Glass Evidence Solving the Crime

Types 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 SiO 2 and NaCO 3 CaCO 3 or CaO added so NOT water soluble Density is 2.44g/cm 3

Soda–lime Glass Surface of soda lime glass

Flint Glass Optical glassOptical glass High refractive index (RI)High refractive index (RI) RI = 1.45 to 2.00RI = 1.45 to 2.00 Contains 4-6% iron oxideContains 4-6% iron oxide Disposal poses a pollution problemDisposal poses a pollution problem Also used to make simulated diamondsAlso used to make simulated diamonds

Flint Glass lenses Lead crystal Optical instruments

Borosilicate Glass Heat-resistant glass First sold under brand name “Duran” Called Pyrex by 1915 Made of quartz, NaCO 3, CaCO 3, and boron Higher melting point & more break resistant Density 2.23 g/cm 3

Borosilicate 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 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 Glass

Refractive 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

Density 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 1 st on a balance Determine volume by placing sample in graduated cylinder of water D = M/V Water displacement tells sample volume

Density Window 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

Glass Breakage Pattern 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: 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 Glass

Shattered 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 Fracture Fractures in a radial pattern point outward from point of impact, forming pie-shaped sections

Concentric 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 types of fractures do you see?

Other Factors in Glass Analysis Heat-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 Holes A 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 1 st shot terminate when they meet fractures from 2 nd 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)