SFR Guidance > Fibres

Guidance Notes: Fibres

Fibre transfer

When an item such as a garment, comes into contact with another surface (such as car seats, POE surfaces, other individuals or weapons) there is the potential for fibres to be transferred from the garment’s fabric to that recipient surface. The number of fibres transferred will depend upon the nature of the fabrics in question, on the type and duration of the contact and on the nature of the recipient surface. Some garments are made of fabrics that are unsuitable for consideration as a source of transferred fibres. For example, garments may shed their constituent fibres but these fibres may be commonly occurring and therefore of low evidential value (e.g. white cotton - colourless fibres lack a key feature of comparison, namely colour, and therefore are generally of little evidential value in cases such as this). Other garments may be made of fabrics that do not tend to shed their constituent fibres. In some instances however, whilst the fabrics involved may not shed their constituent fibres, damage to the surface of a garment may be significant enough to result in fibres or tufts of fibres being shed from the area of damage and thus render a garment suitable as a source of fibres for transfer. The most suitable sources of transferred fibres are those fabrics that readily shed distinctive fibres onto the surfaces with which they come into contact.

Fibre-plastic Fusion

In a collision, materials may be exchanged between the interior surfaces of a vehicle made of thermoplastics and the clothing of the occupants as result of forceful impact between them. As a result, the heat produced from friction during the forceful contact between parts of the clothing and vehicle components produced due to the forceful impact can cause localised melting of the thermoplastic vehicle components and/or synthetic clothing fabrics. During these contacts, which will last only a split second, textile fibres from the clothing can be transferred to and fixed into the temporarily softened plastic. Fibres transferred in this can be compared with the clothing of all occupants and can be attributed to a specific garment.. In addition, fibre plastic fusions can also display features that indicate the direction of the impact that produced them. Hence the outcome of a fibre-plastic fusion examination may not only assist in identifying the seating arrangement of the occupants within a vehicle at the moment of the impact, but also might assist in determiningle the trajectories of the vehicle’s occupants during the incident.

The corresponding trace to the FPFs on the interior plastic surfaces in a car are plastic coating marks on the clothing (garments, shoes) of the occupants. These traces are formed somewhat less frequently – usually in high-speed impacts – by transfer of softened thermoplastic material from the interior of the car to the clothing. They are rather valuable particularly if garments showing a low sheddability are involved (e.g. leather, filament yarn fabric) or if two occupants of a car are wearing garments made of indistinguishable fibres (e.g. blue jeans, white cotton shirts).

Two-way transfer

'Two-way transfer' is a term used to describe contact between two textile items that results in their constituent fibres being transferred in both directions between the two items. Such findings are usually only encountered when both items are seized promptly following any contact being investigated and is most prevalent when any contact between the items is prolonged, repeated or heavy. Two way transfer of fibres is generally considered to be more evidentially significant than a one way transfer.

Secondary transfer

In addition to the above direct transfer of fibres from one item to another, fibres may also be transferred, to a lesser extent, between items which do not come into direct contact with each other. “Secondary Transfer” is a term used to describe the mechanism by which fibres from one surface are transferred to another surface, via an intermediary surface. For example, fibres from a jumper are transferred onto a vehicle seat, and then transferred to another garment making subsequent contact with that seat. Fibres from jumper may therefore then be transferred secondarily to the second garment coming into contact with the seat.

Linking fibres/Fibre populations

Sometimes a direct transfer of fibres between items may not occur because, for example, the items themselves may not shed fibres. However, on occasions, it may be possible to demonstrate that fibres from an unknown source (e.g. a third item) can be found on both the clothing of the individual and located on the ‘scene’ item. These fibres are referred to as ‘linking fibres’ or fibre ‘populations’. For example, a blanket is in contact with a car seat and fibres are transferred from the blanket to the surfaces of the seat. During any subsequent contact between that surface and an individual sitting in that seat, these blanket fibres could be transferred from the seat to the individual’s clothing thus providing a link between them, but without the clothing and the blanket being in direct contact with each other.

Fibre retention

Fibres that have been transferred to another surface will be be lost over a period of time. The rate at which fibres are lost depends upon a number of factors, including the ability of the recipient surface to retain fibres and the degree of disturbance that occurs to that surface. For instance, the extent to which garments are worn or seats are used will affect fibre loss from their surfaces. Coarsely woven and knitted fabrics tend to retain transferred fibres to a greater degree than finely woven materials. As a guide, tests have shown that approximately eighty per cent of fibres transferred to a garment will be lost within three to four hours of normal wear. Fibres that have been transferred to a fixed object, such as a car seat, may persist for longer, provided that the surface is not disturbed too much.

Fibre recovery

Transferred fibres can be recovered from the surfaces of items by systematically applying strips of adhesive tape. Several strips of tape are generally used when taping larger areas to maintain the adhesive properties of the tape and preserve the effectiveness of it’s the recovery process. The tapes can then be attached to thin clear plastic sheets thus securely preserving any recovered fibres for more detailed examination. The tapes used to recover fibres in this way are often referred to as "fibre lifts", "tape lifts" or "tapings".

Fibre identification and comparison

Recovered fibres that appear superficially similar by low power microscopy to those from a particular garment are individually removed from the tapes and, after preparation, the types of fibres are identified and these fibres are then compared with the constituent fibres of the garment by high power microscopy under a range of different lighting conditions. A representative sample of the fibres that are found to be microscopically indistinguishable from the considered potential source may be selected for instrumental colour and/or chemical analysis. Instrumental colour analysis is a technique employed to examine and compare the dyes used to colour fibres. This technique called microspectrophotometry (MSP) measures the absorption of electromagnetic radiation (light) in the visible region and, where appropriate, the ultraviolet region (UV-MSP) of the electromagnetic radiation spectrum. Specific dyes will absorb differently (depending on the structure of the dye and the fibre to which it is applied) and therefore, the resulting spectra can be compared. When the spectra match, the fibres are deemed indistinguishable from each other. Instrumental chemical analysis (FTIR) is a technique whereby the infrared region of the electromagnetic spectrum is used, and is employed to analyse the molecular structure of synthetic (man-made) and some naturally occurring fibres and thus confirms the identity of the fibres.

Evidential significance

When no differences are found between recovered and constituent fibres of a considered potential source by all tests performed, the fibres are said to be ‘indistinguishable’ and can provide support for the assertion that the donor and recipient items have been in contact. In determining the degree of support that the presence of such individual fibres will provide, the Reporting Scientist will evaluate these findings by consider various factors including:

• how readily or otherwise the fabrics involved shed their constituent fibres and the proportion of each fibre ‘type’ shed from a blended fabric;
• how readily or otherwise the fabrics involved would be expected to retain transferred fibres;
• the number of transferred fibres found;
• the frequency of occurrence of the transferred fibres in the general textile fibre population;
• whether the transfer of fibres was in one direction or in both directions;

Other pertinent information (e.g. published scientific literature) when relevant will also be used in evaluating the scientific findings.