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)