Forensic Handbook 2 – Trace Evidence

Dr Allison Jones

May 2002

The value of 'trace' (or 'contact') forensic evidence was first recognised by Edmund Locard in 1910 when he proposed his 'interchange theory'. It basically says that 'every contact leaves a trace'. For example, burglars will leave traces of their presence behind and will also take traces with them. They may leave hairs from their body or fibres from their clothing behind and they may take carpet fibres away with them. Two of the less obvious, but frequent, sources of forensic evidence are glass and paint traces.

Case Study

Paint fragments from a car in a Sudbury, Toronto scrapyard led to the arrest of a man who had been involved in a hit-and-run accident three years earlier. A young boy was hit by a car whilst he was riding his bicycle. The car didn't stop and the boy was left with serious head wounds. There were no witnesses.

Three years later, police were conducting a routine check of scrapyards to see if they could find evidence of cars that may have been involved in crimes. Samples from a Chrysler Coronet matched paint fragments removed from the clothing of the young boy. This suggested they could have shared a common origin. The owner of the car was traced and this evidence was used against him in court. The man was charged with criminal negligence, causing bodily harm and failing to remain at the scene of an accident, and subsequently found guilty.

Paint traces

Paint is a common form of evidence presented to forensic scientists. It can provide valuable evidence that associates a suspect with a crime scene. It might be found on a crowbar, a window or door, on a hit-and-run victim's clothing, on vehicles in a traffic accident, or as flakes on clothing from a break-in entry point.

The nature of paint

Paint is made up of three main components.

The carrier is usually polymeric in nature and 'cures' (becomes solid) on evaporation of the solvent.

The solvent is usually an organic liquid that dissolves the carrier to produce a coating with a consistency suitable for a brush or roller. The solvent is rarely of any forensic value and will usually have long since evaporated.

The pigment gives the colour. Commonly used pigments are titanium dioxide and zinc oxide for white, phthalocyanines for blue, zinc chromate for yellow and copper (I) oxide for red.

Reading the signs

Though visual examination and jigsaw-fitting are employed, paint samples are mainly studied with a range of techniques that analyse the carrier and pigment.

As well as using microspectrophotometry to establish the colour, more in-depth techniques may also be used to establish the composition of the paint samples. Techniques include Fourier Transform Infrared Spectrometry (FT-IR), Pyrolysis Gas Chromatography (P-GC) and Scanning Electron Microscopy (SEM).

The results of such analyses can provide information about what type of surface the paint came from. With car paint, the different layers can be analysed and, through a database, vehicle details may be learnt. Make, model and year of manufacture may be ascertained in this way.

Also, comparison of samples may reveal links to a common origin. For example, a paint sample from the clothing of a hit-and-run victim will be compared with a sample taken from the suspect vehicle to see if they match.

Glass traces

Glass is one of the most common and important substances submitted to forensic science laboratories for analysis. It's frequently encountered at crime scenes, particularly those involving motor vehicle accidents, car theft and burglaries. This is due, in part, to the fact that windows are a common point of entry into buildings for burglars and large quantities of broken glass are produced in traffic accidents. Glass, fragments of which may remain on clothing for a long time, is very stable – it doesn't degrade like biological evidence and doesn't alter over time.

The role of the forensic scientist analysing glass is to unequivocally establish the origin of the sample. In practice this involves comparing fragments obtained from a suspect with samples taken from the scene. Or comparing fragments obtained from the clothing of a hit-and-run victim with samples taken from a vehicle.

How windows break

When struck, flat glass breaks in a very specific way. Glass is weaker under tension than under compression so the window will break on the opposite side to the strike. Radial fractures form initially. These run out from the point of impact on the opposite surface to the applied force. Then concentric fractures form. These run between the radial fractures but on the same side of the glass as the impact.

Fractures formed following impact onto a piece of glass
1 – Radial fracture
2 – Concentric fracture

Most of the glass is projected forward, in the direction of the blow. But some of the fragments will be projected backwards, towards the person breaking the window, up to a distance of 3 metres – known as 'backscatter'. Anyone standing close enough when the window breaks will be covered in tiny fragments of glass which will stick to their clothing and their hair.

Reading the signs

Most of the glass encountered by forensic scientists originates from windows, headlamps, bottles and containers. All this commercial glass is similar in composition and the usual methods of chemical analysis are of little help in distinguishing between them.

The simplest method used to compare pieces of broken glass is to fit them together like a jigsaw, though this is rarely possible due to the small size of samples. However, large fragments of glass that originated from the point of impact do show characteristic 'rib' (or 'heckle') marks along the edges. And these marks can be used to determine the side on which the impact occurred and so help to reconstruct the window.

Edge of broken glass showing rib or heckle marks

Patterns and marks on glass surfaces are also used in the reconstruction process. The surfaces of glass fragments can show characteristic scratch marks, revealing their origin. For example, windscreens can be scratched by windscreen wipers and side windows can be scratched by being wound up and down past abrasive particles.

From an area on a windscreen where the wiper blades cross

From an area on a windscreen over which only one of the wiper blades moves

Scratches on a side window from grit caught in the rubber

Another method of analysis is density measurement. The densities of glass fragments are measured using a technique known as the 'floatation process'. The fragment is placed in a range of liquid mixtures of known density until the fragment just floats. At this point the density of the fragment is the same as that of the liquid mixture. Hence the density of the glass is known.

But the most commonly used method of analysis is RI (refractive index) measurement. Measurement of the RI of glass samples is carried out using GRIM (Glass Refractive Index Measurement) apparatus. The fragment is immersed in silicon oil and mounted on a hot platform under a microscope. The RI of the silicon oil varies with temperature. The temperature of the platform is raised until the glass disappears. At this point the refractive indices of the glass and the oil are the same so the RI of the glass sample can be determined.

With both density and RI measurements, a sample from a suspect can be compared with a sample from the scene to see if they're from the same source. However, there's considerable overlap between the densities and RIs of different types of glass. Therefore such results don't identify the type of glass with total accuracy. Furthermore, even the RIs of glass fragments taken from the same window will vary to some degree. For these reasons, density and RI measurements can't absolutely determine the source of a glass fragment. Though if the samples match they do suggest a possible link between a suspect and the scene. But it's only when two fragments fit together that they can be said, unequivocally, to have come from the same source.

Resources

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Websites

Crime Team
Channel 4's Crime Team lifts the lid on grisly murders from the crime history books.

Forensic Science Society
www.forensic-science-society.org.uk/
The website of the Forensic Science Society including careers in the UK and web links.

Chemical Analysis
http://plus.maths.org/issue19/features/forensic/
In-depth analytical feature based on the work of John Watling, Associate Professor in Forensic Chemistry.

Forensic Handbook 1 – Marks at the Crime Scene
DNA and fingerprints are well-known sources of forensic evidence found at crime scenes. But there are other marks left by criminals. Like the footwear marks and tool marks explained here.

Forensic Detectives and Crime Fiction
The forensically-minded sleuth is a popular character these days. But in fact the crime genre has a had long history of operating on the cusp between art and science. A 150-year-old history. Read about it here.

Books

	Interpreting Evidence by GA Vignaux Looks at the presentation of forensic scientific evidence in court using real cases to illustrate what can go wrong. Transfer evidence and that given by witnesses are considered. Get this book
	Crime Scene to Court by PC White Covers the main areas of a forensic investigation from the scene of the crime to the forensic laboratory and finally to court. It details the latest methods and techniques in use. Get this book
	Crime Scene Search and Physical Evidence by Richard Fox A practical handbook setting out the scientific standards for processing evidence in solving crimes, including fingerprints and residues. Get this book
	Mammoth Book of Murder and Science by Roger Wilkes Forensic science has provided the key to some of this century's most complex and intriguing cases. This book examines cases where traditional methods of detection have failed. Looks at scientific routes of investigation and analysis that have led to the conviction of many dangerous criminals. Get this book

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