1. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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. Glass Evidence: Class or Individual?

2. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Glass is made by heating silica (sand) with soda ash (sodium oxide, Na 2 O) 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. Glass Chemistry

3. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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. Specialty Glass

4. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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 Polymer Glass

5. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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. Windshield Glass

6. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Shattered Windshield

7. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com As a result of hit-and-run or other vehicle accidents, headlights, tail lights and other lamps are frequently broken. Windshield glass, tempered glass, and mirrors break less commonly, but are also possible. Both the scene of the accident and the clothing of the victim can be sources of glass fragments. Glass in Car Accidents

8. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Any person standing close to glass when it is broken can pick up fragments of the broken glass, particularly on clothing. Glass fragments can travel forward and backward from the direction of the breaking force. If a window was broken in a crime, clothing worn by the suspect should be collected and examined. Collection of Glass at Crime Scene

9. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Every effort should be made to collect all the glass found if any possibility exists that glass fragments may be pieced together. When the pieces are too small to individually fit, the crime scene investigator must submit all glass evidence found in the possessions or clothing of the suspect along with a representative sample of broken glass obtained at the crime scene. Collection of Glass at Crime Scene

10. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Glass fragments should be packaged in solid containers (boxes or vials) to avoid additional breakage. Avoid packaging in cotton. Plastic wrap can cause static electricity and make glass fragments difficult to handle. Glass can cut through paper envelopes, so handle carefully. Collection of Glass at Crime Scene

11. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com If the suspect’s shoes or clothing are to be checked for glass fragments, they should be individually wrapped in paper and transferred to the laboratory. Collection of Glass at Crime Scene

12. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Window glass penetrated by a projectile (e.g. bullet or stone) can show cracks which radiate outward (radial fractures) and encircle the hole (concentric fractures). Forensic scientists analyze these radial and concentric fracture patterns to determine the direction of the projectile that impacted the glass. Analyzing Cracks

13. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Bullets and other high-velocity projectiles often leave a hole that is wider at the exit side (like a cone). This is important in determining the direction of impact. Analyzing Cracks

14. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Direction of impact follows the “3 R’s” Rule: Radial cracks form a Right angle on the Reverse side of the force. If several bullets break a sheet of glass, it can be important to determine the sequence of impacts. It is possible to determine the sequence because a fracture always terminates at an existing line of fracture. Analyzing Cracks

15. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Intense heat can also cause thermal fractures. In non-tempered glass, a typical heat crack is curved, has a smooth edge, and has no indication of the point of origin of the crack. Hot glass that is too quickly chilled can also have thermal fractures. Analyzing Cracks

16. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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. Once a single test shows two pieces don’t match, further tests are unnecessary. Evidence can show the fragments are consistent with a match to a class of glass. Forensic Glass Analysis

17. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Less frequently, the forensic examiner determines the end use of the glass or its manufacture (e.g. a lightbulb, eyeglasses). The physical properties used for comparison include glass color, fluorescence, thickness, surface features, curvature and optical properties. These can be assessed using various rapid and nondestructive tests. Forensic Glass Analysis

18. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com It is critical to initially determine if the fragments are silica or polymer glass. Silica glass is much harder and has a higher melting point than polymer glass. Some glass fragments may be too small for analysis. Forensic Glass Analysis

19. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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: 1.Density 2.Refractive index Analyzing Glass Fragments

20. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Density is mass per unit volume (g/cm 3 ). When two samples have the same volume, their weights will differ if the chemical elements that make up the material are different. Higher atomic numbers (# of protons) tend to mean heavier atoms for those elements. Comparing Glass Densities

21. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com One cubic centimeter of silver (atomic number 47) weighs less than one cubic centimeter of gold (atomic number 79). Glass with different elemental compositions will have different weights. Comparing Glass Densities

22. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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. Comparing Glass Densities

23. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com If the glass sinks, it is more dense than the liquid and if floats it is less dense. By comparison, two glass fragments can be shown to share the same density or have different densities, eliminating a match. The density of a suspending liquid mixture can be accurately determined using a density meter. Comparing Glass Densities

24. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com The Refractive Index (RI) of a substance is a measure of the speed at which light travels (v) through that medium. RI is defined as the ratio of the speed of light in a vacuum to the speed of light traveling through the medium being analyzed. RI is denoted with the unitless variable n. Measuring Refractive Index

25. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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. Snell's law is used to determine the direction of light rays through refractive media with varying RIs. Measuring Refractive Index

26. The Angle of Incidence is the angle θ from an imaginary line perpendicular to a surface, called the normal line (dashed). Measuring Refractive Index θ

27. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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. Measuring Refractive Index

28. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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 v 1 / v 2, and equal to n 2 / n 1. 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. Measuring Refractive Index

29. θ2θ2 θ1θ1 n1n1 n2n2 incident ray refracted ray

30. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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. Measuring Refractive Index

31. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com A glass particle can be sequentially immersed in a series of different liquid media whose refractive indices are known. When the refractive index of the liquid matches that of the glass particle, the Becke line disappears. Measuring Refractive Index

32. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Vehicle Headlight1.47 – 1.49 Window1.49 – 1.51 Bottle1.51 – 1.52 Contact Lens1.52 – 1.53 Refractive Indices for Common Items

33. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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. Measuring Refractive Index

34. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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. Measuring Refractive Index

35. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Glass manufacturers control the concentrations of many chemical elements in their product. However, the concentrations of trace elements are rarely controlled unless they alter the glass in some undesirable way. Trace elements can come from impurities in the raw materials and/or from deterioration of the glass furnace. Glass composition analysis can be used to identify: –Different manufacturers, –Different production lines of the same manufacturer –Differences made over a period of time in a single production line Elemental Analysis

36. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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. X-ray Diffraction (XRD)

37. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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. Elemental Analysis

38. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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. Elemental Analysis

39. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com