Which property imparts paint with its most distinctive forensic characteristics

Physical Evidence Paper

Resources: Ch. 5, 8, 9, & 14 of Forensic Science

You are on a team of crime scene investigators. Your team was instructed to collect the physical evidence at a crime scene. Arriving at the crime scene your team observes the following:

Shell casings
Three sets of footprints (two muddy sets and one bloody set) throughout the house
Bloody fingerprints
Tire tracks by the side entrance of the house

Write a 1,050- to 2,100-word paper that includes the following:

Identify the various types of physical evidence encountered at the crime scene.
Describe the preservation and collection of the firearms evidence.
Describe the preservation and collection of fingerprints, footprints and tire tracks.
Describe the legal issues regarding physical evidence encountered at the crime scene.
Identify the significance of physical evidence.

LEARNING OBJECTIVES

After studying this chapter, you should be able to:

• List the most useful examinations for performing a forensic comparison of paint. • Understand the applications of stereoscopic microscopes, pyrolysis gas chromatography, and infrared spectrophotometry in forensic paint comparison and examination. • Define and understand the properties of density and refractive index. • List and explain forensic methods for comparing glass fragments. • Understand how to examine glass fractures to determine the direction of impact of a projectile. • List the important forensic properties of soil. • Describe the proper collection and preservation methods for forensic paint, glass, and soil evidence.

GREEN RIVER KILLER

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This case takes its name from the Green River, which flows through Washington state and empties into Puget Sound in Seattle. Within a six-month span in 1982, the bodies of six females were discovered in or near the river. The majority of the victims were known prostitutes who were strangled and apparently raped. As police focused their attention on an area known as Sea-Tac Strip, a haven for prostitutes, girls mysteriously disappeared with increasing frequency. By the end of 1986, the body count in the Seattle region rose to forty, all of whom were women believed to have been murdered by the Green River Killer.

As the investigation pressed on into 1987, the police renewed their interest in one suspect, Gary Ridgway, a local truck painter. Ridgway had been known to frequent the Sea-Tac Strip. Interestingly, in 1984 Ridgway actually had passed a lie detector test. In 1987, with a search warrant in hand, police searched Ridgway’s residence and also obtained hair and saliva samples from him. Again, because of insufficient evidence, Ridgway was released from custody.

With the exception of one killing in 1998, the murder spree stopped in 1990, and the case remained dormant for nearly ten years. However, the advent of DNA testing brought renewed vigor to the investigation. In 2001, semen samples collected from three early victims of the Green River Killer were compared to saliva that had been collected from Ridgway in 1987. The DNA profiles matched, and the police had their man. An added forensic link to Ridgway was made when minute amounts of spray paint found on the clothing of six victims were compared to paints collected from Ridgway’s workplace. Ridgway ultimately avoided the death penalty by confessing to the murders of forty-eight women.

Forensic Examination of Paint

Our environment contains millions of objects whose surfaces are painted. Thus paint, in one form or another, is one of the most prevalent types of physical evidence received by the crime laboratory.

Paint as physical evidence is perhaps most frequently encountered in hit-and-run and burglary cases. For example, a chip of dried paint or a paint smear may be transferred to the clothing of a hit-and-run victim on impact with an automobile, or paint smears could be transferred onto a tool during a burglary. Obviously, in many situations a transfer of paint from one surface to another could impart an object with an identifiable forensic characteristic.

In most circumstances, the criminalist must compare two or more paints to establish their common origin. For example, such a comparison may associate an individual or a vehicle with the crime site. However, the criminalist need not be confined to comparisons alone. Crime laboratories often help identify the color, make, and model of an automobile by examining small quantities of paint recovered at an accident scene. Such requests, normally made in hit-and-run cases, can lead to the apprehension of the responsible vehicle.

COMPOSITION OF PAINT

Paint is composed of a binder and pigments, as well as other additives, all dissolved or dispersed in a suitable solvent. Pigments impart color and hiding (or opacity) to paint and are usually mixtures of various inorganic and organic compounds added to the paint by the manufacturer. The binder is a polymeric substance that provides the support medium for the pigments and additives. After paint has been applied to a surface, the solvent evaporates, leaving behind a hard polymeric binder and any pigments that are suspended in it.

The most common types of paint examined in the crime laboratory are finishes from automobiles. Manufacturers apply a variety of coatings to the body of an automobile; this adds significant diversity to automobile paint and contributes to the forensic significance of automobile paint comparisons. The automotive finishing system for steel usually consists of at least four organic coatings: electrocoat primer, primer surfacer, basecoat, and clearcoat.

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ELECTROCOAT PRIMER

The first layer applied to the steel body of a car is the electrocoat primer. The primer, consisting of epoxy-based resins, is electroplated onto the steel body of the automobile to provide corrosion resistance. The resulting coating is uniform in appearance and thickness. The color of these primers ranges from black to gray.

PRIMER SURFACER

Originally responsible for corrosion control, the surfacer usually follows the electrocoat layer and is applied before the basecoat. Primer surfacers are epoxy-modified polyesters or urethanes. The function of this layer is to completely smooth out and hide any seams or imperfections because the basecoat will be applied on this surface. This layer is highly pigmented. Color pigments are used to minimize color contrast between primer and topcoats. For example, a light gray primer may be used under pastel shades of a colored topcoat; a red oxide may be used under a dark-colored topcoat.

BASECOAT

The next layer of paint on a car is the basecoat or colorcoat. This layer provides the color and aesthetics of the finish and represents the “eye appeal” of the finished automobile. The integrity of this layer depends on its ability to resist weather, UV radiation, and acid rain. Most commonly, an acrylic-based polymer composes the binder system of basecoats. Interestingly, the choice of automotive pigments is dictated by toxic and environmental concerns. Thus, the use of lead, chrome, and other heavy-metal pigments has been abandoned in favor of organic-based pigments. There is also a growing trend toward pearl luster, or mica, pigments. Mica pigments are coated with layers of metal oxide to generate interference colors. Also, the addition of aluminum flakes to automotive paint imparts a metallic look to the paint’s finish.

CLEARCOAT

An unpigmented clearcoat is applied to improve gloss, durability, and appearance. Most clearcoats are acrylic based, but polyurethane clearcoats are increasing in popularity. These topcoats provide outstanding etch resistance and appearance.

MICROSCOPIC EXAMINATION OF PAINT

The microscope has traditionally been, and remains, the most important instrument for locating and comparing paint specimens. Considering the thousands of paint colors and shades, it is quite understandable that color, more than any other property, gives paint its most distinctive forensic characteristics. Questioned and known specimens are best compared side by side under a stereoscopic microscope for color, surface texture, and color layer sequence (see Figure 14-1).

The importance of layer structure for evaluating the evidential significance of paint evidence cannot be overemphasized. When paint specimens possess colored layers that match in number and sequence of colors, the examiner can begin to relate the paints to a common origin. How many layers must be matched before the criminalist can conclude that the paint specimens came from the same source? Much depends on the uniqueness of each layer’s color and texture, as well as the frequency with which the particular combination of colors under investigation is observed. Because no books or journals have compiled this type of information, the criminalist is left to his or her own

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experience and knowledge when making this determination.

Unfortunately, most paint specimens do not have a layer structure of sufficient complexity to allow them to be individualized to a single source (see Figure 14-2). However, the diverse chemical composition of modern paints provides additional points of comparison between specimens. Specifically, a thorough comparison of paint must include a chemical analysis of the paint’s pigments, its binder composition, or both.

FIGURE 14-1 A stereoscopic microscope comparison of two automotive paints. The questioned paint on the left has a layer structure consistent with the control paint on the right.

Courtesy Leica Microsystems, Buffalo, NY, www.leica-microsystems.com

ANALYTICAL TECHNIQUES USED IN PAINT COMPARISON

The wide variation in binder formulations in automobile finishes provides significant information. More important, paint manufacturers make automobile finishes in hundreds of varieties; this knowledge is most helpful to the criminalist who is trying to associate a paint chip with one car as distinguished from the thousands of similar models

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that have been produced in any one year. For instance, there are more than a hundred automobile production plants in the United States and Canada. Each can use one paint supplier for a particular color or vary suppliers during a model year. Although a paint supplier must maintain strict quality control over a paint’s color, the batch formulation of any paint binder can vary, depending on the availability and cost of basic ingredients.

FIGURE 14-2 Red paint chips peeling off a wall revealing underlying layers.

Jack Hollingsworth\Getty Images, Inc. – Photodisc/Royalty Free

CHARACTERIZATION OF PAINT BINDERS

An important extension of the application of gas chromatography to forensic science is the technique of pyrolysis gas chromatography. Many solid materials commonly encountered as physical evidence—for example, paint chips, fibers, and plastics—cannot be readily dissolved in a solvent for injection into the gas chromatograph. Thus, under normal conditions these substances cannot be subjected to gas chromatographic analysis. However, materials such as these can be heated to high temperatures (500°C –1000°C), or pyrolyzed, so that they will decompose into numerous gaseous products. Pyrolyzers permit these gaseous products to enter the carrier gas stream, where they flow into and through the gas chromatography (GC) column. The pyrolyzed material can then be characterized by the pattern produced by its chromatogram, or pyrogram.

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pyrolysis

The decomposition of organic matter by heat.

Pyrolysis gas chromatography is particularly invaluable for distinguishing most paint formulations. In this process, paint chips as small as 20 micrograms are decomposed by heat into numerous gaseous products and are sent through a gas chromatograph.

As shown in Figure 14-3, the polymer chain is decomposed by a heated filament, and the resultant products are swept into and through a gas chromatograph column. The separated decomposition products of the polymer emerge and are recorded. The pattern of this chromatogram, or pyrogram, distinguishes one polymer from another. The result is a pyrogram that is sufficiently detailed to reflect the chemical makeup of the binder. Figure 14-4 illustrates how the patterns produced by paint pyrograms can differentiate acrylic enamel paints removed from two automobiles. Note the subtle differences between the minor peaks when comparing the two pyrograms.

CLOSER ANALYSIS THE STEREOSCOPIC MICROSCOPE

The details that characterize many types of physical evidence do not always require examination under very high magnifications. For such specimens, the stereoscopic microscope has proved quite adequate, providing magnifying powers from 10× to 125×. This microscope has the advantage of presenting a distinctive three-dimensional image of an object. Also, whereas the image formed by the compound microscope is inverted and reversed (upside-down and backward), the stereoscopic microscope is more convenient because prisms in its light path create a right-side-up image.

The stereoscopic microscope, shown in Figure 1, is actually two monocular compound microscopes properly spaced and aligned to present a three-dimensional image of a specimen to the viewer, who looks through both eyepiece lenses. The light path of a stereoscopic microscope is shown in Figure 2.

The stereoscopic microscope is undoubtedly the most frequently used and versatile microscope found in the crime laboratory. Its wide field of view (i.e., the area of the specimen that can be seen when magnified) and great depth of focus (i.e., the thickness of the specimen that is entirely in focus) make it an ideal instrument for locating trace evidence in debris, garments, weapons, and tools. Furthermore, its potentially large working distance (i.e., the distance between the objective lens and the specimen) makes it ideal for microscopic examination of big, bulky items. When fitted with vertical illumination, or a light source above the specimen, the stereoscopic microscope becomes the primary tool for viewing opaque specimens, to characterize physical evidence as diverse as paint, soil, gunpowder residues, and marijuana.

FIGURE 1 A stereoscopic microscope.

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Mikael Karlsson\Arresting Images Royalty Free

FIGURE 2 A schematic diagram of a stereoscopic microscope. This microscope is actually two separate monocular microscopes, each with its own set of lenses except for the lowest objective lens, which is common to both microscopes.

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Courtesy Foster & Freeman Limited

Infrared spectrophotometry is still another analytical technique that provides information about the binder composition of paint. Binders selectively absorb infrared radiation to yield a spectrum that is highly characteristic of a paint specimen.

FIGURE 14-3 A schematic diagram of pyrolysis gas chromatography.

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SIGNIFICANCE OF PAINT EVIDENCE

Once a paint comparison is completed, the task of assessing the significance of the finding begins. How certain can one be that two similar paints came from the same surface? For instance, a casual observer sees countless identically colored automobiles on our roads and streets. If this is the case, what value is a comparison of a paint chip from a hit-and-run scene to paint removed from a suspect car?

From previous discussions it should be apparent that far more is involved in paint comparison than matching surface paint colors. Paint layers beneath a surface layer offer valuable points of comparison. Furthermore, forensic analysts can detect subtle differences in paint binder formulations, as well as major or minor differences in the elemental composition of paint. Obviously, these properties cannot be discerned by the naked eye.

The significance of a paint comparison was convincingly demonstrated from data gathered at the Centre of Forensic

Science, Toronto, Canada.1 Paint chips randomly taken from 260 vehicles located in a local wreck yard were compared by color; layer structure; and, when required, infrared spectroscopy. All except one pair were distinguishable. In statistical terms, these results signify that, if a crime-scene paint sample and a paint standard/reference sample removed from a suspect car compare by the previously discussed tests, the odds against the crime-scene paint having originated from another randomly chosen vehicle are approximately 33,000 to 1. Obviously, this type of evidence is bound to forge a strong link between the suspect car and the crime scene.

Crime laboratories are often asked to identify the make and model of a car from a very small amount of paint left behind at a crime scene. Such information is frequently of use in a search for an unknown car involved in a hit-and-run incident. Often the questioned paint can be identified when its color is compared to color chips representing the

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various makes and models of manufactured cars. However, in many cases it is not possible to state the exact make or model of the car in question because any one paint color can be found on more than one car model. For instance, General Motors may have used the same paint color for several production years on cars in its Cadillac, Buick, and Chevrolet lines.

FIGURE 14-4 Paint pyrograms of acrylic enamel paints: (a) paint from a Ford model and (b) paint from a Chrysler model.

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Courtesy Varian Inc., Palo Alto, CA

FIGURE 14-5 An automotive color chart of various car models.

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Courtesy Damian Dovanganes\AP Wide World Photos

Color charts for automobile finishes are available from various paint manufacturers and refinishers (see Figure 14-5). Since 1975, the Royal Canadian Mounted Police Forensic Laboratories have been systematically gathering color and chemical information on automotive paints. This computerized database, known as PDQ (Paint Data Query), allows an analyst to obtain information on paints related to automobile make, model, and year. The database contains such parameters as automotive paint layer colors, primer colors, and binder composition (see Figure 14-6). A number of US

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laboratories have access to PDQ. Also, some crime laboratories maintain an in-house collection of automotive paints associated with various makes and models, as shown in Figure 14-7.

COLLECTION AND PRESERVATION OF PAINT EVIDENCE

As has already been noted, paint chips are most likely to be found on or near people or objects involved in hit-and-run incidents. The recovery of loose paint chips from a garment or from the road surface must be done with the utmost care to keep the paint chip intact. Paint chips may be picked up with tweezers or scooped up with a piece of paper. Paper made into druggist folds and glass and plastic vials make excellent containers for paint. If the paint is smeared on or embedded in garments or objects, the investigator should not attempt to remove it; instead, it is best to package the whole item carefully and send it to the laboratory for examination.

FIGURE 14-6 (a) The home screen for the PDQ database. (b) A partial list of auto paints contained in the PDQ database.

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Royal Canadian Mounted Police

FIGURE 14-7 A crime laboratory’s automotive paint library. Paints were collected at an automobile impound yard and then cataloged for rapid retrieval and examination.

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Royal Canadian Mounted Police

When a transfer of paint occurs in hit-and-run situations (such as to the clothing of a pedestrian victim), uncontaminated standard/reference paint must always be collected from an undamaged area of the vehicle for comparison in the laboratory. The collected paint must be close to the area of the car that is suspected to have come into contact with the victim. This is necessary because other portions of the car may have faded or been repainted.

Standard/reference samples are always removed in a way that includes all the paint layers down to the bare metal. This is best accomplished by removing a painted section with a disposable scalpel. Samples 1/4 inch square are sufficient for laboratory examination. Each paint sample should be separately packaged and marked with the exact location of its recovery.

When a cross-transfer of paint occurs between two vehicles, all of the layers, including the foreign as well as the underlying original paints, must be removed from each vehicle. A standard/reference sample from an adjacent undamaged area of each vehicle must also be taken in such cases. Before collecting each sample, an investigator must use a new disposable scalpel in order to prevent cross-contamination of paints.

Quick Review

• Paint spread onto a surface dries into a hard film that is best described as consisting of pigments and additives

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suspended in a binder. • Questioned and known paint specimens are best compared side by side under a stereoscopic microscope for color, surface texture, and color layer sequence. • Pyrolysis gas chromatography and infrared spectrophotometry are used to distinguish most paint binder formulations. • PDQ (Paint Data Query) is a computerized database that allows an analyst to obtain information on paints related to automobile make, model, and year.

CASEFILES THE PREDATOR

September in Arizona is usually hot and dry, much like the rest of the year—but September 1984 was a little different. Unusually heavy rains fell for two days, which must have seemed fitting to the friends and family of 8-year-old Vicki Lynn Hoskinson. Vicki went missing on September 17 of that year, and her disappearance was investigated as a kidnapping. A schoolteacher who knew Vicki remembered seeing a suspicious vehicle loitering near the school that day, and he happened to jot down the license plate number. This crucial tip led police to 28-year-old Frank Atwood, recently paroled from a California prison. Police soon learned that Atwood had been convicted for committing sex offenses and for kidnapping a boy. This galvanized the investigators, who realized Vicki could be at the mercy of a dangerous and perverse man.

The only evidence the police had to work with was Vicki’s bike, which was found abandoned in the middle of the street a few blocks from her home. Police found scrapes from her bike pedal on the underside of the gravel pan on Atwood’s car, as well as pink paint on Atwood’s front bumper, apparently transferred from Vicki’s bike. The police believed that Atwood deliberately struck Vicki while she was riding her bicycle, knocking her to the ground.

The pink paint on Atwood’s bumper was first looked at microscopically and then examined by pyrolysis gas chromatography. This technique provides investigators with a “fingerprint” pattern of the paint sample, enabling them to compare this paint to any other paint evidence. In this case, the pink paint on Atwood’s bumper matched the paint from Vicki’s bicycle.

Vicki’s skeletal remains were discovered in the desert, several miles away from her home, in the spring of 1985. Positive identification was made using dental records, but investigators wanted to see if the remains could help them determine how long she had been dead. Atwood had been jailed on an unrelated charge three days after Vicki disappeared, so the approximate date of death was very important to proving his guilt.

Investigators found adipocere, a white, fatty residue produced during decomposition, inside Vicki’s skull. This provided evidence that moisture was present around Vicki’s body after her death, which did not seem to make sense, considering her body was found in the Arizona desert! A check of weather records revealed that there had been an unusual amount of rainfall during only one period of time since Vicki was last seen alive: a mere 48 hours after her disappearance. This put Vicki’s death squarely within Frank Atwood’s three-day window of opportunity between her disappearance and his arrest. Frank Atwood was sentenced to death in 1987 for the murder of Vicki Lynn Hoskinson. He remains on death row awaiting execution.

Forensic Analysis of Glass

Glass that is broken and shattered into fragments and minute particles during the commission of a crime can be used to place a suspect at the crime scene. For example, chips of broken glass from a window may lodge in a suspect’s shoes or garments during a burglary; particles of headlight glass found at the scene of a hit-and-run accident may confirm the identity of a suspect vehicle. All of these possibilities require the comparison of glass fragments found on the suspect,

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whether a person or vehicle, with the shattered glass remaining at the crime scene.

COMPOSITION OF GLASS

Glass is a hard, brittle, amorphous substance composed of sand (specifically, silicon oxides) mixed with various metal oxides. When sand is mixed with metal oxides, melted at high temperatures, and then cooled to a rigid condition without crystallization, the product is glass. Soda (or sodium carbonate) is normally added to the sand to lower its melting point and make it easier to work with. Another necessary ingredient is lime (or calcium oxide), which is added to prevent the glass, known as “soda-lime” glass, from dissolving in water. Often the molten glass is cooled on top of a bath of molten tin. This manufacturing process produces flat glass typically used for windows. This type of glass is called float glass. The forensic scientist is often asked to analyze soda-lime glass, which is used for manufacturing most windows and glass bottles.

The common metal oxides found in soda-lime glass are sodium, calcium, magnesium, and aluminum. In addition, a wide variety of special glasses can be made by partially or completely substituting other metal oxides for the silica, sodium, and calcium oxides. For example, automobile headlights and heat-resistant glass, such as Pyrex, are manufactured with boron oxide added to the oxide mix. These glasses are therefore known as borosilicates.

Another type of glass that the reader may be familiar with is tempered glass. This glass is made stronger than ordinary window glass by introducing stress through rapid heating and cooling of the glass surfaces. When tempered glass breaks, it does not shatter but rather fragments into small squares, or “dices,” with little splintering (see Figure 14-8). Because of this safety feature, tempered glass is used in the side and rear windows of automobiles sold in the United States. The windshields of all cars manufactured in the United States are constructed from laminated glass. This glass is given strength by sandwiching one layer of plastic between two pieces of ordinary window glass.

tempered glass

Glass to which strength is added by introducing stress through rapid heating and cooling of the glass surface

laminated glass

Two sheets of ordinary glass bonded together with a plastic film.

COMPARING GLASS FRAGMENTS

For the forensic scientist, comparing glass consists of finding and measuring the properties that will associate one glass fragment with another while minimizing or eliminating the possible existence of other sources. Considering the prevalence of glass in our society, it is easy to appreciate the magnitude of this analytical problem. Obviously, glass possesses its greatest evidential value when it can be individualized to one source. Such a determination, however, can be made only when the suspect and crime-scene fragments are assembled and physically fitted together. Comparisons of this type require piecing together irregular edges of broken glass as well as matching all irregularities and striations on the broken surfaces (see Figure 14-9). The possibility that two pieces of glass originating from different sources will fit together exactly is so unlikely as to exclude all other sources from practical consideration.

FIGURE 14-8 When tempered glass breaks, it usually holds together without splintering.

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xyno6/istockphoto.com

FIGURE 14-9 A match of broken glass. Note the physical fit of the edges.

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Courtesy Sirchie Fingerprint Laboratories, Inc., Youngsville, NC, www.sirchie.com

Unfortunately, most glass evidence is either too fragmentary or too minute to permit a comparison of this type. In such instances, the search for individual properties proves fruitless. For example, the general chemical composition of various window glasses has so far been found to be relatively uniform among various manufacturers and thus offers no basis for individualization within the capability of current analytical methods. However, as more sensitive analytical techniques are developed, trace elements present in glass may prove to be distinctive and measurable characteristics.

The physical properties of density and refractive index are used most successfully for characterizing glass particles. However, these properties are class characteristics, which cannot provide the sole criteria for individualizing glass to a common source. They do, however, give the analyst sufficient data to evaluate the significance of a glass comparison, and if the density and refractive index values are not comparable, this certainly excludes the possibility that the glass fragments originated from the same source.

MEASURING AND COMPARING DENSITY

Density is defined as mass per unit volume:

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density

The measurement of mass per unit of volume.

Density is an intensive property of matter—that is, it remains the same regardless of the size of an object; thus, it is a characteristic property of a substance and can be used in identification. Solids tend to be more dense than liquids, and liquids are more dense than gases.

intensive property

A property that is not dependent on the size of an object.

A simple procedure for determining the density of a solid is illustrated in Figure 14-10. First, the solid is weighed on a balance against standard gram weights to determine its mass. The solid’s volume is then determined from the volume of water it displaces. This is easily measured by filling a cylinder with a known volume of water (V1), adding the

object, and measuring the new water level (V2). The difference (V2 – V1, expressed in milliliters, is equal to the

volume of the solid. Density can now be calculated from the equation in grams per milliliter (i.e., mass per volume).

FIGURE 14-10 A simple procedure for determining the density of a solid is first to measure its mass on a scale and then to measure its volume by noting the volume of water it displaces.

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The fact that a solid object either sinks, floats, or remains suspended when immersed in a liquid can be accounted for by its density. For instance, if the density of a solid is greater than that of the liquid in which it is immersed, the object sinks; if the solid’s density is less than that of the liquid, it floats; and when the solid and liquid have equal densities, the solid remains suspended in the liquid medium. This knowledge gives the criminalist a rather precise and rapid method for comparing densities of glass.

In a method known as flotation, a standard/reference glass particle is immersed in a liquid, possibly a mixture of bromoform and bromobenzene. The composition of the liquid is carefully adjusted by adding small amounts of

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bromoform or bromobenzene until the glass chip remains suspended in the liquid medium. At this point, the standard/ reference glass and liquid each have the same density. Glass chips of approximately the same size and shape as the standard/reference are now added to the liquid for comparison. If both the unknown and the standard/reference

particles remain suspended in the liquid, their densities are equal to each other and to that of the liquid.2 Particles of different densities either sink or float, depending on whether they are more or less dense than the liquid.

The density of a single sheet of window glass is not completely homogeneous throughout. It has a range of values that can differ by as much as 0.0003 g/mL. Therefore, in order to distinguish between the normal internal density variations of a single sheet of glass and those of glasses of different origins, it is advisable to let the comparative density approach but not exceed a sensitivity value of 0.0003 g/mL. The flotation method meets this requirement and can adequately distinguish glass particles that differ in density by 0.001 g/mL.

DETERMINING AND COMPARING REFRACTIVE INDEX

Once glass has been distinguished by a density determination, different origins are immediately concluded. Comparable density results, however, require the added comparison of refractive indices. The bending of a light wave because of a change in velocity is called refraction. The phenomenon of refraction is apparent when we view an object that is immersed in a transparent medium such as water; because we are accustomed to thinking that light travels in a straight line, we often forget to account for refraction. For instance, suppose a ball is observed at the bottom of a swimming pool; the light rays reflected from the ball travel through the water and into the air to reach the eye. As the rays leave the water and enter the air, their velocity suddenly increases, causing them to be refracted. However, because of our assumption that light travels in a straight line, our eyes deceive us and make us think we see an object lying at a higher point than is actually the case. This phenomenon is illustrated in Figure 14-11.

The ratio of the velocity of light in a vacuum to its velocity in any medium determines the refractive index of that medium and is expressed as follows:

refractive index

The ratio of the speed of light in a vacuum to its speed in a given medium.

FIGURE 14-11 Light is refracted when it travels obliquely from one medium to another.

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For example, at 25°C the refractive index of water is 1.333. This means that light travels 1.333 times as fast in a vacuum as it does in water at this temperature.

Like density, the refractive index is an intensive physical property of matter and characterizes a substance. However, any procedure used to determine a substance’s refractive index must be performed under carefully controlled temperature and lighting conditions because the refractive index of a substance varies with its temperature and the wavelength of light passing through it. Nearly all tabulated refractive indices are determined at a standard wavelength, usually 589.3 nanometers; this is the predominant wavelength emitted by sodium light and is commonly known as the sodium D light.

When a transparent solid is immersed in a liquid with a similar refractive index, light is not refracted as it passes from the liquid into the solid. For this reason, the eye cannot distinguish the liquid-solid boundary, and the solid seems to disappear from view. This observation, as we will see, offers the forensic scientist a rather simple method for comparing the refractive indices of transparent solids.

This determination is best accomplished by the immersion method. For this, glass particles are immersed in a liquid medium whose refractive index is adjusted until it equals that of the glass particles. At this point, known as the match point, the observer notes the disappearance of the Becke line, indicating minimum contrast between the glass and liquid medium. The Becke line is a bright halo observed near the border of a particle that is immersed in a liquid of a different refractive index. This halo disappears when the medium and fragment have similar refractive indices.

Becke line

A bright halo observed near the border of a particle immersed in a liquid of a different refractive index.

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The refractive index of an immersion fluid is best adjusted by changing the temperature of the liquid. Temperature control is, of course, critical to the success of the procedure. One approach is to heat the liquid in a special apparatus known as a hot-stage microscope (see Figure 14-12). The glass fragments are immersed in a boiling immersion fluid, usually a silicone oil, and illuminated with sodium D light or another wavelength of light. The liquid is then heated at the rate of 0.2°C per minute until the match point is reached. This is the point at which the examiner observes the disappearance of the Becke line on the glass fragments. If all the glass fragments examined have similar match points, it can be concluded that they have comparable refractive indices (see Figure 14-13). Furthermore, the examiner can determine the refractive index value of the immersion fluid as it changes with temperature. With this information, the exact numerical value of the glass refractive index can be calculated at the match point temperature.

Along with varying in density, glass fragments removed from a single sheet of plate glass also may not have a uniform refractive index; instead, these values may vary by as much as 0.0002. Hence, for comparison purposes, the difference in refractive index between a standard/reference and questioned glass must exceed this value. This allows the examiner to differentiate between the normal internal variations present in a sheet of glass and those present in glasses that originated from completely different sources.

FIGURE 14-12 A hot-stage microscope used to view glass chips when determining their refractive index.

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Courtesy Chris Palenik, Ph.D., Microtrace LLC, Elgin, IL

CLASSIFICATION OF GLASS SAMPLES

A significant difference in either density or refractive index proves that the glass fragments examined do not have a common origin. But what if two pieces of glass exhibit comparable densities and comparable refractive indices? How certain can one be that they did, indeed, come from the same source? After all, there are untold millions of windows and other glass objects in this world.

To provide a reasonable answer to this question, the FBI Laboratory has collected density values and refractive indices from glass submitted to it for examination. What has emerged is a data bank correlating these values to their frequency of occurrence in the glass “population” of the United States. This collection is available to all forensic laboratories in the United States. This means that, once a criminalist has completed a comparison of glass fragments, he or she can correlate their density and refractive index values to their frequency of occurrence and assess the probability that the fragments came from the same source.

FIGURE 14-13 Determining the refractive index of glass. (a) Glass particles are immersed in a liquid of a much higher refractive index at a temperature of 77°C. (b) At 87°C the liquid still has a higher refractive index than the glass. (c) The refractive index of the liquid is closest to that of the glass at 97°C, as shown by the disappearance of the glass and the Becke lines. (d) At the higher temperature of 117°C, the liquid has a much lower index than the glass, and the glass is plainly visible.

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Courtesy Walter C. McCrone

Figure 14-14 shows the distribution of refractive index values (measured with sodium D light) for approximately 2,000 glass specimens analyzed by the FBI. The wide distribution of values clearly demonstrates that the refractive index is a highly distinctive property of glass and is thus useful for defining its frequency of occurrence and hence its evidential value. For example, a glass fragment with a refractive index of 1.5290 is found in approximately only 1 out of 2,000 specimens, whereas glass with an index of 1.5180 occurs in approximately 22 specimens out of 2,000.

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The distinction between tempered and nontempered glass particles can be made by slowly heating and then cooling the glass (a process known as annealing). The change in the refractive index of tempered glass upon annealing is significantly greater than that of nontempered glass and thus serves as a point of distinction.

CLOSER ANALYSIS GRIM 3

FIGURE 1 GRIM 3 identifies the refraction match point by monitoring a video image of the glass fragment immersed in an oil. As the immersion oil is heated or cooled, the contrast of the image is measured continuously until a minimum, the match point, is detected.

Courtesy Foster & Freeman Limited, Worcestershire Shine, UK, www.fosterfreeman.co.uk

An automated approach for measuring the refractive index of glass fragments by the immersion method with a hot-stage microscope is to use the instrument known as GRIM 3 (Glass Refractive Index Measurement)* (see Figure 1). The GRIM 3 is a personal computer/video system designed to automatically measure the match temperature and refractive index of glass fragments. This instrument uses a video camera to view the glass fragments as they are being

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heated. As the immersion oil is heated or cooled, the contrast of the video image is measured continually until a minimum, the match point, is detected (see Figure 2). The match point temperature is then converted to a refractive index using stored calibration data.

*

Foster and Freeman Limited, 25 Swan Lane, Evesham, Worcestershire WRII 4PE, UK

FIGURE 2 An automated system for glass fragment identification.

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Courtesy Foster & Freeman Limited, Worcestershire Shine, UK, www.fosterfreeman.co.uk

FIGURE 14-14 The frequency of occurrence of refractive index values (measured with sodium D light) in approximately 2,000 flat glass specimens analyzed by the FBI Laboratory.

Courtesy FBI Laboratory, Washington, DC

GLASS FRACTURES

Glass bends in response to any force that is exerted on any one of its surfaces; when the limit of its elasticity is reached, the glass fractures. Frequently, fractured window glass reveals information about the force and direction of an impact; such knowledge may be useful for reconstructing events at a crime-scene investigation.

The penetration of ordinary window glass by a projectile, whether a bullet or a stone, produces a familiar fracture pattern in which cracks both radiate outward and encircle the hole, as shown in Figure 14-15. The radiating lines are appropriately known as radial fractures, and the circular lines are termed concentric fractures.

radial fracture

A crack in a glass that extends outward, like a spoke of a wheel, from the point at which the glass was struck.

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concentric fracture

A crack in a glass that forms a rough circle around the point of impact.

Often it is difficult to determine just from the size and shape of a hole in glass whether it was made by a bullet or by some other projectile. For instance, a small stone thrown at a comparatively high speed against a pane of glass often produces a hole very similar to that produced by a bullet. On the other hand, a large stone can completely shatter a pane of glass in a manner closely resembling the result of a close-range shot. However, in the latter instance, the presence of gunpowder deposits on the shattered glass fragments signifies damage caused by a firearm.

When it penetrates glass, a high-velocity projectile such as a bullet often leaves a round, crater-shaped hole surrounded by a nearly symmetrical pattern of radial and concentric cracks. The hole is inevitably wider on the exit side (see Figure 14-16), and hence examining it is an important step in determining the direction of impact. However, as the velocity of the penetrating projectile decreases, the irregularity of the shape of the hole and of its surrounding cracks increases, so at some velocities the hole shape will not help determine the direction of impact. At this point, examining the radial and concentric fracture lines may help determine the direction of impact.

FIGURE 14-15 Radial and concentric fracture lines in a sheet of glass.

Courtesy Sirchie Fingerprint Laboratories, Inc., Youngsville, NC, www.sirchie.com

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FIGURE 14-16 A crater-shaped hole made by a projectile passing through glass. The upper surface is the side the projectile exited.

Don Farrall\Getty Images, Inc. – Photodisc/Royalty Free

When a force pushes on one side of a pane of glass, the elasticity of the glass permits it to bend in the direction of the force applied. Once the elastic limit is exceeded, the glass begins to crack. As shown in Figure 14-17, the first fractures form on the surface opposite that of the penetrating force and develop into radial lines. The continued motion of the force places tension on the front surface of the glass, resulting in the formation of concentric cracks. An examination of the edges of the radial and concentric cracks frequently reveals stress markings (Wallner lines) whose shape can be related to the side on which the window first cracked.

FIGURE 14-17 The production of radial and concentric fractures in glass. (a) Radial cracks are formed first, beginning on the side of the glass opposite the destructive force. (b) Concentric cracks occur outward, starting on the same side of the force.

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FIGURE 14-18 Stress marks on the edge of a radial glass fracture. The arrow indicates the direction of force.

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Richard Saferstein, Ph.D.

Stress marks, shown in Figure 14-18, are shaped like arches that are perpendicular to one glass surface and curve to nearly parallel the opposite surface. The importance of stress marks stems from the observation that the perpendicular edge always faces the surface on which the crack originated. Thus, in examining the stress marks on the edge of a radial crack near the point of impact, the perpendicular end is always found opposite the side from which the force of impact was applied. For a concentric fracture, the perpendicular end always faces the surface on which the force originated. A convenient way for remembering these observations is the 3R rule: Radial cracks form a right angle on the reverse side of the force. These facts enable the examiner to determine which side of a broken window was impacted. Unfortunately, the absence of radial or concentric fracture lines prevents these observations from being applied to broken tempered glass.

FIGURE 14-19 Two bullet holes in a piece of glass. The left hole preceded the right hole.

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When there have been successive penetrations of a piece of glass, it is frequently possible to determine the sequence of impact by observing the existing fracture lines and their points of termination. A fracture always terminates at an existing line of fracture. In Figure 14-19, the fracture on the left preceded that on the right; we know this because the latter’s radial fracture lines terminate at the cracks of the former.

COLLECTION AND PRESERVATION OF GLASS EVIDENCE

The gathering of glass evidence at the crime scene and from the suspect must be thorough if the examiner is to have any chance at individualizing the fragments to a common source. If even the remotest possibility exists that fragments may be pieced together, every effort must be made to collect all the glass found. For example, evidence collection at hit-and-run scenes must include all the broken parts of the headlight and reflector lenses. This evidence may ultimately prove invaluable in placing a suspect vehicle at the accident scene, if the fragments can be matched with glass remaining in the headlight or reflector shell of the suspect vehicle. In addition, examining the headlight’s filaments may reveal whether an automobile’s headlights were on or off before the impact (see Figure 14-20).

FIGURE 14-20 The presence of black tungsten oxide on the upper filament indicates that the filament was on when it was exposed to air. The lower filament was off, but its surface was coated with a yellow/white tungsten oxide, which was vaporized from the upper (“on”) filament and condensed onto the lower filament.

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When an individual fit is improbable, the evidence collector must submit all glass evidence found in the possession of the suspect along with a sample of broken glass remaining at the crime scene. This standard/reference glass should always be taken from any remaining glass in the window or door frames, as close as possible to the point of breakage. About one square inch of sample is usually adequate for this purpose. The glass fragments should be packaged in solid containers to avoid further breakage. If the suspect’s shoes and/or clothing are to be examined for the presence of glass fragments, they should be individually wrapped in paper and transmitted to the laboratory. The field investigator should avoid removing such evidence from garments unless absolutely necessary for its preservation.

When a determination of the direction of impact is needed, all broken glass must be recovered and submitted for analysis. Wherever possible, the exterior and interior surfaces of the glass must be indicated. When this is not immediately apparent, the presence of dirt, paint, grease, or putty may indicate the exterior surface of the glass.

Quick Review

• To compare glass fragments, a forensic scientist evaluates density and refractive index. • The immersion method is used to determine a glass fragment’s refractive index. It involves immersing a glass particle in a liquid medium whose refractive index is adjusted by varying its temperature. At the refractive index match point, the visual contrast between the glass and liquid is at a minimum. • The flotation method is used to determine a glass fragment’s density. It involves immersing a glass particle in a liquid whose density is carefully adjusted by adding small amounts of an appropriate liquid until the glass chip suspends in the liquid medium.

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• By analyzing the radial and concentric fracture patterns in glass, the forensic scientist can determine the side of impact by applying the 3R rule: Radial cracks form a right angle on the reverse side of the force.

Forensic Analysis of Soil

There are many definitions for the term soil; however, for forensic purposes, soil may be thought of as any disintegrated material, natural and/or artificial, that lies on or near the earth’s surface. Therefore, forensic examination of soil is not only concerned with the analysis of naturally occurring rocks, minerals, vegetation, and animal matter; it also encompasses the detection of such manufactured objects as glass, paint chips, asphalt, brick fragments, and cinders, whose presence may impart soil with characteristics that make it unique to a particular location. When this material is collected accidentally or deliberately in a manner that associates it with a crime under investigation, it becomes valuable physical evidence.

SIGNIFICANCE OF SOIL EVIDENCE

The value of soil as evidence rests on its prevalence at crime scenes and its transferability between the scene and the criminal. Thus, soil or dried mud found adhering to a suspect’s clothing or shoes or to an automobile, when compared to soil samples collected at the crime site, may link a suspect or object to the crime scene. As with most types of physical evidence, forensic soil analysis is comparative in nature; soil found in the possession of the suspect must be carefully collected and then compared to soil samplings from the crime scene and its vicinity.

However, one should not rule out the value of soil even if the site of the crime has not been ascertained. For instance, small amounts of soil may be found on a person or object far from the actual site of a crime. A geologist who knows the local geology may be able to use geological maps to direct police to the general vicinity where the soil was originally picked up and the crime committed.

FORENSIC EXAMINATION OF SOIL

Most soils can be differentiated by their gross appearance. A side-by-side visual comparison of the color and texture of soil specimens is easy to perform and provides a sensitive property for distinguishing soils that originate from different locations. Soil is darker when it is wet; therefore, color comparisons must always be made when all the samples are dried under identical laboratory conditions. It is estimated that there are nearly 1,100 distinguishable soil colors; hence, color offers a logical first step in a forensic soil comparison (see Figure 14-21).

FIGURE 14-21 A color chart is displayed behind three soil samples

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Courtesy of Gretag Macbeth, Munsell Color

Low-power microscopic examination of soil reveals the presence of plant and animal materials as well as artificial debris. Further high-power microscopic examination helps characterize minerals and rocks in earth materials. Although this approach to forensic soil identification requires the expertise of an investigator trained in geology, it can provide the most varied and significant points of comparison between soil samples. Only by carefully examining and comparing the minerals and rocks naturally present in soil can one take advantage of the large number of variations between soils and thus add to the evidential value of a positive comparison A mineral is a naturally occurring crystal, and like any other crystal, its physical properties—for example, its color, geometric shape, density, and refractive index—are useful for identification. More than 2,200 minerals exist; however, most are so rare that forensic geologists usually encounter only about 20 of them. Rocks are composed of a combination of minerals and therefore exist in thousands of varieties on the earth’s surface. They are usually identified by characterizing their mineral content and grain size (see Figure 14-22).

mineral

A naturally occurring crystalline solid.

Considering the vast variety of minerals and rocks and the possible presence of artificial debris in soil, the forensic

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geologist is presented with many points of comparison between two or more specimens. The number of comparative points and their frequency of occurrence must be considered before concluding that specimens are similar and judging the probability of their common origin.

Rocks and minerals not only are present in earth materials but also are used to manufacture a wide variety of industrial and commercial products. For example, the tools and garments of an individual suspected of breaking into a safe often contain traces of safe insulation. Safe insulation may be made from a wide combination of mineral mixtures that provide significant points of identification. Similarly, building materials such as brick, plaster, and concrete blocks are combinations of minerals and rocks that can easily be recognized and compared microscopically to similar minerals found on the breaking-and-entering suspect.

FIGURE 14-22 A mineral viewed under a microscope.

Courtesy of Chris Palenik, Ph.D., Microtrace LLC, Elgin, IL