The Forensic Examination and Interpretation of Tool Marks

David Baldwin, John Birkett, Owen Facey, Gilleon Rabey

• Publisher: Wiley-Blackwell
• Year: 2013
• ISBN:
  9781118374177
• Edition: 1
• Topic: Medicine
• Subtopic: Forensic Medicine

The Forensic Examination and Interpretation of Tool Marks brings together key techniques and developments in the field of tool marks in forensic science and explains clearly how tool mark analysis can be used within forensic investigation.

The purpose of this book is to bring together as much of this information as possible in an accessible manner. The book deals with all aspects of tool mark evidence from crime scene to courtroom. The examination of a wide variety of different tool marks are discussed, including those made by specific tools such as saws and in complex materials such as bone.

The general principles and techniques used in tool mark examinations can also be applied to some other closely related fields. Therefore, sections on the examination of manufacturing marks, including those on plastic film items, and physical fit comparisons are also included.

The book will be of interest to a wide range of people and not just to tool mark examiners and people studying forensic science. It will be of use to crime scene examiners, officers investigating crimes where tool marks are found and members of the legal professions.

• Brings together key techniques and developments within the field of tool mark investigation.
• Includes material on examining tool marks at the crime scene and in the laboratory, interpretation and evaluation issues and how tool mark evidence should be presented in court.
• Covers specialized tool mark examinations, manufacturing marks, including those on plastic film items, and physical fits.
• Includes a large range of illustrations and photographs.
• Invaluable reference for practicing forensic scientists, students of forensic sciences, members of the legal professions and crime scene investigators, enabling them to recognise the importance of tool marks within an investigation.
• An extremely valuable resource in the on-going debate regarding the evidential value of tool marks in court.
• Part of the 'Essentials in Forensic Science' book series.

Chapter 1

Introduction

1.1 Overview of contents

The term ‘tool mark examination’ is often applied to cover a wide range of possible forensic examinations. However, whilst the term implies that you must have some type of tool or instrument before you can proceed, this is not the case at all as we shall see. Throughout the book we will use the term ‘tool’ to cover all instruments, tools or other objects that have come into contact with another surface.

As the term suggests, it primarily applies to examinations involving marks that have been made by a tool or tools used by a person in order to commit an offence, where usually both the scene mark(s) and suspect tool(s) are available to the examiner for consideration, to determine whether or not the tool submitted was the one used. Chapters 2–6 will mainly deal with this type of typical tool mark examination, concentrating on the important aspects used by the examiner such as how tools are made and what features may be found on their surfaces, the type of tool marks that can be found at scenes of crime, the laboratory techniques that can be used to examine them and how results can be interpreted and evaluated. The principles that we will cover in these chapters, as well as here in Chapter 1, also provide a solid framework for the scientific investigation of other related forensic examinations described.

The basis of all tool mark examinations is that when one object comes into contact with a softer material, then evidence of the contact may result. Put simply, if the contact forces the two objects together without any movement then an impressed mark in the softer material will result; this may be a direct representation of the surface of the harder object that has been in contact. If however, there is some form of movement between the two objects then a dynamic mark will result. The dynamic mark will consist of a series of parallel lines, which are often referred to as striations (made up of ‘striae’). In general terms there are four main types of action that produce striated marks:

• sliding marks, which are sometimes also known as scratch marks, for example where a tool slips across the surface;
• cutting marks, where a single bladed tool such as a knife slices through an object;
• cutting marks, where a double bladed tool is used to sever an object;
• stabbing marks, where a tool is forced into a material.

In many instances a mark will consist of a combination of both impressed and dynamic detail, both of which have been produced by and directly relate to surface features on the harder object at the time the mark was made.

‘Tool marks’ therefore could also be found on the surface of a variety of manufactured items. This includes marks on the surface of those very tools and instruments that have been used to commit an offence. Hand tools such as screwdrivers, case openers and bolt cutters, to name but a few, are mass produced items that are shaped, moulded and finished by various processes during manufacturing. The machines, moulds and other equipment that form a finished hand tool are ‘tools’ themselves (albeit larger ones) and have the ability to leave marks of various descriptions on the finished manufactured item. Assessing how these manufacturing marks on the surface of tools were made and their evidential significance is a fundamental stage in the interpretation and evaluation of routine tool mark comparisons.

Therefore, it follows that other mass produced items will also have ‘tool marks’ upon their surfaces. For example, items such as screws, nuts and nails, where the item has been somehow shaped, moulded, gripped or cut to achieve the final shape and finish. However, for the purposes of this book, these will be referred to as ‘manufacturing marks’ and when considered alongside typical tool marks, the more general term ‘marks’ will be used. (This should not be confused with other forensic disciplines where marks are examined such as fingerprints, footwear marks or tyre marks.) In a forensic examination, such manufactured items may be seized from a crime scene and there is a necessity to compare them with a suspect population of similar items. In these sorts of circumstances, the tool mark detail on the surfaces of the items under examination needs to be assessed, and an evaluation made on whether or not they have come from the same source. For examinations such as these, the same equipment and techniques used for routine tool mark examination can be applied. The main difference is in how the results are interpreted and evaluated, which will be discussed in Chapter 7, although the same fundamental principles that we will see in Chapter 6 still apply.

In this book we will also cover what are known as ‘physical fits’. This term refers to broken or torn items where the pieces can be demonstrated to have once been joined together, and therefore formed part of the same item. What the item is and how it was broken/torn can alter the information and laboratory techniques used in order to reach a conclusion. In general though, physical fit examinations fall into four main categories.

1. Broken items that can be obviously fitted back together; otherwise known as ‘jigsaw’ fits. A tool mark examiner would not necessarily be required to demonstrate this sort of physical fit.

2. Broken items where the pieces require routine tool mark examination techniques to demonstrate that the pieces fit together and thus to form a conclusion. The detail may require a microscopic comparison and use of casting.

3. Broken, torn or cut items where knowledge of manufacture and type of marks left on the surface of the item need to be taken into account in order to support a fit. Typically, these sorts of examinations require techniques more commonly associated with routine tool mark examinations or manufacturing marks, particularly those relating to plastic film items.

4. Items that were originally fitted together or were in contact for a period of time. Typically these examinations involve a consideration of what material has been transferred or is a result of the contact.

The categories of physical fit that fall under the auspices of tool mark or manufacturing mark examinations will be dealt with in Chapter 8.

Plastic manufactured items, such as plastic film, bags and adhesive tape, can also exhibit features of manufacture caused by the machinery and processes used to make them. As with other mass-produced items such as screws, examinations tend to focus on whether crime scene and suspect items came from the same source. However, in this particular area, different examination techniques are normally applied in order to visualise the detail (but the need to understand how the detail has originated during the manufacturing process remains the same). Another critical factor in the interpretation and evaluation of these sorts of manufacturing marks is knowledge of how rapidly the detail changes. Chapter 9 will cover aspects of forensic examination in relation to the manufacture of plastic film items, including physical fits (as described above under category three) involving this sort of material.

Whilst this book aims to cover a wide range of examination types encountered in this particular field of forensic science, it cannot be a catch all for all of the weird, wonderful and unusual examinations that the authors have encountered in their collective 129 years of forensic experience. However, the fundamental principles of more typical marks' examinations, which we shall describe in this book, can be applied to any type of examination that falls, no matter how tenuously, into the categories of marks described. Our aim is to cover a broad selection of more commonly encountered examination types and some tried and tested, best practice techniques and methods based on our knowledge and experience of doing work of this kind. However, such work is not without its limitations and Chapter 10 will attempt to capture some of the ways in which other experts in this field are striving to make improvements.

1.2 A brief history of tool marks

At this juncture, it is useful to put tool mark examination into an historical context, as it is a traditional area of work that has been in use for longer than may be expected. There has been an appreciation that marks can be related back to tools from early times, although there were few written texts on the subject. One frequently quoted example comes from 12th century China, where the different shapes of wound caused by cutting instruments such as sickles were considered but it had little impact on the courts, even in China. The first book to have a major effect was written by Hans Gross in 1891, published in 1893 as two volumes, entitled Handbuch für Untersuchungsrichter als System der Kriminalistik. This was later translated into various languages, including English (see Gross, 1907) and has been republished many times.

Gross was a professor of law at the University of Graz in Austria and was also a practising judge. The book was written from his experience as a judge and details some of the best physical, as distinct from circumstantial, evidence that was presented. Tool marks are not given in a specific chapter but feature under the heading of Theft – Burglary and House Breaking, in Chapter 17. In this Gross says that it is necessary to describe, record (here by drawing) and take mouldings of all the damage done by the thief. The example is given of a tool, where the impressed tip detail indicated that a screwdriver, rather than a chisel, had been used and examination with a hand lens showed that one corner was damaged. A sketch was made and later a screwdriver used in another burglary was connected to this scene using the recorded information. There is no mention of using microscopes here but they are mentioned in Chapter 5 Section vii, dealing with firearms together with a comparison microscope. This goes with the caution that ‘…microscopic examinations can only be made by really skilled experts’.

All the basic steps of tool mark examination are present in this work and it remained in print until 1934. During the period 1891–1934, the courts increasingly recognised the value of scientific evidence but there were no large laboratories in existence and equipment was both limited and often awkward to use. After 1945 there was a large surge in scientific research, both in universities and industry. Forensic science benefited from this and the investigation of tool marks and firearms was improved through the development of better designed equipment, especially optical equipment.

In 1953 Paul L. Kirk, who was by that time a Professor at the University of California in forensic science, wrote an influential textbook called Crime Investigation, which includes sections on tool mark examination. He recognised the need to cast marks found at scenes of crime, when the item with the mark cannot be taken to a laboratory and suggested ways of achieving this. By the time of the last update and reprint in 1974, two-pack silicone materials were being suggested for casting, with the added comment that there was the need to colour the surface to obtain good reflectivity for comparison purposes. There are detailed discussions relating to types of tools and the importance of what are now called class characteristics together with individual characteristic detail.

The book makes a clear distinction between ‘compression marks’, called impressed marks in this book, and ‘sliding marks’, here called dynamic or striated marks or further divided into sliding, cutting or stab marks depending on the action used. The method given for the examination of impressed marks is to use macro photography of the individual scene mark and the test mark, then to use side by side comparison of the photographic images. A comparison microscope (also known as a comparator or comparison macroscope¹) with long focal length lenses is suggested for examining striated marks, with the comment that photography using the comparator is not always successful. Examination of physical fits is also included in the text.

There is a discussion of what is meant by a ‘tool mark match’, together with an increasing set of references to papers dealing with this topic in the reprints. There is recognition of the problems involved in ‘matching’ and the need for training and experience. In relation to striated marks, although by implication impressed marks as well, Kirk says ‘There is a need for conservatism – no witness can truthfully state “This is the only tool on earth that could have made this mark”’. In the 1974 edition there is a reference to the work by Biasotti (1964), The Principles of Evidence Evaluation as Applied to Firearms and Tool Mark Identification, which contains some of the first references for objective methods for evaluating striated marks.

In 1958 J.E. Davis published An Introduction to Toolmarks, Firearms and the Striagraph. This gave more information than Kirk's book but, apart from introducing the striagraph, which did not enter general use, added little new.

While Kirk makes clear in his introduction that his book was written for ‘laboratory criminalists’, most of the later authors write for more general audiences. H.J. Walls, a director of the Metropolitan Police Forensic Science Laboratory, London, UK, wrote a book entitled Forensic Science: An Introduction to Scientific Crime Detection, the first edition appearing in 1968. He states in the introduction that the book is intended for non-specialists. There is a general discussion of tool marks, which does not go into any great detail about the methods used in their examination. In dealing with the problem of matching marks, he noted that there is rarely a ‘perfect’ match between test and questioned marks. In an unreferenced aside on striated marks he notes that tests in the United States showed that a correspondence of over 70% of the striae could be found for suitable marks made by the same tool but was less than 25% in marks made by different tools.

In 1969 the Association of Firearms and Tool Mark Examiners (AFTE) was formed in the United States. This provided a forum for those examiners, separate from the various forensic science groups already in existence. Its journal and publications did much to raise the profile of this area of forensic wor...