Time zero
Fibres Deep Dive Pt 6. Numbers of fibres transferred at the point of contact, where fibres are transferred during contact, and what happens to them next
Introduction
In casework, we will never know how many fibres were transferred onto a surface when contact occurs. The only fact we will ever establish with certainty is how many fibres we find on a surface at the point we recover them. The persistence curve 1 tells us nothing but the rate of loss over time for a surface whilst that surface is active, but it is used by scientists to predict whether a fibre examination is likely to be successful, given case circumstances provided by investigating officers which may or may not be reliable.
A significant factor in solving the case of the murder of Stephen Lawrence was scientists understanding what assumptions were made when the strategy for performing forensic examinations were first undertaken in 1993. At that time it was understood that the attack was so fast (less than 10 seconds), that any fibres transferred onto the clothing of attackers would likely to have been minimal and lost in the period between the attack and the seizure of the suspect’s clothing. For that reason clothing from the suspects were not examined for fibres from Stephen Lawrence’s clothing.
But the case was cracked really by scientists challenging those assumptions, and even though they were not equipped with proper scientific data to fully lend scientific weight to help them, they considered fibre examinations worth pursuing. In the end, finding fibres, led to finding everything else. Sometimes it shows that it doesn’t matter how good your DNA technology is, it is still reliant on scientists finding evidence to put in the tube. One of the key items in the Stephen Lawrence case was examined 6 times by 6 of the most competent, well-trained and experienced forensic scientists that England and Wales had to offer, from 4 different laboratories. None of them found blood. It was only after a fibre from Stephen Lawrence’s polo-shirt found on that item which had within microscopic traces of blood, that a 7th and final examination found a tiny, tiny speck of blood on the clothing.
Fibre Transfer During Physical Assaults
Thanks to the research teams at Northumbria Uni and UTS Sydney we are beginning to better understand time zero and the mechanisms and dynamics of fibre transfer and persistence that flow from that point. I’ve included some papers in the footnotes, but the PhDs associated with them are also well worth a read. Also Victoria’s webinar at the top of this post is worth a listen.
Both the teams at Northumbria and UTS performed similar experiments involving analysing fibre transfer through martial arts settings. I will try and do them both some justice below.
Pairs of volunteers from the Northumbria University2 Jiu Jitsu team were tasked to carry out different assault scenarios, wherein one acted as an assailant (orange suit) and other as a victim (purple suit). The assailant was tasked to adopt an aggressive, dominant, stance whilst the victim a more defensive, protective stance. Each simulation measured two parameters: duration (30 or 60 s) and intensity (low or high).
So its thousands of fibres being transferred at time zero, with a logical correlation between the intensity of the contacts and the duration.
The team also looked at where the fibres were deposited and somewhat unsurprisingly, the fibres were deposited mostly where the contacts were heaviest.
Fibre distribution maps, generated using a zonal taping approach, showed higher quantities of fibres transferred to the upper, front body, in line with expectations, as those areas received the most contact. This demonstrated in casework, inclusive of packaging of the garments prior to examination, inference with intense activity can be made from areas where large fibre quantities are found, although the inverse may not be the case with small numbers of fibres.
What about other fibre types?
The team at UTS were performing their work independently of the team at Northumbria and their findings corroborated that research, whilst adding information regarding transfer of polyester fibres and the persistence of fibres transferred post assault. At time zero, polyester fibres were found to transfer in fewer numbers than cotton (not surprisingly), but the numbers were still in the hundreds - and in the context of the source garment comprising only 20% of polyester fibres by composition 3. A reminder that textile labels tell you little about the fibres shedding from a garment.
The findings from the team at UTS in relation to where the fibres were deposited also were similar to the findings from the team at Northumbria.
UTS, however took the experiment further looking at persistence4 . This was their experimental design:
This is what they found, and we’ve seen this so many times over the last 50 years:
But it’s in this next bit that there’s a proper little additional nugget of information, and that is what appears to happen to fibres on garments as time progresses:
There is a shift towards finding shorter cotton fibres over time. One of the possible reasons for that is longer cotton fibres breaking up into smaller ones over time as they are subjected to physical forces on the surface during wear (such as friction). On that basis it does not appear that short cotton fibres that are transferred at time zero are necessarily persisting for longer, but that they are a product of longer fibres fragmenting.
This is what the team concluded:
Variations in fibre length distribution profiles were shown to be influenced by a dynamic interplay between both donor fibre and recipient textile characteristics, in addition to the balance between concurrent mechanisms of loss and fragmentation (of natural fibres including cotton). Fibres < 1.0 mm in length were more readily lost than short fibres (1.0 – 3.0 mm), contrary to previous research.
The findings suggest a clearer association between physical activity and length, rather than quantity, of recovered fibres. In practice, the evidential value of such findings may be underestimated in certain circumstances if higher length thresholds are used to guide examination.
Overall, the persistence of fibres is a complex and dynamic multifactorial phenomenon. Persistence in initial stages appear governed by characteristics of the donor fibre, with fragmentation of hoody and T-shirt cotton being a major contributor. This was markedly illustrated by increased recovery of T-shirt cotton after high intensity activity. At later timepoints, persistence is largely determined by attributes of the recipient textile.
In the empirical context of this study, the recovery of fibres within two hours of transfer may prove greater evidential value in informing about the nature of the transfer activity and subsequent actions of the wearer. Additionally in practice, examination of fibre length may prove more informative than numbers of fibres alone regarding activities of the wearer. This is particularly pertinent where cotton (and other natural fibres) and highly retentive recipient substrates are implicated.
The study clearly shows that there is something going on with the profile of the lengths of transferred target fibres found on clothing that may provide an insight into the activity of the wearer since time zero.
The issue for caseworkers attempting to squeeze information out of fibres beyond the mere linking of two surfaces together, is the harsh reality that whilst the behaviour of human beings is predictable outside of the context of a criminal assault, as soon as an assault begins, it is chaos. The chaos continues post-assault, people step outside their normal behaviours. Information surrounding their precise activity prior to seizure of clothing can be difficult to clarify in most cases. In terms of the microscopic behaviour of fibres in that chaos, every case is unique.
So if caseworkers really want to attempt to reconstruct activities by looking at numbers of fibres and/or lengths of fibres or any combination of other measurable factors, they are faced with a research project of their own that is defined by the case circumstances. This makes caseworkers uncomfortable, because of the assumptions they will have to make and the fact that they will never be sure that the information that they have is reliable. They could expend a huge amount of resources on a model that delivers an outcome they believe is robust enough to report to court, only to get to court and have key assumptions they have made rendered invalid as new information comes to light. If that happens, whilst scientists will say that was always a risk and they acted only as any other scientist would under the circumstances, the reality is that those who pay the bills will likely never commission fibre work again, because that will be the case that they remember.
I’ll come back to this later in the deep dive, when I wrap up all of these posts in one single case example. Next up, “secondary transfer” by the numbers and what it tells us about target fibre studies.