Fibres are Lost Quickly, What Does That Mean?

Fibres Deep Dive Pt 2. The persistence curve. Least understood, but most important, curve for understanding fibre evidence

Introduction

We first learned about the persistence of fibres in the brilliant work by the FSS in the 1970s. In a series of legendary papers, authors Pounds and Smalldon, laid the foundations for the principles of forensic fibre examination. Every scientific paper published since which looked at those principles has served to reinforce their original findings, that fibres transferred to a surface are lost rapidly after transfer but as the numbers drop, the rate of loss slows. It is the classic exponential decay curve:

The curve is remarkably consistent across different types of fibres, different types of recipient surfaces (clothing, shoes, skin, indoors and outdoors) and environments, although obviously the rate of loss is faster in certain scenarios.

All too frequently this information is used erroneously to determine whether or not a fibre examination is worth performing based on the single metric of time it has taken police forces to seize an item of clothing after an offence has alleged to have occurred. I have seen Standard Operating Procedures (SOPs) for strategies which make statements like this one:

Fibre examinations should not be undertaken if clothing is seized beyond 2-4 hours after the offence has occurred, owing to the likelihood that fibres have been lost.

This is a fundamental misunderstanding of the science. Although fibres can be lost rapidly, particularly in the early phases, fibre loss is predominantly the result of a physical activity on the surface where the fibres have been transferred, particularly in the later stages of the decay curve. If there is no physical activity to cause the loss, the fibres are likely to still be there.

The X-axis of the curve, is time that the clothing has been worn after transfer, not merely time since transfer. Transferred fibres do not have an expiry date where they self combust after 8 hours of being transferred. That is why a victim of homicide found 8 hours after their death will still contain fibres from the suspect on their clothing, because in spite of the elapse of 8 hours, the victim is inactive. That is why fibres from a victim’s clothing can be found on clothing seized from suspects weeks or even years after an offence has occurred, because the suspect may have hidden their clothing away moments after the crime and it remained undisturbed since, or perhaps like in this case below, the transferred fibres perhaps became lodged in a part of a garment that protected them from being lost.

Similarly the Y-axis is also misunderstood. It is a measurement of the percentage of fibres lost from a surface, meaning that after 2-4 hours of “normal wear” 80% of fibres transferred to that surface are likely to have been lost. The issue is that the real number of precisely how many fibres are transferred at “time zero” is unknowable. It might be 10, it might be 10,000. That makes a huge difference when assessing whether fibres will be detectable on the surface over time.

It’s a really difficult job predicting the outcome of a forensic fibre examination, so if you’re not going to pursue one, you should be sure that you have a very good argument not to do it. Cost is not enough. Think of the amount of money wasted in the cold case review above chasing a DNA hit that never came. Spend the money, look for the evidence. If you don’t look you will not find.

The Often Overlooked ‘Nugget’

The persistence curve tells us so much more than simply what happens to fibres after they are transferred onto something. It means that the microscopic world that make up the surfaces of our clothing is in a constant state of change. Reading the fibre populations on a garments surface at any given time, it’s like reading a diary of the life of that garment. When we tape a garment to recover fibres, we are taking a snapshot in time. Time stands still on the taping.

Think of a single day of your life in terms of the different fibres on your clothing. You wake up in the morning, grab some clean clothes from the wardrobe and get dressed. At that point on the surface of your clothing are fibres that our likely to have come from your home environment, predominantly from the other items of clothing which were laundered with the clothes you’ve just put on. There’ll be fibres from your partner’s clothing and your kids clothing, fibres from soft furnishings in your home and many, many small populations of fibres from unknown sources that have been brought into your home from elsewhere.

Putting on your clothes you’ll be transferring fibres from you nightwear and bedding to the clothing, socks you put on may have fibres from the bedroom carpet transferred to the soles, these will transfer to the inside of any shoes you put on.

By the time you’ve had breakfast new fibre populations from your kitchen will have worked their way onto your clothing and many fibres will already have been lost. Fibres from your clothing will transfer onto the seat you’ve taken to have your cornflakes, some of these fibres will later transfer onto your partner or your kids when they have their breakfast later.

Getting in the car to work sees another change in fibre populations as fibres from your clothing and all the other fibres that you have recently picked up transfer into the vehicle and vice versa.

At this point as a human being you exist within your own unique environment. No-one else in the world, apart from the people you live with, share all of the fibre populations on their clothing in common with you. Not your neighbour even though they may live only metres away from you. Not your close family who may have visited you at your home a few days previously. The microscopic populations of fibres present on your clothing when you leave the house are entirely a function of the individual choices you and your family have made and the individual fibres you and they have picked up and brought back into that environment.

By the time you and your partner get to work, and the kids have got to school, the fibre populations have changed so much on the clothing of you and your family, that except for only the most numerous and persistent of fibre populations, the populations of fibres on your clothing and your family’s clothing are now completely different.

As we step outside of our home environment, perhaps making contact with a stranger during the commission of a crime, whether we are a perpetrator of a crime or a victim, there’s a massive change on the surface of a garment. Not only are the fibres from the clothing the two parties are wearing transferred from one to another and vice-versa, but their whole unique jungle of everything else is transferred too. It’s an interaction between two unique microscopic worlds and it’s the job of a forensic scientist to try and make sense of it all. No-one should expect that will be anything other than an incredibly challenging thing to do and no-one should underestimate the power of the information contained within that single exchange.

When we return home from work and school, we bring back into the home populations of fibres we have picked up. They get redistributed around our clothing and our family’s clothing as they get washed and stored together. The cycle begins again for a new day. This is why its incredibly difficult for fibre traces to be lost. The fibres don’t spontaneously combust, there’s always a pathway to identifying likely sources or environments where garments were stored. No criminal will be able to get rid of all of them, they don’t know where they are and they can’t see them.

This is what makes textile fibres powerful forensic evidence and it’s utter madness that we are not using them as a tool. Paul Kirk said it best.

Only human failure to find it, study and understand it, can diminish its value.