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Glass Evidence

Glass Evidence. Solving the Crime. Ms. Scribner’s Forensic Crime Science Class Eisenhower High School. What is glass?. Made from fused inorganic materials Amorphus; not crystalline (molecules not arranged) Varies in elemental formula Many shapes and colors. What is Glass?.

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Glass Evidence

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  1. Glass Evidence Solving the Crime • Ms. Scribner’s Forensic Crime Science Class Eisenhower High School

  2. What is glass? • Made from fused inorganic materials • Amorphus; not crystalline (molecules not arranged) • Varies in elemental formula • Many shapes and colors

  3. What is Glass? • No specific melting point • Softens over a temperature range. • Uniform amorphous solid (irregular atomic structure – middle picture). • Because of this, glass breaks in a variety of fracture patterns.

  4. What’s in Glass? • Formers (form glassy, non-crystalline structure): SiO2, B2O3, P2O5, GeO2, V2O5, As2O3, Sb2O5 • Fluxes (lowers melting point): Na2O, K2O, LiO, Al2O3, B2O3, Cs2O • Stabilizers (Chem./Corrosion Resistance): CaO, MgO, Al2O3, PbO, SrO, BaO, ZnO, ZrO

  5. How is glass involved in crime? • Small fragments can be used as trace evidence (link objects/people) • Crime scene reconstruction

  6. How is glass collected? • For reconstruction, detailed photographs can be taken or the entire pane of glass is collected.

  7. How is glass collected? • For trace evidence, usually visually collected with forceps and packaged in plastic bags.

  8. Types of Glass

  9. 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

  10. Soda–lime Glass • Surface of soda lime glass

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

  12. Flint Glass • Lead crystal • Optical instruments • lenses

  13. Borosilicate 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

  14. Borosilicate Glass • Question: • In lab, what label(s) should be on any test tube that you are going to heat? • Answer: Pyrex or Kimex

  15. Why is Glass important in forensics?

  16. 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

  17. Properties of Glass

  18. Glass Properties • Combinations of formers, fluxes, and stabilizers creates unique glass types with different properties: • Density • Refractive Index (RI)

  19. 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

  20. Refraction Index • https://www.youtube.com/watch?v=eX4JqYHjthE • https://www.youtube.com/watch?v=s3EK1lGkf2s • Disappearing glass video: https://www.youtube.com/watch?v=qH1S83Bkttw

  21. Refractive Index • The direction of the light forms two angles with the normal . • If the light passes into a denser medium (the gray area), its direction will bend toward the normal.

  22. Populations of Glass RIs • Because of all the different materials in glass, there will be multiple RI’s  a major one will stick out, like above.

  23. Snell’s Law of Refraction • Can figure out the RI of materials based on the angle the light hits and bends. • n1sin(θ1) = n2sin(θ2) • n1 = RI of incident light • θ1= angle of incident light • n2 = RI of refracted light • θ2= angle of refracted light

  24. Snell’s Law Example 1 • Light traveling in air (n1=1) hits a piece of glass at an angle of 30 degrees. The light refracts in the glass at an angle of 15 degrees. What is the index of refraction (RI) of the glass? • n1=1, θ1= 30, θ2= 15, n2=? • 1sin(30)=n2sin(15) • 0.5=0.2588n2 • n2 = 1.93

  25. Snell’s Law Example 2 • Light traveling in air (n1=1) hits a piece of glass at an angle of 45 degrees. If the glass has a refractive index of 1.5, what angle should the light bend at? • n1=1, θ1= 45, θ2= ?, n2=1.5 • 1sin(45)=1.5sin(θ2) • 0.7071=1.5sin(θ2) • .4714= sin(θ2) • θ2= 27 degrees

  26. Snell’s Law Practice • 1. Light traveling in air (n1=1) hits a piece of glass at an angle of 37 degrees. The light refracts in the glass at an angle of 30 degrees. What is the index of refraction (RI) of the glass? • 2. Light traveling in air (n1=1) hits a piece of glass at an angle of 60 degrees. The light refracts in the glass at an angle of 15 degrees. What is the index of refraction (RI) of the glass? • 3. Light traveling in air (n1=1) hits a piece of glass at an angle of 25 degrees. If the glass has a refractive index of 1.75, what angle should the light bend at? • 4. Light traveling in air (n1=1) hits a piece of glass at an angle of 55 degrees. If the glass has a refractive index of 1.3, what angle should the light bend at?

  27. Density The formula for calculating density is: D = m / V • The mass (m) of a fragment of glass can be found using a balance beam device. • Place the fragment of glass into a beaker filled with water and measure the volume (V) of overflow. • Divide the mass (in grams) by the volume (in milliliters) to find the density (D) of the glass fragment. Forensic Science: Fundamentals & Investigations, Chapter 14

  28. 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 1964-1997.

  29. Density Practice (Left Page) • Find the density for the following pieces of glass: • 1. Mass: 4g, Volume Displaced: 2mL • 2. Mass: 15g, Volume Displaced: 3mL • 3. Mass: 1g, Volume Displaced: 2mL

  30. 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

  31. 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?

  32. Fractured Glass

  33. 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

  34. Radial Fracture • Fractures in a radial pattern point outward from point of impact, forming pie-shaped sections

  35. For reconstruction: • Radial cracks • Concentric cracks

  36. Concentric Fractures • Secondary breaks/fractures • Occur between the radial fractures • On side of glass where force was applied • Similar to spider web

  37. For reconstruction: • Determine the direction of projectile. • When looking at a radial section, conchoidal fractures point back to the origin of the break • Core usually on opposite side of the origin

  38. For reconstruction: • Determine the order of projectiles when dealing with more than one. • 2 • 1 • 3

  39. For reconstruction: • Determine the order of projectiles when dealing with more than one. • 2 • 1 • 3

  40. For trace analysis: Associate unknown glass found at the crime scene with known samples to determine the source. Done with microscopic analysis • Color, curve, thickness, etc. • Refractive index: Determined by placing glass in oils of different refractive indexes

  41. Refractive index: • Low relief = no outside line R.I. of glass and oil are similar • High relief = thick outside line (Becke line) R.I. of glass and oil are different

  42. Question: What type or types of fractures do you see?

  43. 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

  44. 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 1st shot terminate when they meet fractures from 2nd shot

  45. 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)

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