Glass Fragment Identification

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.

Glass Chemistry 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.

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.

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

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.

Shattered Windshield

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.

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.

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.

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.

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.

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.

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.

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.

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.

Next Step Read and OUTLINE the Intro Glass Examinations article Make sure you turn in your pre-lab/post lab questions from yesterday!

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.

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.

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.

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

Comparing Glass Densities 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 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 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 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.

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.

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.

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.

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 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 θ 2 n 2 n 1 θ 1

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

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

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

Next Steps Read Glass Fragment Identification background and Lab Procedure Answer Pre-lab questions Get started on LAB 1