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Glass Fragment Identification 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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 Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Shattered Windshield Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Glass in Car Accidents • 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Collection of Glass at Crime Scene • 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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. • 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Collection of Glass at Crime Scene • 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Collection of Glass at Crime Scene • 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Analyzing Cracks • 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Analyzing Cracks • 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Analyzing Cracks • 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Analyzing Cracks • 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Next Step… • Read and OUTLINE the Intro Glass Examinations article • Make sure you turn in your pre-lab/post lab questions from yesterday! Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Forensic Glass Analysis • 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Forensic Glass Analysis • 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Forensic Glass Analysis • 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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: 1. Density 2. Refractive index Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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. • Higher atomic numbers (# of protons) tend to mean heavier atoms for those elements. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Comparing Glass Densities • 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Comparing Glass Densities • 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Measuring Refractive Index • 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Measuring Refractive Index • 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Measuring Refractive Index The Angle of Incidence is the angle θ from an imaginary line perpendicular to a surface, called the normal line (dashed). θ 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Measuring Refractive Index θ2 n2 n1 θ1 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Measuring Refractive Index 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. Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com 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 Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com Next Steps… • Read Glass Fragment Identification background and Lab Procedure – Answer Pre-lab questions • Get started on LAB 1 Copyright © 2013 Crosscutting Concepts, LLC. All Rights Reserved. www.CrosscuttingConcepts.com