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2 CHAPTER Forensic Analysis 5 of Glass OBJECTIVES FEATURES In this chapter you should Jones gain an & understanding Bartlett Learning, of: LLC On the Crime Scene Glass The composition of different F types of glass Fragments Solve Hit-and-Run The optical and nonoptical properties of glass On the Crime Scene Forensic Techniques to determine the way in which glass has fractured Analysis of Glass Windshield Techniques Jones & to Bartlett match glass Learning, fragments LLC Fragments The use F of scanning electron microscopy and Back At the Crime Lab X-ray fluorescence to determine the elemental composition of glass WRAP UP Chapter Spotlight Key Terms Putting It All Together Further Reading..

3 YOU ARE THE FENSIC SCIENTIST When a pane of glass shatters, small, sharp pieces called shards are thrown over a wide area. Larger pieces travel in the direction of the blow Jones and are & Bartlett usually found Learning, close to the LLC original location of the glass pane. Jones Smaller & Bartlett shards can Learning, be LL propelled up to 10 ft from F the pane, also in the direction of the blow. If a pane of glass is shattered F by a violent blow, DISTRIBUT hundreds of tiny backscattered shards will inevitably become caught in the hair or clothing of the person who broke the pane. Because these shards are so small (less than 1 mm long), they are easily dislodged. The speed at which they fall off the perpetrator depends on the type of clothing worn by the individual and his or her subsequent activities. Most shards are lost Jones in the & first Bartlett hour after Learning, the event, and LLCthe probability of finding glass Jones evidence & on Bartlett a suspect Learning, decreases over LLC time. Investigators collect these tiny shards from a suspect by combing the suspect s hair and shaking his or her clothing over a clean piece of paper. If two or more glass shards from a suspect s hair or clothes are found to be indistinguishable from a control sample of glass from the scene, they can be considered significant associative evidence. 1. A car has its driver s window smashed during an attempted robbery. A suspect who is running down the street is Jones & Bartlett stopped Learning, by police. He LLC claims to have nothing to do with Jones the car. & Bartlett What should Learning, be the officers LLCnext step? 2. The lab finds glass shards on the suspect s sweatshirt. Which tests should now be done on the glass fragments? Introduction Glass has been shown to be very useful evidence because it is often encountered in criminal investigations. For example, when a burglar breaks a pane of window, small fragments of glass are often showered Jones onto & Bartlett his or her Learning, hair, clothing, LLCor shoes, and these F fragments can later be found on the suspect as transfer evidence. This chapter describes starts to soften when it is heated to a temperature of the many different types of glass commonly found more than 650 C, a fact that can prove useful when at crime scenes and explains how glass fragments investigating fires. For example, if the windows of a can be placed into specific classes through the use burned building are found to be deformed (melted), Jones & Bartlett of optical Learning, and nonoptical LLC analysis methods. In Jones the & temperature Bartlett Learning, of the fire LLC must have exceeded addition, the chapter describes how to individualize a glass fragment by making a fracture 650 C. Common window glass fractures when its match. Types of Glass calcium oxide (CaO), and 5% other oxides. This type of glass is made by heating together sodium carbonate (baking powder), calcium F oxide (lime) DISTRIBUT or calcium carbonate (limestone), and silicon dioxide (sand). Soda-lime glass has a green to yellow tint, which is most easily seen by looking at the edge of the Jones pane. This & Bartlett color is Learning, caused by an LLC iron impurity that is F present in the sand. Soda-lime glass surfaces or edges are placed under tension, and an edge fissure may propagate into visible cracks. A variety of metal oxides can be added to this basic recipe to give glass a Jones special appearance. & Bartlett For Learning, LL example, the addition of lead oxide F (PbO) will give DISTRIBUT Glass is a solid that is not crystalline but rather has an amorphous structure. The atoms of an amorphous solid have a random, disordered arrangement, unlike the regular, orderly arrangement that Jones is characteristic & Bartlett of crystalline Learning, solids. LLC Another characteristic property of glass is that it softens the glass a high brilliance because of its greater internal reflection of light; for this reason, lead glass is used for expensive crystal dinnerware. The addition of cobalt oxides will make the glass blue, manganese Jones oxide will & Bartlett make it Learning, purple, chromium LLC oxide will make F it green, and copper oxide will over a wide temperature range rather than melting make it red or blue-green. sharply at a well-defined temperature. In 1912, the Corning Glass Company found Soda-lime glass is the glass commonly used in that the addition of 10% to 15% boron oxide Jones & Bartlett most windows Learning, and bottles. LLC It consists of 70% silicon dioxide Jones (B 2 O& 3 ) Bartlett to glass Learning, made the resulting LLC product more (SiO 2 ), 15% sodium oxide (Na 2 O), 10% shock and heat resistant. This borosilicate glass 116 CHAPTER 5 Forensic Analysis of Glass..

4 on the CRIME SCENE Glass Fragments Solve Hit-and-Run On June 24, 1995, on the island of Providenciales in the Turks and Caicos Islands, a passerby reported to police that a woman s body was lying on the side of the Leeward Highway. The police found the 42-year-old woman lying face down. Upon investigation, police concluded that the victim had been struck by a car sometime after midnight while walking home from her job as a waitress. The local constable carefully documented the area surrounding the body. His report of items scattered around the body included earrings, a watch, a pendant and chain, eyeglasses, debris from the undercoating of a vehicle, and nine large glass fragments. The constable photographed the items in their original positions, measured distances Jones from the & body Bartlett to the found Learning, objects, LLC and collected soil samples from the Jones surrounding & Bartlett area. These Learning, items were LLC packaged and sent to the Miami-Dade Police Crime Laboratory in Florida for analysis. F Eleven days later, a suspect was identified when neighbors reported that his car was missing a headlight. The suspect denied being involved in the accident and requested that his attorney be present for any further questioning. Upon gaining access to the suspect s car, the police found considerable damage to the driver s-side front fender as well as a missing headlight on that side of the vehicle. Because this was an older car, each side had two headlights, each of which contained glass lenses. Because the car had been washed, a careful examination of the vehicle did not reveal any biological material. However, glass fragments were found lodged in the bumper and inside the lamp assembly of the missing light. The constable collected these fragments and samples of debris from the undercarriage of the car for further analysis. The Miami-Dade Crime Laboratory analyzed the glass fragments in particular to determine whether an association existed between Jones the glass & Bartlett fragments Learning, found on the LLC crime scene and the glass fragments Jones found & Bartlett in the Learning, LL suspect s vehicle. At the lab, F investigators visually inspected the fragments for fracture matches but F did not find DISTRIBUT one. Later, glass fragments stamped with the markings e-a-l-e-d found at the crime scene were matched to those taken from the suspect s car. Equipped with a GRIM2 refractive index measurement apparatus (which is discussed in this chapter), the police lab found that nine of the crime scene fragments had similar qualities to those of the suspected car enough to be statistically significant. Furthermore, the lab established that all the fragments came from a common source by using elemental analysis with an inductively coupled plasma-optical emission spectrometer (ICP-OES). was given the trade name Pyrex and was subsequently found to resist attack from virtually all ter into pieces with sharp edges, but rather breaks When tempered glass breaks, it does not shat- chemicals except hydrofluoric acid (HF), which into dices (i.e., small pieces without sharp etches its surface. edges). Tempered glass is used in the side and rear windows of automobiles, in large commercial Tempered Glass windows, in doors, and even in shower doors and Tempered glass (also known as safety glass) is more home windows where the window is less than 1 ft than four times stronger than window glass. During from the floor. its manufacture, the sand, lime, and sodium carbonate are heated together, and the hot glass that is formed is rolled Jones into sheets. & Bartlett Its upper Learning, and lower LLC surfaces are then cooled F rapidly with jets of air. This process leaves the center of the glass relatively hot compared to the surfaces and forces the surfaces and edges to compress. Tempered glass is stronger because wind Jones & pressure Bartlett or impact Learning, must first LLC overcome this compression before there is any possibility of fracture. Windshield Glass Automobile windshields Jones & Bartlett are made from Learning, laminated LLC glass ( FIGURE 5-1 ). F Today, windshields are made with two layers of glass, with a high-strength vinyl plastic film such as polyvinyl butyral (PVB) being sandwiched in between the layers. The three pieces Jones are laminated & Bartlett together Learning, by applying LLCheat and pressure in a special oven called an autoclave... Types of Glass 117

5 FIGURE 5-1 Automobile windshields are made from laminated glass whereas the side and rear windows consist of tempered glass. This type of glass is ideal for automobile windshields because of its strength and shatter resis- particularly difficult challenge given that glass To individualize the glass to one source a tance. The plastic film holds the glass in place is so ubiquitous in modern society when the glass breaks, helping to reduce injuries To pinpoint the source of the glass evidence, Jones & Bartlett from flying Learning, glass. The LLC film also can stretch, yet the Jones the & forensic Bartlett examiner Learning, needs LLC the two usual samples: glass fragments collected from the glass still sticks to it. Laminated safety glass is very crime difficult to penetrate as compared to normal windowpane scene and glass fragments taken from some item glass. The glass sandwich construction belonging to the suspect. The examiner must then allows the windshield to expand in an accident compare these samples (often side by side via a without tearing, which helps hold the occupants stereomicroscope) by identifying Jones & their Bartlett characteristics for example, their Learning, LL in a vehicle. Banks use a similar bullet-proof glass color, F fracture pattern, DISTRIBUT that has multiple layers of laminated glass. scratches and striations (irregularities) from manufacturing, unevenness of thickness, surface wear (outside versus inside surfaces), surface film or Forensic Examination of Glass dirt, and weathering patterns. In particular, the Jones Evidence: & Bartlett An Overview Learning, LLC examiner Jones tries to & fit Bartlett the pieces Learning, of the puzzle LLC together by matching F the irregular edges of the For the forensic scientist, the goals in examining broken glass samples and finding any corresponding irregularities between the two fragments glass evidence are twofold: To determine the broader class to which the ( FIGURE 5-2 ). Finding a perfect match is tantamount Jones & Bartlett glass Learning, belongs, thereby LLC linking one piece of Jones to individualizing & Bartlett Learning, the glass to LLC a single source with glass with another complete certainty. 118 CHAPTER 5 Forensic Analysis of Glass..

6 Courtesy of Jon Girard. adhering materials might suggest how the pieces fit together. Second, chemical analysis of the adhering F materials might further individualize the pieces and prove their association. Thickness Thickness can be measured to a high degree of accuracy with a micrometer. One must be careful, how- ever, in assuming that the thickness is constant it is not, particularly in curved pieces of glass. For this reason, the forensic examiner must take several representative Jones measurements & Bartlett Learning, of both the LLC FIGURE 5-2 Matching F broken pieces of glass. Finding a known and the questioned F samples. Determination perfect match is tantamount to individualizing the glass to a of curvature can distinguish flat glass from container, decorative, or ophthalmic glass. Thickness single source with complete certainty. is a very useful way of proving that two pieces of glass, which are otherwise extremely similar, are not actually from the same source. Nonoptical Physical Properties of Glass Hardness A number of scales are used to describe the hard- Many nonoptical physical properties can be used to compare a questioned specimen of glass to a known sample. These nonoptical Jones & Bartlett physical properties include surface curvature, texture, and special ness of substances. Geologists and mineralogists Learning, LLC often employ the Mohs scale, which indicates a treatments. Clearly, frosted glass cannot be a match substance s hardness relative to other substances. to a clear window glass. Similarly, a curved piece On the Mohs scale, the softest common mineral (such as a fragment from a bottle) cannot come talc is assigned a relative value of 1, and the hardest common mineral diamond is assigned a from the same source as a flat piece (such as from a window). Jones And & finally, Bartlett laminated Learning, glass would LLC not relative value of Jones 10. Each & Bartlett of the remaining Learning, values LLC is assigned to another F appropriate common mineral. compare to wire-reinforced glass. Thus these sorts of comparisons are most useful in proving the two pieces cannot be associated. For example, quartz is assigned the Mohs value 7 and topaz is assigned the Mohs value 8. The relative positions of the minerals on the Mohs scale reflect their scratching power: A harder Jones & Surface Bartlett Striations Learning, and LLCMarkings Jones substance & Bartlett will scratch Learning, a softer one. LLCThus diamond When sheet glass is rolled, the rollers leave parallel will F scratch everything else on the list; topaz will striation marks, called ream marks, on the surface. scratch quartz and everything lower on the Mohs Even polishing does not completely remove these scale, down to talc. Talc, by contrast, will not marks, and their presence can be enhanced by lowangle illumination and photography. These ream scratch anything else on the list. For an unknown Jones & Bartlett Learning, mineral LLC or substance, its relative Jones hardness & Bartlett is determined by using it to try to scratch F the bench- DISTRIBUT Learning, LL marks may hint at how various pieces should be oriented in the case of an indirect F physical match mark minerals. Its position on the scale is between where an intervening piece may be missing. The the benchmark mineral, which it scratches, and relative spacing might also be useful as a means of the next mineral on the list, which scratches it. individualization. Surface scratches, etchings, and For instance, an unknown that scratched talc and other markings Jones & might Bartlett be employed Learning, a similar LLC way quartz but was Jones itself & scratched Bartlett by Learning, topaz would LLC as the forensic F examiner tries to piece together the be assigned a relative F position between 7 and 8. puzzle. In this same fashion, all other materials can be ordered appropriately. Surface Contaminants The Mohs scale is not very useful for glass Jones & The Bartlett presence Learning, of such impurities LLC as paint and putty Jones samples, & however, Bartlett because Learning, all glasses LLCtend to fall in is useful in two ways. First, the patterns of the the F same range, between 5 and 6. Thus the Mohs.. Nonoptical Physical Properties of Glass 119

7 scale is too insensitive for forensic work, as are all strain where the load meets the pane. The side of the other standard hardness scales. Generally, holding the weight is called the loaded side, designated as side L, and the unloaded side is designated the forensic lab establishes relative hardness by F referring to glass samples in its collection. The as side U. The deformation induced by the load will relative scratching power of the known and questioned samples is established by trying to scratch a tensile strain. If the tensile strain is sufficient to cause side U to expand, so side U will experience these samples with Jones glass in & Bartlett the lab s Learning, collection. LLCovercome the tensile strength Jones of the & pane, Bartlett the pane Learning, LL Either the scratching powers of the known or will develop cracks on the unloaded F side. Several DISTRIBUT unknown samples are similar or they are not. Glass Fractures Second, they will radiate outward, away from the Elasticity is the ability of a material to return to its load point; they are therefore called radial cracks. previous shape after a force is exerted on it. For The radial cracks F form several pie-shaped (or triangular) sectors radiating from the point of loading. example, when a force is exerted on a pane of glass, it stretches (this bending may not be visible to the If the load is suddenly removed, these sectors will naked eye). If the force is not too high, the glass stay in place because the third side of each of the Jones & Bartlett will then Learning, return to its LLC original state and no damage Jones triangular & Bartlett sections Learning, is still solid LLC glass. occurs. If the force exceeds the glass s elasticity, If the load persists, however, each sector will however, the glass fractures. continue to be forced outward. This movement The forensic examiner may be able to analyze causes compressive strains on side U and concurrent tensile strains on side L. These strains will cause fractured window panes and determine the direction of an impact and the amount of force applied new cracks to develop on the loaded side. As before, to them, suggesting what actually happened at these cracks grow in two ways: first from the loaded the scene. For example, it is often important to establish whether a window was broken from the inside or the outside. At the scene of a homicide, a broken window near the door latch may be an attempt Jones to disguise & Bartlett the crime Learning, as a burglary. LLC In the case of F a burglary, the window would have been broken from the outside. However, if the homicide loaded side. In contrast, tangential cracks grow was deliberate, the perpetrator may have broken from one radial crack to another and from the the window from the inside in an attempt to mislead investigators into thinking burglary was the the weight was placed statically on a pane of glass. loaded side to the unloaded side. This is the case if Jones & Bartlett intruder s Learning, primary goal. LLC Jones & By Bartlett contrast, Learning, when a bullet LLCis shot at the pane of glass, the load is a projectile. The load side is known as the entrance side, and the unloaded side Characteristics of Glass Fractures Glass may be subjected to three types of forces (strains): Compressive force Jones squeezes & Bartlett the material. Learning, LLC Tensile force expands the material. Shear force slides one part of the material in one direction and another part in a different direction. Each Jones of these & Bartlett forces causes Learning, a deformation, LLC which is resisted F by the internal cohesion (stress) of the of these cracks may appear, and they will grow or travel in two directions simultaneously. First, they will grow from the unloaded to the loaded side. to the unloaded side, and second until they connect two radial cracks. These new cracks are called tangential cracks or concentric cracks, and the resulting pattern has a spider web appearance ( FIGURE 5-3 ). Note that Jones radial & Bartlett cracks grow Learning, from the LLC load point outward and from the unloaded side to the Concentric Crack Jones & Bartlett Learning, Concentric LLC Crack material. Glass breaks when a tensile strain is FIGURE 5-3 Radial cracks grow from the loaded point applied that is sufficient to overcome the natural outward and from the unloaded side to the loaded side. tensile stress limit of the material. Tangential (also know as concentric) cracks grow from one Jones & Bartlett If a Learning, person places LLC a weight on a horizontal Jones radial & Bartlett crack to another Learning, and from the LLC loaded side to the sheet of glass, the pane will experience compressive unloaded side. 120 CHAPTER 5 Forensic Analysis of Glass Applied Force.. Radial Crack

8 is called the exit side. The same cracking occurs, exit side. As can be seen in FIGURE 5-5, the angle and the same hole formation happens as when at which a bullet pierces a pane of glass can help a static load is applied. As the initial velocity of identify F the position of the shooter. If the bullet the projectile increases, however, the central hole came at an acute angle from the left, glass fragments will be sprayed to the right and the exit hole becomes smaller, the cracking patterns become simpler, and the central hole develops a pattern will be an irregular oval. If the bullet came from wherein the exit hole is Jones invariably & Bartlett wider than Learning, the an LLC acute angle from the right, Jones glass fragments & Bartlett will Learning, LL entrance hole ( FIGURE 5-4 ). F be sprayed to the left and the exit F hole will be an DISTRIBUT Examination of the edges of broken pieces of irregular oval. This test works best when the hole glass will reveal a set of curved lines known as rib is made by a high-speed projectile. In the event marks (or stress marks). These arcs are always the hole was made by a low-speed projectile (such nearly perpendicular to the surface at the side on as a hammer), this test will not be very meaningful. In this case it is usually best to examine the which the break started and curve until they are rib marks. Of course, F to make this examination nearly parallel to the surface on the opposite side (e.g., the side to which the break grew). In a radial crack, the rib marks will be nearly perpendicular to the unloaded (or exit) side and nearly parallel Bartlett to the loaded Learning, (or entrance) LLC side. Things will Jones & be exactly reversed for a tangential crack, which grows in the opposite way. The 3R rule helps in remembering this pattern: Radial cracks give rib marks, which make Right angles on the Reverse side from where F the force was applied. The direction of lateral propagation of the crack is always from the concave sides of the rib marks toward their convex sides. Thus, in a radial fracture, the rib marks will be oriented with their concave sides cupped Jones toward & Bartlett the load Learning, (or entrance) LLC point. Forensic Examination of Glass Fractures If all the glass pieces are present, the first thing to check for is the hole made by the load or projectile Jones &(e.g., Bartlett bullet, Learning, hammer), which LLCwill be wider on the Gunshot from the right Courtesy of Jon Girard. FIGURE 5-4 The bullet entered from the backside (entrance side) making a smaller hole, and passed through the glass pane leaving a wider hole at the front surface (exit side). Gunshot from the left FIGURE 5-5 The angle at which a bullet pierces a pane of glass can help identify the position of the shooter. Glass Fractures 121..

9 meaningful, each edge must be determined to When there are several bullet holes, analysis be either a radial or a tangential crack (which is can determine the sequence of the impacts. The why it is so important that all pieces be collected), first shot will cause fractures that simply run out and inside and outside sides of the pieces must be (terminate) wherever the original strains have been identified (which is why it is so important that the sufficiently relieved in the material. The radial investigators mark the proper orientation of each fractures associated with a second shot will run piece directly on the Jones item, as & well Bartlett as documenting Learning, LLCout when they meet a fracture Jones from & the Bartlett first shot, Learning, LL all orientations in their F notes and photos). and so on for all subsequent shots F ( FIGURE 5-6 ). DISTRIBUT Thus, if the forensic scientist is examining the edge of a radial fracture, whichever side shows nearly perpendicular rib marks will be the unloaded (or exit) side, meaning the side away from the force that caused the break. Alternatively, if the scientist is examining a tangential fracture, the side showing her clothing. For example, wool sweaters will retain the nearly perpendicular rib marks will be the glass fragments longer than a leather jacket. The loaded (or entrance) side, meaning the side from size of a fragment may be so small that individual which the original breaking force was applied. characteristics cannot be found. In these cases the Jones & Bartlett In the Learning, event that LLC the investigator or evidence Jones forensic & Bartlett examiner Learning, turns to measurements LLC of density technician neglected to mark which side of the and refractive index to characterize glass evidence. glass was inside and which side was outside, it is sometimes possible to figure out this information in the lab. Traces of window putty would indicate Glass Density Tests an outside side, for example, and paint traces of different colors might Jones also & be Bartlett used to distinguish Learning, LLC Density tests are often performed Jones on & glass Bartlett fragments. Learning, LL between the two sides. Of course, the preceding discussion assumes that the glass is not tempered. When tempered glass breaks, it produces small pieces; the fractures cannot Jones be categorized & Bartlett as radial Learning, or tangential, LLC so the kind of analysis F mentioned previously is not applicable. The majority of fragments recovered from a suspect s clothing or hair will likely be very small (0.25 to 1 mm). Most glass evidence adhering to a suspect is lost fairly rapidly, depending on the suspect s subsequent activities and the texture of his or When a forensic scientist measures the density of a glass fragment, he or she is measuring one of its physical properties. Density is a class characteristic, so it cannot serve as the sole criterion used for individualizing Jones the glass & Bartlett evidence Learning, to a single source. LLC Such measurements can, however, give the forensic (A) (B) FIGURE 5-6 In these two bullet holes in one piece of glass, the formation of (B) preceded the formation of (A). 122 CHAPTER 5 Forensic Analysis of Glass..

10 scientist enough data to warrant further testing of known and questioned specimens should be made other evidence or to provide enough evidence to using fragments of approximately equal size. exclude the glass fragments as having originated The Federal Bureau of Investigation (FBI) has somewhere other than the crime scene. In addition, reported density results for 1,400 glass samples if a sufficient amount of separate class characteristic received from 1964 to From this information, evidence can be gathered against a suspect, collectively the evidence may Jones make a & strong Bartlett circumstan- Learning, container LLC glass, and tableware Jones glass all & overlap. Bartlett Learning, LL it is known that the range of densities for flat glass, tial case, which may result F in conviction. When the density tests are concluded, F any evidence that does not match the known specimen DISTRIBUT To see how this works, consider decorative glass. This type of glass is made by adding different minerals to the glass recipe as the basic are found to have comparable densities, however, can be excluded. If questioned and known samples ingredients sand, lime, and sodium carbonate further testing is still required. A refractive index are being heated. The density of the resulting glass test is usually performed to support the compari- will vary with the type and amount of minerals son. If the density measurement indicates that the added. If a recovered glass fragment is placed in a specimen from the crime scene matches the reference liquid that has a higher density than the glass, the material, a refractive index test that also glass fragment will float. If the liquid is less dense indicates a match will improve the discrimination Jones & than Bartlett the glass Learning, fragment, the LLCglass will sink. When Jones capability & Bartlett by approximately Learning, twofold. LLC the density gradient column method is used to determine the density of glass, the forensic scientist uses a density gradient tube filled with a liquid that has been especially prepared to have a density gradient. The gradient is prepared Jones such & Bartlett that the density Learning, Color LLC Comparing the color of a suspect F piece of glass DISTRIBUT at any level is less than that of any level lower in the tube and greater than that of any level higher in the tube. The gradient is prepared by mixing bromoform and bromobenzene, two dense organic liquids, in different Jones proportions. & Bartlett When Learning, glass fragments LLC are poured in the F top of the column, they fall through the liquid until they become suspended in the liquid Given that sample size may affect the apparent at the level that is the same density as the particular color, side-by-side comparisons should be made glass fragment. Fragments of different densities will, with fragments of approximately the same size. therefore, settle at different levels in the column. These fragments should be visually compared by Jones & The Bartlett questioned Learning, glass fragment s LLC density may then Jones placing & them Bartlett on edge Learning, over a white LLCsurface using be compared with a glass sample from the crime natural F light. Viewing the glass in this way allows scene to prove (or disprove) that it is a match. for the optimal observation of color. It also allows Density measurements should not be performed the examiner to distinguish between the true color on fragments of glass that are cracked of the glass and any coatings or films that might be or contain an inclusion, Jones because & Bartlett these flaws Learning, will present on the glass s surface. In addition, observing the glass using both fluorescent and incandes- LLC make the glass seem less dense than it really is. cent light is often helpful in distinguishing F colors. DISTRIBUT (An inclusion is a defect that forms when a particle or bubble becomes embedded in the main body of the glass.) Window glass, in particular, does not have a uniform density. For this reason, the Jones variation & Bartlett in density Learning, of the known LLC sample should be determined F with samples taken from different locations in the window or door whenever possible. Likewise, because the surface or edge of tempered glass is denser than at its interior, care Jones & must Bartlett be taken Learning, with tempered LLCglass to measure several known samples. Density comparisons between Optical Physical Properties of Glass with the color of a reference sample can distinguish whether the two samples share a common source. As a consequence, significant color differences between Jones glass fragments & Bartlett can Learning, be used as the LLC basis for exclusion of a suspect. Refractive Index Light has wave properties. That is, a beam of light traveling from Jones a gas (such & Bartlett as air) into Learning, a solid (such LLC as glass) undergoes a decrease in its velocity, such that the beam bends downward as it passes from the air into the glass. The application of this phenomenon allows the determination of the glass s Jones refractive & Bartlett index, a measure Learning, of how LLCmuch the light is bent F (refracted) as it enters the glass... Optical Physical Properties of Glass 123

11 The bending of a light beam as it passes from Light one medium to another is known as refraction. The refractive index, η, is the ratio of the velocity θ of light in the air to the velocity of light in the glass Air Incident being measured. The velocities of light in both Beam media are related to the angles that the incident Interface and refracted beams Jones make & with Bartlett a theoretical Learning, line LLC drawn vertically to the F glass surface ( FIGURE 5-7 ). Glass Refracted η x D = V air /V glass = sin θ air /sin θ Beam glass where θ' V air = Velocity of light in air Jones & Bartlett Learning, LLC V glass = Velocity of light in glass FIGURE 5-7 Refraction F of light through glass. The refractive θ = Angle of light in air θ = Angle of light in glass x = Temperature D = Light from sodium D line (589 nm) Jones & Bartlett The velocity Learning, of light LLC in a liquid sample is always less than that of light in air, so refractive index values for solids are always greater than 1. index a measure of how much the light is bent (refracted) as it enters the glass can be used as a basis of comparison for glass samples. on the CRIME SCENE Forensic Analysis of Glass Windshield Fragments Twenty-one-year-old Jones & Bartlett Iowa Learning, State University LLCstudent Danny Peterson died Jones June 8, & 2002, Bartlett 6 days Learning, after being hit LLC by a vehicle. F Peterson and two friends were walking along the shoulder of a road F when a vehicle veered off the pavement, fatally injured Peterson, and sped away. After Danny was transported to the hospital, Detective Jack Talbot of the South Lake Minnetonka Police Department investigated the scene of the accident. He recovered small glass slivers from a windshield on the road and fragments from a headlight in a nearby culvert. Fragments of windshield glass also were collected from Danny s clothes. Jones & Bartlett There Learning, was no straightforward LLC match for the windshield Jones fragments, & Bartlett but the Learning, glass from the LLC headlight matched a specific make and model of car the Mercury Cougar. F Even though the headlight glass seemed important, it was not probative. In fact, the headlight turned out to belong to a Mercury Cougar that was in the repair shop on the day of the accident. All investigative leads were followed but to no avail. Three months after the accident, however, a young woman called the police Jones with & a Bartlett promising Learning, lead. She suggested LLC that her husband, Guido Vivar-Rivera, Jones & Bartlett might have Learning, LL been involved in the accident. F On the night of the accident, Rivera was drunk and upset, saying F that someone DISTRIBUT broke his Pontiac s windshield with a rock. The windshield was repaired soon after the accident. The police inspected the outside of Rivera s 1996 Pontiac Grand Am and saw stress fractures on the bumper, possibly caused from the impact of hitting Danny. Detective Talbot questioned employees at the repair shop who replaced the windshield. The employees said it was unlikely the windshield damage was caused by a rock. Jones Detective & Bartlett Talbot and Learning, his team arrested LLCRivera and got a search warrant Jones for the & Pontiac Bartlett Grand Learning, Am. In addition LLC to the F stress fractures on the bumper, they found a dent on the hood and F pieces of broken window glass on the inside of the car. This glass was a match with the windshield fragments found at the scene of the accident and on Danny s clothes. Rivera was convicted of a felony hit-and-run and served his time at the Hennepin County Adult Correctional Jones & Bartlett Facility. Learning, LLC 124 CHAPTER 5 Forensic Analysis of Glass..

12 The temperature and wavelength of the light Jones being & Bartlett refracted Learning, influence LLC the refractive index for any substance. The temperature of the sample affects its density, and the density change affects the velocity of the light beam as it passes though the sample. Therefore, the temperature at which the refractive index is determined Jones & Bartlett is always Learning, specified by a superscript in the F notation of η. Likewise, LLC the wavelength of the light used affects the refractive index because light of differing wavelengths bends at different angles. The bright yellow light from a lamp containing sodium, which produces a beam with a wavelength of 589 nm (Figure 5-6), is commonly called the sodium D line. This lamp provides the standard wavelength of light, denoted FIGURE 5-8 A hot stage microscope is a key instrument in η D. Thus the refractive index of a liquid measured the forensic examination of glass. The temperature of the at 20 C using a sodium lamp that gave a reading of sample affects its density, and the density change affects Jones & Bartlett would Learning, be reported LLC as η 20 D = the velocity of the light beam as it passes through the sample and hence its refractive index. Single sheets of plate glass, such as those com- monly used for making windows, do not usually have a uniform refractive index value across the entire pane. Because the index of refraction can glass fragments and immersion oil are placed on vary as much as from one side to another, a microscope slide, which is then inserted into a the difference in the refractive Jones indices & Bartlett of the Learning, ques- hot LLCstage microscope ( FIGURE Jones 5-8 ). The & Bartlett stage of Learning, LL such a microscope is fitted with a F heater that can DISTRIBUT tioned plate glass fragment and the reference sample must be smaller than if the forensic scientist is to be able to distinguish the normal variations in a pane of glass from variations that would rule Jones out & a match Bartlett altogether. Learning, LLC The refractive F index is one of the most commonly measured physical properties in the forensic laboratory, because it gives an indication of the refractive index of the oil by about per 1 C. the refractive index of the glass but decreases the composition and the thermal history of the glass. When the glass fragments are initially observed Two methods are used to measure the refractive through the microscope, they will produce a bright Jones & index Bartlett of glass: Learning, the oil immersion LLC method and the Jones halo around & Bartlett each fragment, Learning, known LLC as the Becke Emmons procedure. line F ( FIGURE 5-9 ). As the temperature increases, Oil Immersion Method When using the oil immersion method, the forensic examiner places the Jones questioned & Bartlett glass fragments Learning, LLC in specialized silicone oils whose F refractive indices have been well studied. The refractive index of the oil is temperature dependent: As its temperature increases, its refractive index decreases. Silicone oils are chosen for this task because they are resistant to Jones decomposition & Bartlett at high Learning, temperatures. LLC The warm the sample slowly while accurately reporting the temperature to ±2 C. A filter inserted between the lamp and the sample allows light with a constant 589-nm Jones wavelength & Bartlett to Learning, reach the sample. Increasing the temperature has little effect on LLC refractive index of virtually all window glass and most bottles can be compared by using silicone oil as the comparison liquid and by varying its temperature between 35 C and 100 C. FIGURE 5-9 Oil immersion is one technique used to determine the Jones & Bartlett An easy Learning, way to vary LLC the refractive index of Jones & refractive Bartlett index Learning, of glass. The Becke LLCline appears as the immersion oil is to heat it. The suspected a bright halo around the glass fragment... Optical Physical Properties of Glass 125 Courtesy of Foster & Freeman Ltd. Courtesy of Foster & Freeman Ltd.

13 the refractive index of the oil decreases until the uses a hot stage microscope in conjunction with Becke line and the glass fragments disappear. different source lamps. It measures the index of At this point (called the match point), the refractive indices of the oil and the glass fragment are the refractive index measurements are recorded refraction at a variety of wavelengths. Most often the same. The examiner can compare suspect and by first taking a measurement with a sodium lamp known samples in this way to determine whether (the sodium D line at 589 nm) and then by using they have the same Jones match & point; Bartlett alternatively, Learning, he LLCa hydrogen lamp (which Jones produces & two Bartlett lines, Learning, the LL or she can estimate the F refractive index of the glass C line at 656 nm and the F line F at 486 nm). The DISTRIBUT from graphs that report the refractive index of the oil as a function of temperature. Automated systems are also available for making refractive index measurements using the immersion method. The Glass Refractive Index microscope converts the difference in the refractive indices between the particle of glass and the silicone oil to a difference in brightness contrast, and it enhances the Becke line. This procedure increases the precision of the refractive index mea- Measurement (GRIM) system, for example, combines surements taken on the glass particles. a hot stage microscope with a video camera The questioned glass is crushed and placed that records the behavior of the glass fragments in the silicone oil on the hot stage. As the temperature as they are being heated ( FIGURE 5-10 ). That is, the of the hot stage increases, measurements Jones & Bartlett camera shows Learning, the contrast LLC between the edge of Jones are & taken Bartlett at the Learning, three different LLCwavelengths (486, the glass fragment and the immersion oil as the 589, and 656 nm). Lines representing the refrac- temperature increases, until it reaches the match tive index of the glass as a function of wavelength point. The GRIM system s computer then converts are recorded for each temperature. These data are this temperature to a refractive index using reference then superimposed on a complex graph, known information stored in a database. as the Hartmann net. The Hartmann net contains the correlation between the Jones refractive & Bartlett index and Learning, LL Emmons Procedure The Emmons procedure, which was developed by the Association of Official Analytical Chemists, the wavelength at fixed temperatures for the silicone oil. The point at which the dispersion lines for the glass samples intersect the dispersion lines for the silicone oil is where the refractive index Courtesy of Foster & Freeman Ltd. FIGURE 5-10 The GRIM 3 system is an automated technology to measure the refractive index of glass. 126 CHAPTER 5 Forensic Analysis of Glass..

14 of the glass sample is determined. Three separate in the FBI database, while glass with a refractive indices of refraction are recorded: η C, η D, and η F. index of was found in more than 100 of Because three separate measurements are taken on the F 2337 specimens. The forensic scientist can each sample, this method, although more difficult access this FBI database whenever he or she needs to carry out, gives more precise refractive index to compare the refractive index of a questioned measurements. glass fragment to refractive index information and, Jones & Bartlett Learning, thereby, LLC calculate the probability Jones that & two Bartlett such Learning, LL Refractive Index of Tempered F samples might be matches as a F result of sheer DISTRIBUT versus Nontempered Glass chance ( FIGURE 5-11 ). The FBI also has correlated the relationship Often a forensic examiner needs to determine between their refractive indices and densities for whether the questioned glass sample is tempered 1,400 glass specimens ( FIGURE 5-12 ). The results or nontempered Jones & glass. Bartlett Tempered Learning, glass can LLC be distinguished from nontempered glass by heating the show that once the refractive index of a glass speci- men is known, the subsequent measurement of its glass fragments in a furnace at a temperature higher density will improve the discrimination capability of the measurements by approximately two- than 600 C in a process known as annealing. If the questioned glass sample is large enough, it can be fold. Most forensic examiners prefer to measure broken in two. Each piece is heated separately in Jones refractive & Bartlett index simply Learning, because LLC refractive index the oven, is allowed to cool, and then has its refrac- measurements F are faster and easier to make than tive index measured. Because annealing alters the density measurements, and often the glass fragment size is too small to get an accurate density optical properties of the glass, the change in refractive index between the two annealed pieces can be measurement. If the glass fragment is large enough, used to determine if it is tempered or nontempered both the refractive index and the density should be glass. After annealing, Jones the change & Bartlett in refractive Learning, determined LLC unless other discriminating Jones & Bartlett measurements such as elemental analysis are F performed. DISTRIBUT Learning, LL index for tempered glass is F much greater than the change observed for nontempered glass. fragments from different sources may have similar indices of refraction or similar densities. Thus individualizing glass samples accurately is particularly challenging. To Jones assist crime & Bartlett labs in Learning, making such LLC distinctions, the FBI F has compiled density and refractive Variations in Density and Elemental Analysis of Glass Refractive Jones Index & Bartlett Learning, LLC The physical and Jones optical & methods Bartlett for Learning, forensic comparison of glass fragments F are well established in LLC As with other F types of evidence, the properties of glass are more often used to exonerate suspects crime labs and widely accepted in courts throughout the world. These analytical methodologies than to individualize samples and definitively prove a connection between a suspect and a crime scene. have two other advantages: (1) These tests are nondestructive, so the evidence is preserved for addi- Indeed, if either the densities or the refractive indices Bartlett of a questioned Learning, glass LLC specimen and a reference Jones tional testing, & Bartlett and (2) Learning, the tests are LLC performed using Jones & glass sample do not match, then the forensic scientist can easily prove that they did not share a com- that these tests will remain the principal methods inexpensive F instruments. These advantages ensure mon origin. However, glass is so ubiquitous, and for the comparison of glass. Methods of elemental so many manufacturers use the same processes to analysis particularly those in which the specimen produce each type (e.g., Jones rolling & molten Bartlett glass Learning, into is LLC consumed during the analysis should Jones & Bartlett be used Learning, LL flat sheets to make windows), that sometimes even only after all nondestructive methods F of examina- DISTRIBUT tion have been completed and in cases in which additional discrimination is necessary. The elemental composition of glass can be measured by surface techniques such as use of a scanning electron Jones microscope & Bartlett (SEM) Learning, or X-ray LLC fluorescence (XRF). F The SEM has several disad- index data about glass from around the world. vantages that limit its value in the analysis of glass These data indicate how widespread the use of a fragments. Primary among these is that, because of glass with a specific refractive index is. For example, the irregular shape of the glass fragments, precise Bartlett a glass fragment Learning, having LLCa refractive index of Jones quantitative & Bartlett determination Learning, of element LLC concentra- Jones & was found in only 1 out of 2337 specimens tion F is not possible... Optical Physical Properties of Glass 127

15 Number of Specimens Refractive Index (n D ) FIGURE 5-11 The frequency of occurrence of refractive indices of glass specimens has been determined by the FBI and is available to forensic examiners in an FBI database. The XRF, by contrast, is routinely used for elemental analysis of glass. For instance, the glass industry Jones uses & Bartlett XRF as an Learning, accurate, precise LLC method of enforcing F quality control during glass manufacturing. The XRF instrument focuses a beam of X-rays on the surface of the glass and then measures the energy of the X-rays that are emitted from the glass. The energy of the emitted X-rays can be correlated to the presence of specific elements. In Jones study, & XRF Bartlett was used Learning, to measure LLCthe ratios of 10 elements F in window glass samples that had virtually identical indices of refraction. When the elemental ratios determined by XRF were compared, the source of 49 of the 50 glass specimens could be correctly determined. One major Jones advantage & Bartlett of XRF Learning, is that it LLC does not destroy the sample. The elemental composition of glass also can be measured by flameless atomic absorption spectrophotometry (FAAS) or inductively coupled plasma (ICP) methods. There are two Jones major & disadvantages Bartlett Learning, LL of using these methods for the F analysis of glass DISTRIBUT fragments. First, the glass fragment must be dissolved in acid and small samples of the resulting solution then injected into the instrument, so the original sample is destroyed. Second, these methods entail Jones use of & hazardous Bartlett chemicals, Learning, such LLCas hydrofluoric acid. F FIGURE 5-12 The FBI has correlated the densities and refractive indices for a wide variety of glass specimens. 128 CHAPTER 5 Forensic Analysis of Glass Despite these disadvantages, the ICP method, when coupled with an optical emission spectrometer (ICP-OES), has been shown by the FBI to Jones be & a dependable Bartlett Learning, method for LLC the determination of..