Expert Witness (15 page)

Just out of interest, the sleepy drunk lady was found guilty. The breath readings she provided at the police station were among some of the highest I've ever encountered: 1640. The legal limit is 400. She was no more than 152 centimetres tall (about 5 foot) and weighed 38 kilograms (6 stone). The smallest ones are always the biggest worry.

O
nce upon a time, an expensive charter vessel was moored at a wharf. Berthed close by was a scruffy trawler. The charter vessel's captain noticed that the topsides of his beautiful vessel, including the instrument mast (which, for those of a non-boating inclination, is the highest point of the vessel above sea level) were speckled with tiny brown dots of material that weren't supposed to be there. The paintwork was ruined and she needed a repaint. Who was going to pay for it? Aha! It should be the owner of the scruffy trawler because some maintenance contractors had been using an angle grinder and the swarf (the fine metallic shavings that shower like a firework when someone uses an electric cutting tool on metal) must have scattered over the nice white paintwork of the charter vessel.

Thus followed a lengthy and expensive battle about the cause of the damage — was it angle grinder swarf or was it some thing else?

Enter the forensic science team. I spent several chilly hours on board the charter vessel, togged up in a white, blouson scene suit but not enough in the way of thermal undergarments. To stop further damage to the boat's surfaces and also so that I didn't slip around on said surfaces, I had no shoes or socks on my feet, which didn't endear me to the occasion. By the time I got back in my car, my hands were an attractive mottled bluey orange and I was frozen to the core.

We collected specimens of the brown material and took them to the laboratory for analysis. To determine the chemistry and structure of the material, standard analytical techniques were used including scanning electron microscopy and X-ray diffraction. It turned out that the brown specks were at least in part associated with what was probably ash from the active, pluming White Island volcano in the Bay of Plenty; it later transpired the charter vessel had been fishing in the vicinity of the volcano at a time before the brown specks had been noticed.

It wasn't just the chemical analysis of trace material that solved the problem for the court. The fact that the brown flecks were found high up on the instrument mast meant that the angle grinder swarf would have to have been carried high and wide on the wind, some thing the court thought unlikely given the wind and tide conditions on the day in question. The court also considered it was very unlikely that the trawler's maintenance contractors would have been at work at the time when the weather conditions were right for distribution of the angle grinder swarf over the charter vessel's surfaces: 4 p.m. on a Friday afternoon. The court took the view that it was unlikely the maintenance contractors
would list workaholism among their attributes.

This is a potted, limited summary of an enormously involved case that took several years, court dates and many hundreds of thousands of dollars to resolve. It's also a good example of why casework context (tides, wind speeds, working habits of cash-paid contractors) is so important in resolving issues.

Traces of all sorts of things are left behind at crime scenes. Clothes leave traces of themselves as fibres and some times as patterns. People leave traces of themselves as skin cells, semen, spots of blood, saliva, fingernails, ear wax and other body material. Broken windows leave traces of themselves on people's clothing and shoes as tiny fragments of glass. Cars leave traces of themselves as paint flakes or fragments of bumpers or window glass on other cars, people or items such as lampposts, trees and bicycles. Drugs leave traces of themselves as well, albeit so small they're often invisible to the naked eye. More unusual types of trace material might include soil, pollen, ash, nail polish, lipstick, oil, seed pods, sand, and as we've just seen, volcanic ash or angle grinder swarf. Basically, it includes anything that can be left behind and then later collected for analysis. The skill in applying findings from trace material often comes not just from the collecting of the sample, but also recognising what has been collected and how to get the most information from those collected samples.

If traces of material can be collected and analysed then they can be used in all kinds of casework including insurance,
criminal and civil. Traces of items can be used in a number of ways: to provide associations or links between items, places and people; to provide information on circumstances using some form of reconstruction of events; or perhaps as part of an investigative approach such as determining possible provenances of samples of unknown origin.

Glass fragments are very common in casework, because they turn up in such a wide range of situations — car crashes, burglaries, car break-ins, fights and other assaults or murders involving broken bottles; the list goes on. Have a look at how much broken glass is lying around on a daily basis — near curbs, at road intersections, on main streets outside pubs and clubs, on waste ground, on beaches, in parks.

When we consider glass fragments in casework, it's not just about the circumstances of a case. Other things about the history of glass fragments have to be taken into account:

Types of glass

There's a lot of information available about different sorts of glass, what chemical compounds are added to them, how glass is manufactured, shaped and heat-treated and its various uses. Some countries manufacture glass but, as far as I'm aware, all of New Zealand's sheet glass is imported. Not all glass is created equal and there are different types: vessel glass (pint glasses, juice glasses, wine glasses and so on), domestic window glass, industrial glass (such as metal mesh reinforced glass), vehicle glass (windscreens are made of two layers of glass with a polymer layer sandwiched between them; side and rear window glass is toughened so it breaks into cubes but it's not layered) — the list is long and varied.

What happens when glass breaks

When a window or sheet of glass is broken by impact with another hard object (as opposed to just cracking through application of pressure), the majority of the glass fragments travel in the same direction as the projectile. If a brick is thrown from outside a window into a house, the brick and most of the glass shards travel into the house. However, there is also backward fragmentation of tiny glass fragments, many of which are microscopic in size. If the brick thrower is close to the window when they throw the brick then they'll be showered with tiny splinters of glass. The best forensic evidence is obtained where significant quantities of recently broken glass are found in hair combings or on the surfaces of clothing.

Persistence

Once the glass fragments have landed on our brick thrower, we have to consider how long the fragments will remain on their clothes or in their hair. People wearing woolly jumpers and jeans will retain these fragments for a relatively long period of time, particularly if they saunter away from the broken window, as opposed to running away as fast as their legs will carry them.

How much glass might be on a random member of the general population?

In order to be able to interpret scientific findings relating to glass fragments we have to under stand how much glass might be found on a member of the general public. Otherwise, how will we know whether or not finding 10 glass fragments on someone is unusual? Studies have been undertaken to consider
this, which also take into account where the brick thrower lives. For example, in 1970s and 1980s Northern Ireland, any member of the general public would have had more glass fragments on their clothes than someone from central New Zealand, just because of the number of bomb blasts occurring at that time.

Accidental contamination

As with any other traces, the possibility of accidental contamination should always be considered. This involves assessing how the items were seized by the police, what happened to them after they were seized and how they were examined at the laboratory.

Once we have all this information, the forensic scientist is in a position to start making sense of the circumstances of a case and what is most likely to have happened. Here are a couple of examples.

A shop window was kicked in on a Saturday night by someone walking past on the way home from the pub. Mr Spud was arrested. He said the window was already broken when he came along and all he did was go into the shop through the broken window, have a look about and come out again. He's probably broken the law in some way but is he guilty of damaging property?

The glass fragments found on his clothing confirmed he had been in the immediate vicinity of breaking or broken glass, which probably originated from the broken shop window. Did he break the window?

The glass results alone will not be able to answer this question. Glass fragments can't tell us whether or not he broke the glass or whether the fragments on his clothes got there when he walked through the freshly broken window and brushed against the glass, which would have been coated with tiny fragments. However, his downfall in this case was the fact that he was the only one seen on CCTV in the immediate vicinity at about the right time. Combining the glass results with the CCTV results was good enough for the court to convict.

In another example, sometime between 7.45 p.m. and 10.45 p.m. the side windows of two different vehicles were smashed with a brick (the first opportunity for glass fragments to be transferred onto the perpetrator) in the car park of a public house. The perpetrator then leaned into the vehicles (a Mini and a Toyota) and removed items from them (the second opportunity for glass fragments to become attached to the clothing of the perpetrator).

Mr Sheep was arrested at midnight, approximately one to four hours after the windows were smashed. Hair combings were collected and a T-shirt seized from Mr Sheep 50 minutes later. Glass samples were collected from the two damaged vehicles.

No glass was found in his hair combings; however, four fragments of fresh-looking broken glass were recovered from his T-shirt. Two of the fragments matched the broken glass from the Mini, one fragment matched the broken glass from the Toyota; the final fragment of glass originated from another source. The scientific findings provided strong support for the view that Mr Sheep had been in contact with breaking or
broken glass from the broken windows of both the Mini and the Toyota.

Mr Sheep denied having broken into the two cars but said that he had spent the evening in the pub with the person who
had
broken into them. The question I was asked was whether the glass found on Mr Sheep's T-shirt could have been transferred to him from the perpetrator via a mechanism called secondary transfer. This would mean glass was transferred from the broken car window via an intermediate surface, such as the clothing of the perpetrator, onto another surface, in this case Mr Sheep's T-shirt.

In Mr Sheep's case, all four of the recovered fragments were small, being less than 0.5 millimetres in maximum dimension. The cloth of the T-shirt was reasonably good at retaining glass fragments. Given that Mr Sheep was arrested between one and four hours after the incidents occurred, he would have had to acquire the three matching glass fragments in that time period.

It was also likely that in order for three matching fragments to be recovered from his T-shirt, the number of fragments originally transferred via secondary contamination (i.e. contact with the ‘contaminated' surface) would have been more than three as some will have been lost through general activity, the nature of the contact Mr Sheep had with the perpetrator and also the type of clothing being worn by that individual.

Overall, I agreed with the prosecution's expert in this case; his scientific findings had been fairly and accurately reported. Most importantly, he was correct in his assertion that Mr Sheep had probably been in contact with breaking or broken
glass that originated from the Mini and the Toyota. What the prosecution's expert's statement didn't say was how the glass got onto Mr Sheep's clothing or that Mr Sheep must have been the one who broke the windows. The reason it didn't say that is because it's not possible to make that distinction on the basis of the scientific findings alone.