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Glass : A Multitasking Class of Evidence
Introduction q Glass is an important class of associative evidence. § It is found in multiple crime types. From the smallest of splinters to an intact window with bullet holes, glass is evidence that tells a story about its relationship to the crime. Example q A burglar breaks a window in the basement of a house and climbs through. § He figures out the house is alarmed but a quick inspection reveals that the basement windows were not. q Perhaps the owners decided that the basement didn’t need alarming, rationalizing the windows were too small for someone to crawl through. Perhaps it was a financial decision. Regardless, the window is broken and there has been a burglary. q Broken glass provides specific pieces of investigative information, such as the § Direction of the break and the § Location of the entry point for the burglary.
Why is Glass Important? q Leads to other classes of evidence: footwear impressions, fingerprints, DNA, etc. q Glass Offers Information: § For example, maybe the robbery had been staged by the people living at the house. § If that was the case, inspecting the glass might reveal the direction of the break – outside in or inside out. ü If the break had been inside-to-out, it narrows the possibilities to two - assuming the window had not been broken prior to the burglary. q People in the house broke the window to stage burglary, q OR was exit point for burglar.
Always Consider Glass From It’s Forensic Value q Value as forensic evidence § A means to answer investigative questions § As a way to add to the puzzle of the investigation ü Pinpointing entrance/egress points ü Identify characteristics of the crime o Direction of gunshot o Shooter positions o Direction of window breaking Investigator Concerns q Finding glass evidence, q Making important on-scene determinations & interpretations q Collecting and preserving.
What Is Glass
Defined: “Any of various amorphous materials formed from a melt by cooling to rigidity without crystallization: as a : a usually transparent or translucent material consisting typically of a mixture of silicates b : a material (as obsidian) produced by fast cooling of magma. ” q Any amorphous material that melts and forms a rigid structure without crystallizing after cooling is a “glass. ” This means a glass is not necessarily made of silicon. Wikipedia gives plastics and resins as examples. Why understand in the composition of glass? q Different types of glass have different purposes and thus different chemical compositions … valuable because knowing the general composition of glass can give scene investigators information about how to search for it. q Certain types of glass that appear uncolored and transparent to the unaided eye have elemental compositions that can make them appear colored or fluoresce under UV light or the ALS.
Glass Types q Different types of glass depending on the manufacturing process § Flat glass: ü Sheet Glass o Made by rolling through rollers o Used for house windows ü Plate Glass o Made by rollers o Thicker than sheet glass & highly polished ü Float glass discriminated from double-ground & polished plate o Manufactured by “floating” molten glass onto surface of bath of melted tin o Tin diffuses into the glass & fluoresces under UV Ø Blue to yellowish glow & not on both surfaces o Very smooth & polished § Leaded Glass § Blue-white under UV § Rings like a bell after pinging with forefinger § Old Glass ü Antique glass (yellow/Vaseline glass - Uranium oxide) o Fluoresces bright yellow/green under UV ü Often has undissolved silica & other imperfections
Category of Common Glass Soda-lime glass – Very common Common Uses A type of flat glass used for containers, light bulbs. More than 90% of all glass manufactured. About 60 -75% silica, 12 -18% soda and 5 -15% lime and other minor ingredients. All glass manufacturers use the same basic soda-lime composition. Soda-lead glass – crystal – decorative items – common with differences among quality item. Called crystal or lead, lead oxide is used in place of calcium oxide, and is often the type Borosilicate glass – cooking glass common This is heat resistant glass – three times better than soda-lime glass - known by the of glass used for cut glass decanters, vases and bowls. The lead gives the glass a brilliance which makes it suitable for decorative glass. This type was typical of the collectable antique glass characteristic of the brilliant glass period in the mid to late 1800 ’s into the early 20 th century. Importantly, leaded glass will fluoresce a bright blue-white color under UV light. trade names Pyrex and Kimax. It contains about 80% silica, 4% sodium oxide, 2$ alumina and 13% boric oxide. This glass is used for ovenware and laboratory glass – beakers, test tubes, etc. Fused Silica Glass – found mostly in laboratories Highly heat-shock resistant, this glass is used for laboratory glassware and optical fibers. It can be heated to extremely high temperatures and then plunged into cold water without cracking. 100% silica. 96% Silica Glass – uncommon Resists heat nearly as well as fused silica glass but is less expensive to manufacture. It has a borosilicate composition made porous by chemical treatment, which shrink on heating. The glass is sold under the trade name Vycor. Colored glass – decorative common Glass gets its color from various oxides added to the glass during its manufacture. Nickel oxide gives glass a tint ranging from yellow to purple. Cobalt oxide yields blue glass and gold, copper or selenium oxides produce red glass. Some yellow antique glass contained uranium oxide – yes, it is radioactive – that fluoresces bright yellow-green under UV light.
Type of Glass Flat Glass Uses Type of Glass Uses Windows, mirrors, room dividers, furniture, windshields Eyeglasses, microscopes, telescopes, camera lenses Glass Containers Laminated Safety Glass Alternating layers of flat glass and plastic. Windshields Bullet-resistant glass Tempered safety glass Single piece & stronger than flat glass. All-glass Colored structural glass doors, side & rear windows of automobiles, basketball backboards. Breaks into small, cubes of glass. Small particles in the glass that disperses light Foam Glass making it appear milky. Contains fluorides & is used for lighting fixtures and tableware. Heavy plate glass. Building exterior facing, interior walls, partitions and tabletops. Two hollow-sections sealed at high temperature. Insulation made into walls & other structures. Withstands rapid change in temperature. Laboratory ware. Heat resistant glass Contains boric oxide. Withstands temperature changes. Cookware & resistance to chemical attack. Electrical applications. Resistant to flow of electric current & seals tightly to metals without cracking. Light bulbs, TV picture tubes. Glass optical fibers Transmit information as pulses of light, control board displays. Glass tubing Made from different types of glass. Fluorescent lights, neon signs, glass piping, chemical tubing. Radiation-absorbing and radiation-transmitting glass Transmits, modifies or blocks heat, light, Xrays & other radiant energy. UV light absorbs UV. Polarized glass cuts glare of brilliant light. One-way glass coated can see only one way. Laser glass Contains small amount of substances that allow it to generate laser beams. Invisible glass Coatings on cameras and eyeglasses. Chemical Photochromic glass film that decreases normal loss of light by reflection. Exposed to UV light and heat & reproduces Heavy metal fluoride pattern or photograph in the body of the glass Darkens when exposed to UV light. Sunglasses, windows, instrument controls. Made from elements of the chalcogen group – selenium, sulfur & tellurium. Transparent to IR light. Semiconductor in electronic devices. Used as protective coating on solar collectors or as insulating material. Optical Glass Opal glass Glass building blocks Laboratory glass Photosensitive glass Chalcogenide glass Fiberglass Glass for electrical uses Sol-Gel glass Packaging food, medicines, chemicals, jars, bottles. Fine, solid rods of glass. Insulation, glass yarn, tape, cloth and mats. electrical insulation, chemical filtration firefighters suits, automobiles, airplanes. Thick, multilayer laminated. Windshields, bank teller windows, military windshields, aircraft. Filled with many cells of gas. Light & floats on water. Heat insulation in buildings, steam pipes. Can be cut with a saw. Transparent glass used in optical fibers that infrared rays.
Glass in a Case q Characteristics § Many uses § Possesses different qualities § Breaks easily q Glass at the Scene § Ideal transfer evidence ü Ejects very small particles in different directions – Retained by fabrics, footwear Shards of glass from Crime Scene
Glass as Evidence Multitasking & Associative Evidence An example of multitasking, associative evidence … it occurs in many crime types AND provides associative forensic information that can often significantly help the on-scene investigation q Homicide/burglary/rape § Associative evidence Associative Transfer ü Origin estimation ü Broken glass tied to assailant/Burglar q Hit & Run § Associative evidence ü Glass transfer o Window glass § Headlight glass § Direction of break q Shooting Incident Scenes § Direction of shot § Sequence of shots Sequencing Shots Homicide Rape Burglary Shooting Hit & Run Direction of Break Direction of Shot
How Glass Breaks Understanding is Critical to Determining Direction of Force
Direction of Force q How Glass Breaks: Understanding how glass breaks gives insight into how to determine the direction of the force that broke the glass, say a window. q First it bends and then begins to fracture, beginning with the formation of radial fractures, R 1. As the stress on the glass continues, concentric fractures begin, C 1. Fracturing continues alternating with radial (R 2) and concentric (C 2), etc, until the breaking process ends. R 2 R 3 C 2 C 3 C 1 R 1
Glass Failure (Breaking) Annealed glass failure heat-strengthened glass failure fully tempered glass failure http: //www-g. eng. cam. ac. uk/gft/media/PG%20 projects/Mauro%27 s%20 publication/Diagnostic_Interpretation_of_Glass_Failure_DRAFT. pdf
Determining Direction of Force q When the first radial fracture begins, stress occurs along the edges closest to the point of impact, § Stress lines form called conchoidal fractures. § These form right angles opposite the side from where the force is applied,
Determining Direction of Force Conchoidal Stress Fractures on Edge of Radial Fracture Procedure Conchoidal Fracture Crack starts on side of glass that is opposite the applied force. 1. Remove piece of glass. 2. Locate edge corresponding to radial fracture near break 3. Examine edge for Conchoidal Fractures 4. Find (right angle) at extreme edge of fracture 5. Force came from opposite side of the right angle Direction of Force Edge of Radial Fracture Right Angle Formed By Chochoidal Fracture And edge of Radial Fracture
Conchoidal Striations Forensic Examination of Glass and Paint. Caddy, B. Ed. ; Taylor & Francis: London, 1999.
Radial Edge Technique Can Fail Not valid for all types of glass. q. Tempered glass is not easily reconstructed. Actually it is a daunting task. Reconstructing hundreds or thousands of similar looking chunks of sharp-edged glass is like mission impossible … though possible … takes time, perseverance. q. Laminated glass (automobile windshields) have special problems because of the plastic lamination holding the sandwiched glass together … radial edges are not available to examine because they are held together by the plastic laminate. q. Thermal glass… fracture is curved … smooth edge … § No conchoidal fractures and thus no determinable point of origin
Bullet Through Window q Premise: §Window fractures but remains intact as the bullet passes through it carrying shards of glass with it. q Bullet removes cone-like chunk of window … § Cone forms on the opposite side of the bullet path (exit side of bullet path. Shape of the cone q Helps elucidate direction of bullet. q A perfectly shaped (symmetrical) indicates a straight-through bullet q Skewed (asymmetric) cone shows bullet path at angle to the window
Debris Trail & Direction of Travel q The direction of force & approximation of the impact angle … determined by locating the trail of small shards/splinters a bullet carries with it when it exits the window. q Investigator must locate shards/splinters by highlighting the glass: ALS wavelength, a UV light or flashlight --- determined by chemical composition of the glass. The trail can be elusive … but it should be there. q The impact angle … track shard trail. § Bullet creates shard trail by carrying splinters of glass after passing through window. This shard trail follows bullet’s path at angle it hit the window.
Determining Approximate Angle of Impact of Bullet through Window Procedure q Locate glass shard trail using ALS, flashlight and/or UV light source. q Measure distance the smallest shards/splinters travel. q Using zero edge protractor, determine approximate angle bullet exited from window. q Confirm approximate angle by measuring length and width of bullet hole and calculating angle using sine function Perspective: Looking Down onto Glass Shard Trail Direction of Shard Trail A act le of Imp A A=Ang Direction of Bullet Zero Edge Protractor ge Top Ed w do of Win
Sequencing Bullet Shots in Plate Glass q. Locate the radial fractures and tracing them to their endpoint. If the radial fracture from, bullet 1 ends at a radial fracture from another bullet, bullet 2, then bullet 1 entered the glass second. The radial fractures (green) fired by the second bullet – on the left – were stopped from continuing by the radial fractures coming from the bullet hole on the right (blue radial fractures). Unless there is reason to believe these radial fractures are artifacts, such as elongation of radial fractures because the car had been moved, the radial fractures in green (2 nd bullet) impacted second.
Tempered Glass Automotive Glass q Collect all Pieces @the scene § Window can remain intact after shooting üVery fragile o Rarely together v. Try to collect intact – use tape to hold together v. Crater @ impact point rarely present üCan be put back together if very patient – complex jigsaw puzzle q Determining if shards are tempered glass § Interference patterns from strain in the glass üObserved with polarized light o Place over light source w/polarizing filter o Observe with second polarizing filter
Shot Sequence in Tempered Glass: 00 Buckshot First Pellet Strike Only strike that produces Radial fractures Radial Fractures from 1 st Pellet
Direction & Sequencing Through a Windshield
Direction Through A Windshield q. Bullet though a windshield § Passes through first pane, the laminate and the second pane before exiting q. Laminate changes dynamics § Restricting usual bending movement of glass and altering the visual shape of the cone on exit side of the glass. q Two cones form. § Different subtle visual characteristics that can be identified at the scene.
Sequencing Laminated Glass Two Layers of Glass With Plastic Laminate Between
Shot Sequence in Laminated Windshield q Identify intersecting radial fractures § Determine which were “stopped” by the radial fractures. q Shots into laminated windshield glass gives fractures on both sides of the laminate. § Track only fractures on one side of the glass to find intersecting fractures § Be wary of artifacts from movement of the car or from the laminate … consider as part of shot sequence.
Laminated Windshield Crater Hard Edge Sequence? Hard edge of bullet hole (arrow) is visible. This is an indication that the bullet was moving from the outside to the inside of the vehicle.
Understanding Distribution of Glass Shards 358 173 300 133 46 51 26 10 35 11 11 29 93 18 35 66 Direction Of Force Double pane basement window
Collecting and Preserving Glass Evidence The first step in preserving glass evidence is to understand how it fractures and the distribution it can take at the scene, which depends largely on the type of break.
Archiving Glass Evidence Photography/Video/Sketching q Purpose: Capture & preserve patterns … § Tempered glass with bullet holes is fragile … ü tape both sides to prevent mass of glass nuggets during transit. ü It could shatter even after taping. q Capturing intact pattern; § Photograph properly before glass falls apart § Photograph entire patter of fragments ü On ground/in vehicle, etc
Collecting, Packaging and Preserving Glass Evidence q. Operative rule for collecting, packaging and preserving glass evidence is § Common sense … Understanding the type of break. q Objective: Prevent alteration of the evidence during packaging and transit. § Care when collecting the evidence so that damage does not occur … archiving @ the scene before packaging. q Mark the interior and exterior sides of glass, § Before packaging ID direction of the force, if done @scene. q Tape the edges of glass for safety and evidence protection. Label the larger, outer container “GLASS FRAGILE” and case identification information.
Collecting and Preserving Glass Evidence q. Large Pieces: § Place between pieces of rigid cardboard & tape the cardboard sides together. § Place taped cardboard into larger container & secure to prevent movement. q Small shards § Place in proper container, ü Glass vial or small cardboard box ü Do not allow shifting during transit.