The Jagged Edge

1. The Jagged Edge Glass Fragment Identification

2. Forensic Materials Science • Scientific Working Group for Materials Science • SWGMAT provides Guidelines/Best Practices that have application to Fibers, Glass, Paint, Hair, and Tape.  • SWGMAT also develops standards to accredit laboratories and expert witnesses. • http://swgmat.org/

3. Evidence – Class vs. Individual • Class evidence shares physical characteristics with a group of objects or individuals. • Individual evidence originates with a single person or source. • Some categories of physical evidence can be only be class evidence while others can be either class or individual evidence, including glass.

4. Glass Evidence: Class or Individual? • Individual: Broken glass pieces can be fitted together like a puzzle. A specific fragment can be uniquely placed at a crime scene. • Class: Small fragments of glass can transfer to a victim or perpetrator of a crime or their vehicle. • After a hit and run accident, glass fragments consistent with a vehicle class can be identified, even if the specific vehicle is not known.

5. Glass Chemistry • Glass is made by heating silica (sand) with soda ash (sodium oxide, Na2O) and lime (calcium oxide, CaO) to a molten mass, then cooling it so quickly that large crystals do not form. • Glass is processed by rolling it into sheets or by blowing or molding to desired shapes.

6. Specialty Glass • Metal oxides are added to make colored glass. • Frosted glass has surfaces treated with acid or a plastic film. • Tempered glass is stronger than normal glass. It is made by a rapid heating and cooling process. • Pyrex® baking dishes • Corelle® dinnerware.

7. Polymer Glass • Polymer glasses are strong transparent plastics which can replace silica glass in eyeglasses, drinking glasses, windows or vehicle tail lights. • Polymer glass is molded from several different plastics • Acrylic • Polycarbonate • Polyethylene terephthalate

8. Windshield Glass • Car windshields are made with laminated safety glass. • Safety glass has a layer of plastic between two pieces of ordinary glass. • Windshields are placed in cars using gaskets to keep them rigidly in place. • Modern windshields are designed not to fall out of the vehicle even if they shatter. • The laminated glass can break if an object is thrust into the windshield.

9. Shattered Windshield

10. Collection of Glass at Crime Scene • Every effort should be made to collect all the glass found if any possibility exists that glass fragments may be pieced together. • Typically two or more glass fragments are compared to determine if they originated from different sources. • Unless there is an exact fit between two pieces of broken glass, it isn’t possible to prove the glass pieces came from the same source.

11. Analyzing Glass Fragments • Forensic glass comparison requires the scientist to identify and measure properties that will match one glass fragment with another while minimizing or eliminating other glass sources. • Forensic scientists primarily examine two physical properties: • Density • Refractive index

12. Comparing Glass Densities • Density is mass per unit volume (g/cm3). • When two samples have the same volume, their weights will differ if the chemical elements that make up the material are different. • Glass with different elemental compositions will have different weights.

13. Comparing Glass Densities • The flotation method is a precise and rapid method for comparing glass densities. • A glass fragment is immersed in a series of liquids of varying densities. The glass chip will neither sink nor float in the liquid medium of the same density.

14. Measuring Refractive Index • The Refractive Index (RI) of a substance is a measure of the speed at which light travels (v) through that medium. • When light travels through two media with differing RIs, the light becomes refracted, or bent. • This occurs because when the speed of the wave of light changes, the direction of that wave also changes.

15. Measuring Refractive Index • As light passes the border between media, depending upon the relative RIs of the two media, light will either be refracted to a lesser angle, or a greater one. These angles are measured with respect to the normal line. • In the case of light traveling from air into water, light is refracted towards the normal line, because the light is slowed down in water; light traveling from water to air refracts away from the normal line, since light speeds up.

16. Measuring Refractive Index • Snell's Law states: For a given pair of materials, sine of angle of incidence θ(in material 1)sine of angle of refraction θ(in material 2) is equal to v1 / v2, and equal to n2/ n1. • Sine (sin) is a trigonometric function. It is the ratio of the length of the side opposite an angle in a triangle to the length of the hypotenuse. • A scientific calculator will easily convert an angle into its sine.

17. Measuring Refractive Index θ2 Normal line refracted ray n2 n1 incident ray θ1

18. Snell’s Law N=1.52 N=1.33 The higher the n, the more the light bends

19. 2. Theory of Refraction, (Angular)

20. Measuring Refractive Index The Becke line is a bright halo near the border of a particle that is immersed in a liquid of a different refractive index. When the two RI are the same (the match point) the Becke line disappears and minimum contrast between liquid and particle is observed.

21. nglass >nmedium nglass < nmedium nmedium  = 1.525 nglass    = 1.60 nmedium = 1.525 nglass    = 1.34 Becke line:

22. Becke Line Kendall/Hunt Publishing Company

23. Refractive Indices for Common Items Vehicle Headlight 1.47 – 1.49 Window 1.49 – 1.51 Bottle 1.51 – 1.52 Contact Lens 1.52 – 1.53

24. Refractive index of household liquids Refractive index at 20°C • Baby oil: 1.45 • Canola oil: 1.465-1.467 • Olive oil: 1.467-1.4705 • Soybean oil: 1.470-1.472 • Grape Seed Oil: 1.471-1.478, • Castor Oil: 1.4750 - 1.4850 • Corn Oil: 1.4735 - 1.4785 • Xylene: 1.505 • Clove Oil: 1.543

25. FBI Refractive Index vs Density Data • The FBI has compiled density and refractive index data for glass from around the world. • The FBI has identified a relationship between their refractive indices and densities for 1400 glass specimens that is better at classification.

26. Measuring Refractive Index • Glass Refractive Index Measurement (GRIM) system employs a phase-contrast microscope with a temperature-controlled hot stage that allows for precise heating and cooling of the sample being analyzed. • The GRIM method uses a microscope slide containing glass fragments placed in special heatable immersion oil. This slide is placed on the heated stage and the microscope is focused and aligned.

27. Measuring Refractive Index • The temperature of the microscope stage is set so the RI of the oil is higher than that of the glass sample. • The temperature is then automatically lowered and the contrast between the glass shard and the oil is monitored. The match point (temperature of minimum contrast) is recorded. • This is then repeated by gradually heating the slide and this match point is also recorded. • The two numbers are averaged and this number can be compared between two fragments of glass to determine if they match or are differ.

28. X-ray Diffraction (XRD) • In XRD analysis a glass sample is bombarded with X-rays, and the atomic composition of the glass is determined through detection of the characteristic scattering of those X-rays by the electron clouds of the individual atoms comprising the sample. • The scattering pattern is then analyzed by specialized computer software to determine these atoms.

29. Elemental Analysis • Laser Ablation- Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) • A small portion of the sample is removed from the surface by laser irradiation. • This removed sample portion is then converted to atomic ions (charged atomic particles). • An instrument called a mass spectrometer is then used to separate and analyze the characteristic ions produced based upon the mass-to-charge ratio of those ions.

30. Elemental Analysis • The elemental analysis of the evidence glass is then compared to the known glass from a crime scene (such as a broken window). • If no known glass exists, the evidence can be compared with other types of glass (headlamp, window, drinking glasses). • It is also possible that the company that made the glass or even the geographical location where the glass was made can be determined from the composition.