
• If shooter fired from an angle at the right,
  glass is forced to the left irregular oval
• If bullet fired perpendicular to glass, entry
  hole of bullet will be round.
• High-speed bullet fired from a great distance
  will show characteristics similar to a
  slower-speed bullet fired from a closer range.

13
Special Glass Situations

• Bulletproof Glass Hard and soft glass soft
  glass stretches instead of shattering RIs are
  same thickness is ¾ to 3 in.
• Tempered Glass/Safety Glass 2 glass layers with
  plastic layer in the middle larger pieces with
  less concentric circles
• Backscatter Pieces that project backwards b/c
  fragments collidetrace evidence.
• Heat Fractures Produces wavy lines if wasnt
  broken before fire, no radial/concentric circles
• Scratch Patterns Fast moving objectless
  concentric circles slowermore

14
Handling Glass

• Identify/photograph pieces before moving
• Collect largest fragments
• Identify outside/inside surfaces
• Show window pane positions in a diagram
• Note trace evidence found with glasswill be
  separated in lab
• Properly package
• Separate by physical properties (size, color,
  texture)
• Objects thought to have broken the glass should
  be collected also

15
Cleaning Glass

• Note/collect trace evidence before cleaning
• Cleaning solvents/ultrasound cleaners
• Nondestructive methods such as compound
  microscope examination
• Avoid using all of the evidence in testing in
  case some is destroyed