Forensic Examination of Glass and Paint

Analysis and Interpretation

Brian Caddy, Brian Caddy

• Publisher: CRC Press
• Year: 2001
• ISBN:
  9781135740542
• Edition: 1
• Topic: Derecho
• Subtopic: Ciencias forenses

This volume represents an approach to the analysis of glass and paint as they occur as trace evidence in forensic cases. Each chapter is written by an expert in their particular area. The book is divided into two sections: one referring to paint and one referring to glass. Each section covers an introduction to the composition of these materials an

1
What is trace evidence?

PETER R. DE FOREST

1.1 Preliminary definitions

1.1.1 Trace evidence and physical evidence

The term trace evidence refers to a very broad category of physical evidence. The trace evidence category covers a larger variety of physical evidence types than any other division [1–3]. A discussion of the attributes of physical evidence that make it trace evidence will follow in a later section of this chapter. Anything or any material can be physical evidence. This term has both a legal and a scientific meaning. In the legal context it refers to tangible objects that are brought into court and formally entered into evidence as exhibits (or productions). Obviously, such objects need not be entered into evidence in a legal proceeding to constitute physical evidence from the scientific point of view. Any objects and materials from a crime scene (or suspect, victim, etc.) that may shed light on an investigation are physical evidence from the scientific perspective. They become physical evidence as soon as they are recognised as having possible significance. The process of evidence recognition is more complex and challenging than is generally appreciated. It requires considerable scientific knowledge and skill as well as experience. Evidence recognition, both at the crime scene and in the laboratory, needs to receive more attention from scientists in cooperation with non-scientist investigators.

1.1.2 Forensic science and criminalistics

Forensic science can be defined as the application of the sciences to matters of the law. That branch of forensic science which focuses on physical evidence is known in many quarters as criminalistics. Although these definitions are not universally agreed upon, forensic science is clearly the more general term. In addition to criminalistics, disciplines such as forensic pathology and toxicology fall within the more general field of forensic science. The American Academy of Forensic Sciences (AAFS) has about ten sections, the largest of which is criminalistics with about 800 members. For reasons that are primarily economic, only a fraction of the eligible criminalists in the United States are members of this section of the American Academy of Forensic Scientists. Many more are members of about eight regional associations of forensic scientists whose meetings are geographically convenient. A similar pattern holds on the international scene. There are many more scientists, whether they are called criminalists or not, who are employed by forensic science laboratories to analyse physical evidence from crime scenes than in any other branch of the field.

1.2 Concepts and the production of physical evidence

A fundamental principle of forensic science or criminalistics is that a criminal act, or more generally a human-initiated event, produces a record of itself. The record, however imperfect, is the result of human actor(s) and the events they set in motion producing interactions that result in changes in the immediate environment. These changes can be expected to follow well-established laws of nature. As a consequence of this, they can be studied scientifically. Objects get moved or broken, marks are made, and materials are changed or transferred. In the preface to his classic 1953 textbook, Crime Investigation, Paul L. Kirk [4] had this to say about physical evidence:

Wherever he steps, whatever he touches, whatever he leaves, even unconsciously, will serve as silent evidence against him. Not only his fingerprints or his footprints, but his hair, the fibres from his clothes, the glass he breaks, the tool mark he leaves, the paint he scratches, the blood or semen he deposits or collects – all these and more bear mute witness against him. This is evidence that does not forget. It is not confused by the excitement of the moment. It is not absent because human witnesses are. It is factual evidence. Physical evidence cannot be wrong; it cannot perjure itself; it cannot be wholly absent. Only its interpretation can err. Only human failure to find it, study and understand it, can diminish its value. (The italics are in the original.)

The term trace evidence is difficult to circumscribe and define. In some ways the adoption and well-established use of this term is unfortunate. However, it is difficult to come up with a completely satisfactory alternative term. Two very different notions contribute to concept of trace evidence. One notion is related to size. Small amounts of material may be referred to as traces, and evidence of concern in forensic investigations is often very small. Here the notion of trace seems appropriate, but size is not the defining characteristic. On occasion, relatively large amounts of material may be trace evidence. The word trace also has the connotation of a vestige or of something remaining behind after something else has been removed, as in the phrase ‘he vanished without a trace’. This is closer to the other dimension of trace evidence which embodies the idea of an interaction which results in an exchange of material or the production of a pattern or both. This aspect is reflected in the term transfer evidence which is sometimes used. Despite the extensive and broadly inclusive realm of trace evidence, clearly not all physical evidence is trace evidence. As noted above and further illustrated in the quotation from Dr Kirk, physical evidence can be anything. The field of trace evidence has only a somewhat narrower purview. To sum up, it is the consequences of marks being made and material being transferred that are of concern in the area of trace evidence investigations. They are useful in evaluating possible associations and in reconstructing events.

Logically, physiological stain evidence such as blood and body fluid stains can also be regarded as trace evidence but are normally not discussed in references on trace evidence, because this subject is given specialised treatment in other sources. Conceptually, however, it is useful to consider these stains as a category of trace evidence. Physiological stains are set apart in common practice only because a different, and a more-or-less self-contained, set of analytical techniques is used for their characterisation. In forensic science casework it is essential to avoid focusing on techniques to the neglect of a larger view which encompasses all of the evidence and the context in which it occurs. This point should always be borne in mind.

1.3 Uses of trace evidence

Trace evidence has three distinct uses in forensic science. These are as investigative aids, as associative evidence, and in reconstructions. The least exploited of these uses is the employment of trace evidence to assist in giving direction to an investigation in its early stages. This can also be referred to as investigative intelligence. Unfortunately, historically scientific work on trace evidence in cases has been peripheral to investigations rather than being integrated with them.

Investigative aids

Although the skilful examination and the knowledgeable interpretation of trace evidence has a great inherent potential for contributing much useful information to investigations, this potential is rarely realised in practice for a number of reasons. Principal among these is the dearth of forensic scientists skilled in the examination and interpretation of trace evidence and the general lack of early involvement of forensic scientists in investigations in most jurisdictions. Examples of investigative aids or intelligence supplied by trace evidence interpretations would include suggesting a geographic provenance for an unknown sample, determining the origin of a manufactured material, determining the approximate date of manufacture of a material, providing information to eliminate suspects and thus limit the number of candidate suspects, as well as helping to develop suspects.

Associative evidence

In current practice trace evidence is most commonly used as evidence of association or as associative evidence. In this way it is used to investigate the existence of a link between individuals or between individuals and scenes after suspects have been developed, thus providing information of value at the adjudicative phases of a case.

Reconstruction

Trace evidence may contribute information to the understanding of how an event ofTrace evidence may contribute information to the understanding of how an event of concern took place. This point can be illustrated by considering situations commonly arising in the investigation of motor vehicle accidents. In addition to their value as associative evidence in linking the vehicle to the victim or the scene, paint transfers resulting from the contact of a motor vehicle with some object or another motor vehicle can be studied to provide geometric details concerning the manner in which the contact took place. This is particularly true where there have been multiple contacts and two-way exchanges of different kinds of complementary trace evidence. Beyond the motor vehicle example, there are many examples of trace evidence being used in reconstructions. At times the associative evidence, combined with the reconstruction its geometry affords, can greatly strengthen the association. In some of these cases the association can be so strong that there is no credible alternative, and the association becomes absolute even though the individual associations, provided by each component trace evidence comparison, are only suggestive. An example of this involving paint and other trace evidence will be given later in the chapter.

1.4 The history of trace evidence

Like the early history of forensic science in general, the beginnings of trace evidence examination are rather murky. Anecdotal accounts concerning the utilisation of trace evidence may date back millennia. Suspicious wives have undoubtedly noticed hairs and other traces on a husband’s clothing that in their minds seemed to defy innocent explanation. It is all but certain that similar ad hoc recognitions of trace evidence occurred in early criminal investigations as well. However, trace evidence examinations as we know them had to await the development of modern science and the refinement of the microscope over the course of nearly two centuries by Anton van Leeuwenhoek, Robert Hooke, Ernst Abbe and others. The first articulated general recognition of trace evidence would appear to have occurred in the fictional exploits of Sherlock Holmes in the Sir Arthur Conan Doyle stories. Shortly after the appearance of the Holmes stories, the Austrian Magistrate Hans Gross published a book in which he advocated the use of trace evidence in criminal investigations [5]. Later Edward Oscar Heinrich in the United States, Edmund Locard in France, and Georg Popp in Germany were among the first to make use of trace evidence in casework. Interesting details of the work of some of these pioneers have been discussed by Palenik [6–8]. The details of this history will not be presented here, but the history, such as that recounted in these articles, is well worth reading. The articles present trace evidence insights as well as a historical perspective.

1.5 The scope of trace evidence

1.5.1 Introduction

For trace evidence produced by contact, two general kinds of evidence are produced, pattern evidence and transferred material. The material transferred can be important evidence from two principal perspectives. From one of these, it is the transferred material itself that becomes the evidence. This consideration is applicable where the comparison of the transferred material with its suspected source is used as the evidence of association or common origin between the transferred material and the donor surface. In the second way that trace evidence resulting from contact can be used, it is the pattern of the transferred matter on the recipient surface or the pattern of matter removed from the donor surface that is important. This pattern may be compared to one on the corresponding surface to study the question of commonality of origin, or it may be used to shed light on the way in which the interaction took place between the objects bearing the two interacting surfaces. The latter is an issue involving reconstruction. The former can be considered part of the domain of trace evidence comparisons but will not be the subject of this book.

The focus in this volume will be on paint and glass evidence and thus on the properties and characterisation of these two types of material when they appear as transfer evidence. However, in practice the other perspective or broad class of trace evidence cannot be ignored or important information may be lost. The material and the pattern or context must be considered together. Serious dilemmas can arise with respect to decisions regarding documentation and sampling. For example, in the case of paint evidence, if flakes or chips are encountered, these need to be evaluated for their potential for physical matching to a known source. In addition, layer structure, if present, needs to be considered.

The idea of material transfer across the contact boundary is often referred to as the Exchange Principle in recognition of the early contributions of Dr Edmund Locard of Lyon, France, to the theory and practice of trace evidence examinations. This is discussed by Nickolls [9]. It does not address the situation where the significant evidence is not the material transferred but is instead a pattern produced as a result of the interaction between the two surfaces. A more general exposition of this idea is the one contained within the quotation from the preface of Kirk’s Crime Investigation which was included earlier in this chapter. Here contactless transfers as well as marks are considered. More general yet is the idea of viewing the crime scene as a recording medium where human activities and humaninitiated events produce a latent record in the form of alterations to the environment. These alterations, whether they are material transfers, markings, fractures, or mere displacements of objects, are physical evidence. The totality of the physical evidence, if recognised and properly interpreted, is the record. It should be appreciated that the record is often largely a latent one. It must be recognised, decoded and interpreted.

This section will be concerned with three major divisions of trace evidence, namely contact transfers of material where the material itself takes on importance as the evidence, contactless transfers of material of evidentially useful material, and patterns of value resulting from contact. These three broad divisions encompass the scope of trace evidence. These are subdivided and discussed in the subsections that follow.

1.5.2 Evidence consisting of contact transfers of material

Much trace evidence is the result of the transfer of material across the contact boundary as two surfaces come into contact. Evidence produced in this way comprises a class of evidence known as contact transfer evidence. The material is transferred from what can be called a donor surface to a receiving or recipient surface. For certain combinations of pairs of materials and areas of contact, a given surface may play both the donor and recipient roles. In this case we would have what is termed a two-way transfer. The nature of the materials and the nature of the interaction can influence whether or not this takes place. Two-way transfers are among some of the most evidentially significant transfers. There is a need for more research regarding the details of these interactions and the ways in which they can influence trace evidence interpretations.

1.5.3 Evidence consisting of contactless transfers of material

Contact between donor and recipient surfaces is not always necessary for there to be a transfer of trace evidence. Many situations involving glass transfer provide a good example. A would-be burglar who breaks a window may get numerous microscopic bits of glass on his clothing even though his clothing did not come into direct contact with the window. During the act of breaking the window small fragments of glass are projected from the area of impact. Those projected in a retrograde fashion with respect to the direction of the applied force may be deposited and retained as trace evidence in the hair and clothing of the burglar. Stains produced by airborne blood droplets and deposits of gunshot discharge residue provide additional examples.

1.5.4 Pattern evidence from contact transfers of material

At times the nature of the material transferred may not be as important as evidence of association or prior contact as is the pattern that results from the transfer of the material. It is the pattern that the material makes on the recipient surface that has the most value as evidence of association. This can be called static contact transfer pattern evidence. Material present on a donor surface may be transferred to a receptor surface in a manner that preserves some characteristic geometric features of the donor surface. The pattern left by an inked rubber stamp is a good example of this. A similar situation would be the pattern left on a fingerprint card by a suitably inked fingertip or by a bloody fingertip at a crime scene. Of course in the latter case, depending on case circumstances, the blood transferred could be as important evidence as the pattern produced.

Other situations where the transferred material carries the pattern of interest are the reverse of the above. Here it is the pattern of removal of material from the donor surface compared to the geometry of the recipient surface that is utilised. The patterned rubber sole of a shoe may lift dust from a thin film that exists on a floor to form a detailed void pattern that would have value as evidence. Parts of assemblies that have remained in contact for a period of time before becoming separated present another example. A rubber seal or gasket which has pulled away from a painted surface may take an irregular pattern of paint with it. The pattern of voids on the surface from which the paint has been removed compared with the pattern of paint adhering to the rubber may provide the basis for a conclusion of a unique association at a later time. This is somewhat like evidence produced by physical separation to be discussed in the next section.

1.5.5 Pattern evidence from physical separations of material

The physical separation described above where a pattern of paint was removed from one surface by another could be viewed as a special case of pattern evidence produced by physical separation. The more general case would be where an object is cut apart or fractured producing two or more pieces. Fractures are most often random, resulting in fragments that possess detailed and distinct topology on the fracture surfaces. Such fragments have the potential of being uniquely associated with the surfaces from which they were separated.

1.5.6 Pattern evidence from contactless transfers of material

Patterns can be produced by contactless transfers of material. Unlike the contact patterns discussed in this section of this chapter, most of these contactless transfer patterns seldom have value as evidence of association. Examples of this type would include gunshot discharge residue patterns or airborne blood droplet stain patterns. These kinds of patterns more commonly have value in reconstructions rather than as associative evidence.

Those more exceptional circumstances where a contactless transfer could have value as associative evidence could occur where a patterned void is left where a characteristically shaped object has intercepted the material being deposited during the process of material deposition. The characteristically shaped object might intercept paint spray, bloodspatter, or airborne powder or dust. Here the pattern of the void itself may become evidence if there is a need to evaluate whether specific objects were present during deposition and were the cause of the void. If the object is recovered later at some other location, the possibility of its former presence can be assessed.

1.5.7 Contact pattern evidence independent of transfers of material

Marks resulting from contact between two surfaces where one surface distorts, deforms or reshapes the other comprise the evidence in this category. The marks are to some degree three-dimensional and are not due to material transfer. Material is undoubte...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• List of contributors
• Preface
• 1 What is trace evidence?
• 2 The composition and manufacture of glass and its domestic and industrial applications
• 3 Microscopic techniques for glass examination
• 4 Elemental analysis of glass fragments
• 5 Statistical interpretation of glass evidence
• 6 Interpretation of physical aspects of glass evidence
• 7 Composition, manufacture and use of paint
• 8 The role of colour and microscopic techniques for the characterisation of paint fragments
• 9 Pyrolysis techniques for the characterisation and discrimination of paint
• 10 Use of infrared spectroscopy for the characterisation of paint fragments
• 11 Scanning electron microscopy and energy dispersive X-ray spectrometry (SEM/EDS) for the forensic examination of paints and coatings
• 12 The interpretation of paint evidence