Glass Fragment Identification

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. 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. Forensic Materials Science

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. 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. Evidence – Class vs. Individual

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. 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?

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. 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

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. 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

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. 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

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. 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

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. Shattered Windshield

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. 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. Collection of Glass at Crime Scene

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. 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

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. 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. Glass with different elemental compositions will have different weights. Comparing Glass Densities

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. 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

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. 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. Measuring Refractive Index

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. 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

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. 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

θ2θ2 θ1θ1 n1n1 n2n2 incident ray refracted ray Normal line

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. Snell’s Law N=1.52 The higher the n, the more the light bends N=1.33

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved Theory of Refraction, (Angular)

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. 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

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. Becke line: n glass >n medium n glass < n medium n medium = n glass = 1.60 n medium = n glass = 1.34

Becke Line Kendall/Hunt Publishing Company22

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

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. Refractive index at 20°C Baby oil: 1.45 Canola oil: Olive oil: Soybean oil: Grape Seed Oil: , Castor Oil: Corn Oil: Xylene: Clove Oil: Refractive index of household liquids

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. FBI Refractive Index vs Density Data

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. 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

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. 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

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. 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)

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. 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

Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. 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