Glass EvidenceSolving the Crime
Types of GlassTypes of Glass
Soda–lime Glass
•Most prevalent type of glass• Inexpensive to process•Used for window panes, bottles,
mirrors, data storage disks, light bulbs, etc.
•Made by melting SiO2 and NaCO3
•CaCO3 or CaO added so NOT water soluble
•Density is 2.44g/cm3
Soda–lime Glass
Surface of soda lime glass
Flint GlassFlint Glass• OpticalOptical glass glass• HighHigh refractive index (RI) refractive index (RI)• RI = RI = 1.45 to 2.001.45 to 2.00• Contains Contains 4-6% iron oxide4-6% iron oxide• Disposal poses a Disposal poses a pollution pollution
problemproblem• Also used to make Also used to make
simulated diamondssimulated diamonds
Flint GlassFlint Glass
lenses
Lead crystal
Optical instrumen
ts
Borosilicate GlassBorosilicate Glass
•Heat-resistant glass•First sold under brand name
“Duran”•Called Pyrex by 1915•Made of quartz, NaCO3,
CaCO3, and boron•Higher melting point &
more break resistant•Density 2.23 g/cm3
Borosilicate GlassBorosilicate Glass
Question:
In lab, what label(s)
should be on any test tube that you are going to
heat?Answer: Pyrex or Kimex
Why is Glass important in forensics?
Trace EvidenceTrace Evidence
•CSI investigators must work backwards from evidence at the crime scene
•Collect blood, hair, fibers, fingerprints, & broken glass
•Used to establish a link between the suspect and the crime
•Often present in burglaries and hit and run accidents
Properties of GlassProperties of Glass
Refractive IndexRefractive Index
• Refers to how light passes through the glass
• Variations in the composition of the glass cause bending of light rays
• Extent to which light bends is called refractive index
• Standards used to determine RI
DensityDensity• Affected by the type & amount of
material used to make the glass• Different types of glass have
different densities• NOT affected by the size or shape of
the material (glass fragment)• Mass the sample 1st on a balance• Determine volume by placing
sample in graduated cylinder of water
• D = M/V• Water displacement tells sample
volume
DensityWindow glass does not have uniform
density• Take samples from different
locations
Edge of tempered glass is denser than the interior
FBI has reported density results for 1400 glass samples recovered from 1964-1997.
Glass Breakage Glass Breakage PatternPattern
•Glass breaks or shatters in patterns
•Less quantitative (numerical data) and more qualitative
•Glass fragments can fit together like a jigsaw puzzle
•Composition determines how much the glass will break
Question:Question:
• In a court of law, which type(s) of glass evidence do you think would be the most helpful in convicting a suspect?
• In a court of law, which type(s) of glass evidence do you think would be the least helpful in convicting a suspect?
Fractured GlassFractured Glass
Shattered GlassShattered Glass•When glass is
struck, it breaks in a specific manner
•Applied force compresses the glass
•Glass begins to break OPPOSITE the applied force
•Forms 2 types of fractures – radial & concentric
Radial FractureRadial Fracture
•Fractures in a radial pattern point outward from point of impact, forming pie-shaped sections
Concentric FracturesConcentric Fractures
•Secondary breaks/fractures
•Occur between the radial fractures
•On side of glass where force was applied
•Similar to spider web
Question: What type or Question: What type or types of fractures do you types of fractures do you
see?see?
Other Factors in Glass AnalysisHeat-tempered glass (safety glass)
breaks in small, cube-like pieces• Used in car windows
Imperfections in manufacturing glass
Distinct marks/scratches (i.e. caused by debris on windshield wiper
Bullet HolesA high-speed projectile leaves exit hole
larger than entrance hole• helps determine direction of impact
Examine fracture lines to determine order in which penetrations of the glass occurred• fractures from 1st shot terminate when
they meet fractures from 2nd shot
Other Properties of Glass• Surface striations and markings
• Surface contaminants
• Thickness
• Hardness
• Color
• Elemental analysis• scanning electron microscope• X-ray analysis• Flameless atomic absorption
spectrophotometry (destroys sample)