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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? 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. 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 How is glass involved in crime? Small fragments can be used as trace evidence (link objects/people) Crime scene reconstruction How is glass collected? For reconstruction, detailed photographs can be taken or the entire pane of glass is collected. How is glass collected? For trace evidence, usually visually collected with forceps and packaged in plastic bags. 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 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 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 Flint Glass Lead crystal Optical instruments lenses 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 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 Glass Properties Combinations of formers, fluxes, and stabilizers creates unique glass types with different properties: Density Refractive Index (RI) 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 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 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. 21 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. 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 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 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 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? 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 27 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 19641997. 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 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 For reconstruction: Radial cracks Concentric cracks Concentric Fractures • Secondary breaks/fractures • Occur between the radial fractures • On side of glass where force was applied • Similar to spider web For reconstruction: 1. 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 For reconstruction: 2. Determine the order of projectiles when dealing with more than one. 2 1 3 For reconstruction: 2. Determine the order of projectiles when dealing with more than one. 2 1 3 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 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 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 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)