Fibres May Be Mass Produced, But They Are Not All The Same
Fibres Deep Dive Pt 3. How many garments are made of the same stuff?
Introduction
Textile manufacturing is a business. That business, like any other, exists solely to make profit for the owners. The global textile market makes billions of textiles every year in huge quantities.
Manufacturers sell textiles in a variety of ways, but generally speaking, the fibres that go into these textiles, that end up as populations of tiny microscopic fibres on our skin and clothing, begin life as a part of a large piece of fabric ready to be sold to any customer for any purpose.
But that large piece of fabric requires raw materials to make it and it is made on demand. It is a product of market conditions that are determined at the point of order. Manufacturers despise holding large stocks of stuff because it costs them money, there is no benefit to them of storing stock that does not sell quickly.
Every time a new order comes in and there is insufficient stock to fill it, manufacturers have to make or source more, and it is in that balance between supply and demand, that forensic scientists can detect tiny, subtle, often microscopic differences between orders that are tolerated by both textile manufacturers and customers alike.
Let’s say I am in charge of buying the all of the uniforms for every nurse in the National Health Service. There’s approx. 400,000 of them. Let’s imagine that all of the nurses are required to wear the exact same branded nurses uniform1.
I can predict the number of uniforms every year I need pretty accurately, but there can be unplanned spikes in demand. I have several suppliers who are all required to deliver a particular coloured uniform, made from a fabric with a particular construction of fibres which takes into account all of the needs of the NHS - from robustness for everyday wear, to particular HAZMAT requirements. I have several suppliers because I need contingencies in the event that any one supplier is unable to supply me at any given time.
So I go and make an order for 10,000 uniforms. To meet that order I have to use three suppliers because 10,000 uniforms are never held in stock anywhere at once. To meet that demand the suppliers have to make or source new stock. Each supplier may have its own individual supply chain, they have to order raw materials (fabric), have them dyed to the NHS standard colour and then the uniforms have to be constructed in the factory. The NHS, apart from a requirement stating the uniform should be made from a 60/40 polyester cotton blend and dyed to a particular shade of blue, do not specify where the polyester or cotton comes from, nor do they specify which dyes or combination of dyes are required to impart the NHS blue colour. The suppliers take advantage of this and source their raw materials as cheaply as they can, whilst meeting (hopefully) the quality standards the NHS requires. The result is this: 10,000 uniforms that all pretty much look the same, but are in fact made up of multiple batches of uniforms, some perhaps containing only a few hundred uniforms, that are all dyed slightly differently and made from different types of polyester. Forensic scientists can detect those differences.
This is what forensic fibre experts call batch-to-batch variation, or as I like to call it, the “priest socks” phenomenon.
Here’s an example from one of my cases involving uniform jumpers provided to staff of the London Underground by Transport for London. Each fabric sample was taken for each consignment ordered by the London Underground over a few years. They all look identical but could be classified as coming from three distinct batches of fabric using a combination of microscopy and micro-spectrophotometry.
So when barristers pursue questions for fibre experts quoting annual sales figures of a particular Adidas hooded top (other tops are available) in an attempt to make the jury believe that there are so many garments in the world made of the same stuff, they should be prepared that the answer may not deliver the slam dunk they think their argument deserves.
This brings me back to the comfort zone for forensic scientists - the science - and the group of studies known as “colour blocks”.
Colour Blocks
I haven’t done this deep dive into the science underpinning forensic fibre examination in chronological order, partly because that’s not the way science works. In fact were I to do all this chronologically I would have had to talk about target fibre studies in this part, and also find a place for shedding and transfer and other stuff too. It just doesn’t tell the story in the way I like to tell it.
Looking back I wish that the profession that is (was) forensic fibre examination had looked at colour blocks sooner, but as a profession performing research largely in independent groups doing their own thing, the field was left to evolve more organically without much by way of a steer. Perhaps that was a good thing.
Colour block studies were a fantastic idea and if I’ve got my history right started in the FSS in Wetherby classifying MSP spectra of red cotton fibres, unfortunately, as was the case for many a good FSS study, it didn’t make it beyond internal research reports within the FSS. Cue the brilliant Mike Grieve, lord rest him, and the team at the BKA in Germany with the first published colour block study - “The evidential value of black cotton fibres”.
Colour block studies looked at a particular type of fibre, in this case black cotton, and sampled from hundreds of different sources of the fibres and individually compared all of them, pairwise, under the microscope and by Micro-spectrophotometry (UV-VIS Spectra of the fibre). The purpose was simply to understand just how well or otherwise forensic scientists could discriminate between black cotton fibres from different sources.
What made this study particularly ground breaking was the fact that for the first time, forensic scientists could confidently identify black cotton fibres that could be worth spending the time to look for on a garment, in spite of the challenge of finding them amongst the huge numbers of other black cotton fibres that would likely be present on it. This from a paper published 24 years ago shows what it’s like to find a “good” type of black cotton:
A single black cotton fibre was trapped under a fingernail of a female found murdered in a wood in the Netherlands. The body was naked. The fibre matched those in a pair of sweatpants belonging to a suspect. The spectrum was identifiable as being that of a reactive dye and was compared to the spectra from the 225 reactively dyed black cotton sample from textiles examined in this project. One match and two similar spectra were found. It could be said on the basis of this study that the spectrum was not of a commonly occurring form, being found in less than 1.5% (a conservative estimate) of the samples examined in this project. It should not be forgotten that, nevertheless, such a textile could be produced in quantity, but this finding is far from "black cottons are all the same and therefore evidentially worthless".
The suspect admitted the offence.
Conversely and equally as important, we could also now identify when fibres from a black cotton garment from a suspect were not worth the time to look for them on a victim’s clothes. In 2001, 40% of the sources of black cotton were predominantly dyed with a type of dye that rendered it practically impossible for forensic scientists to distinguish between the different sources made from black cotton dyed with that particular dye.
BUT there remains no way for anyone except a forensic scientist to be able to determine a “good” black cotton target from a “poor” one without using a microspectrophotometer and paying for the expertise to use it, which renders any SOPs in police forces making sweeping generalisations about clothing and fibres and the likelihood of success completely pointless.
More colour blocks were to follow addressing the most abundant fibre types found on any given surface, blue cotton, red cotton, blue polyester amongst others.
The natural fibres remain the most challenging owing to the fact that any given source sheds thousands of plant or animal based fibres. Unlike synthetic fibres, natural fibres are highly variable and generally, the only manufacturing process which imparts discrimination upon them comes from the dyeing processes.
In relation to blue cotton, including indigo dyed cotton (“ blue denim” fibres to the lay person”), a similar situation emerged, some blue cotton fibres were worth looking for, some were not2, the authors made this comment, true then, true now.
The results of this and similar studies, show that the perception of non-denim blue cotton fibres having little or no evidential value is grossly flawed and appears to have been based upon a generic evaluation, rather than upon the discriminating power of the instrumentation used to compare them.
In relation to a colour block study concerning sources of blue polyester fibres, the most abundant broad type of synthetic fibre population found on tapings, the authors came to the conclusion that the likelihood of two blue polyester fibres from different sources matching is extremely low, with only three random matches identified out of 32,385 comparisons, which is about 0.009%.
Next up target fibre studies - just how likely are we to find fibres out there in the world that are indistinguishable from a single garment?
This isn’t what tends to happen in the NHS, nurses may wear different uniforms depending on their speciality and NHS Trust employer. Just a hypothetical example for context.