The Identification of Fibres

JULIUS GRANT

Cltemical Co~iszdtant, Hehner and Cox Ltd., 107 Fe~tchztrclt Street, London E.C.3, England

This $aper zwas fircsentrd at the foz~rth Symposium of the Society ON Saturday, 28th October, 1961

This subject is such a wide one that treatment of it in this paper must be very restricted both in breadth and depth. In general, therefore, only fibres whid are used for commercial or domestic purposes will be considered ; thus, natural hairs as they occur on the human person or on animals will be excluded.

On this basis fibres may be classified into the following categories :-

1 . Animal (particularly wool and silk) textiles, but also other animal hairs in less common use, such as mohair and cashmere.

2. Vegetable fibres used in textiles ; the many fibres used in paper making (wood, straw, bamboo, jute, linen, hemp, etc.) and the very important category of true wood fibres comprising sawdust.

3. Mineral. Asbestos and glass are the only fibres of practical importance, the former being the only naturally occurring mineral fibre.

4. Man-Made Fibres. This category comprises the very wide range of synthetic or man-made fibres.

Methods of fibre identification technique will be discussed with special reference to the above categories, with examples of forensic interest drawn from experience.

I t is assumed in the present paper that the samples have already been taken and have been received in the laboratory. The technique of selecting or collecting fibre samples for forensic purposes is a study in itself. Many (in fact the majority) of the forensic applications of fibre identification involve matching or comparison rather than actual identification as such. A very common and obvious example is the comparison of fibres found on the person or belonging to an accused with those found at the scene of the crime. In sucli cases it is always a great advantage to be able to identify all the fibres present, but there is no necessity always to do so in order to prove that the two exhibits had originally the same origin. A particular instance of this nature familiar to forensic science laboratories is the sawdust filling used for safes which is scattered when the safe is opened by explosives and which inevitably finds its way into tlle crevices of clothing and footwear of those present. Sawdust can be and is usually a homogeneous mixture of woods of different species ; and if one or more of tlle constituents present is an uncommon wood species then the evidence can be very convincing. Evidence of this nature frequently plays an important part in securing convictions (and occasionally acquittals) in bank robbery cases.

As a matter of routine when making the preliminary sorting tests one should preferably use filtered ultra-violet light as well as visible light illumination. The former does not necessarily offer any advantages, and indeed on occasion little more of diagnostic interest is to be seen than in visible light. On the other hand however, some startling and very useful differentiations can be made,

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especially when a number of constituent fibres otherwise very similar is present. In the latter case of course the non-fibrous constituents can be very important contributors to the identification of the exhibit by the comparison method. Examples of a forensic nature are textiles containing starch as dressing or dyed with fluorescent dye-stuffs or optical whiteners ; and certain types of common paper-making fibres, such as unbleached sulphite coniferous wood pulp, which has a strong and characteristic violet fluorescence.

An interesting case of this nature arose when it was alleged that the fibres found on the clothes of a man arrested on a charge of burglary were jute fibres identical with those from a sack in which the the "instruments of the pro- fession" had been carried. On examination it was found that the fibres were in fact jute, but that their fluorescence in ultra-violet light was quite unlike that of the jute fibres from the sack. I t was however, very similar to those from a rug which had been given to the accused as bedclothes to soften his overnight accommodation at the police station after his arrest.

There are three principal methods for the identification of fibres and fibre remains, namely : 1, Microscopical ; 2, Chemical ; 3, Anatomical. They will be dealt with in this order.

Microscopical Methods

Of these the microscopical method and its numerous variations in technique is the most generally applicable, and it can often supply all the information that is required for the identification of a fibre. Frequently however, it must be supplemented by the other methods, particularly chemical tests. This applies especially to man-made fibres which seldom have microscopical features of great diagnostic value. Two other very weighty reasons for the importance of microscopical methods in this work are firstly that they can be applied to extremely small exhibits ; and secondly that the test is as a rule non-destructive as compared, for example, with chemical tests. This latter point is of special significance in connection with documents, from which the necessary specimen of fibres can easily be removed without affecting the value of the exhibit either as evidence or intrinsically.

The preliminary examination should be carried out under low-power lens without mounting the exhibit or treating it in any way apart from teasing the fibres apart. The proportions and types of individual fibres should be ascertained, and some attempt should be made to isolate the various types as species. In the case of fabrics the warp and weft must be noted and taken apart ; the composition of each if it is a single fibre and particularly if it is a blend, is important. In general a preliminary sorting can be made in this way from a general knowledge of the microscopical appearance of the fibres, such as the scales on wool, the twist of cotton, the acicular appearance of asbestos, the bordered pits and ducts of coniferous wood, and the relatively featureless structure of most man-made fibres. I t is the sub-division of these broad cate- gories which is more difficult and which requires special experience and technique.

The microscopical identification of fibres is indeed largely a matter of ex- perience and without this the best of apparatus and fibre atlases are of limited value. A "fibrary" (that is, a library of authentic fibre specimens) is however, of considerable help. Thus comparisons between authentic specimens of certain fibres and the exhibit can help identification to a far greater extent in cases of doubt than comparison with illustrations or drawings. Even so, the methods must be used with circumspection. The majority of fibres encountered in forensic work have undergone some manufacturing process and this can often alter their characteristic features. Thus mercerisation and the preparation of fibres for paper-making can remove the characteristic twist from natural cotton fibres.

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The following are some aids commonly used in microscopical technique. Staining is of course well known. The stains are chosen according to the subject, but a good universal stain which may be used for sorting purposes is Herzberg's stain, which is made by mixing a saturated solution of zinc chloride with a solution containing equal weights of iodine and potassium iodide. This stain gives a dark blue colour with starch, yellow with lignified fibres, red with cotton or linen textile fibres, and purple with chemically-treated vegetable fibres; mineral fibres are unaffected. If the use of the stain is preceded by an alkaline hyposulphite bleach it can be used to differentiate between viscose or cupram- monium (blue) and acetate rayon (yellow) fibres.

After the preliminary examination, and separation of individual fibres the many operations of identification follow. Paper fibres may be teased out by wetting a corner of the document and removing them with mounted needles on to a microscope slide. This may be done without damage to the exhibit. Also one can count on the sample being representative of the whole document because paper, though essentially heterogeneous in physical composition, always has the same degree of random distribution of fibres throughout the length and breadth of the sheet.

Man-made fibres present special problems, but their identification is becoming more and more important because of their increasing frequency of occurrence and their increasingly great variety. In general they are very similar micro- scopically in their uniformity of width and absence of characteristic diagnostic structural features. The only natural fibre they resemble to any degree in this respect is natural silk. In their original form they are of course continuous, but when cut into staple fibre this feature is lost, except that the pieces are fairly uniform in length and they have two cut ends which are usually easy to recognise.

The shapes of their cross-sections are often more valuable as diagnostic features but these should be assessed cautiously because the cross-section depends on the spinning technique and the pattern of the spinning jet, and this can vary considerably from one manufacture to another. However they usually show some symmetrical shape. For this reason the proof of identity by a comparison test with a preparation of known origin is all the more valuable in such cases.

A number of special microscope accessories exist to facilitate the above type of work. In most cases they are fairly well-known, and they will therefore be referred to here only very briefly.

The comparison microscope is an extremely useful instrument because it enables two specimens to be viewed side-by-side, and so compared detail for detail. The exhibits are mounted under separate microscopes and a prismatic optical device brings the two fields into juxtaposition in a single eyepiece. The uses of polarised light, phase-contrast and dark-ground illumination to bring out structural details are now also well-known. Section-cutting can also yield results of value, but in general it is of rather less importance in connection with fibres than with many other types of microscopical technique. Indeed, unless the results are interpreted expertly they can be definitely misleading. This arises partly from the fact that the conventional method of section-cutting, in which the specimen is embedded in wax or a suitable plastic medium before it is cut, affects the internal structure of the fibre ; and partly because with synthetic fibres, as already pointed out, the cross-sections can be more characteristic of the method of spinning used, than of the origin of the fibre.

Surface illumination is another aid to microscopical technique, but it is of more use for the examination of materials made up of fibres than for the fibres themselves ; examples are of course, fabrics and paper. A projector is also very useful because the fibres can be enlarged and thrown on to a sheet of paper and

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drawn, and tlieir dimensions ~neasured. Also two preparations may be thrown on to tlie same screen in adjacent positions as an aid to identification by com- parison.

Some interesting results have also been obtained with electron micrograpliy although unfortunately tliis technique is be)-ond the facilities of most labora- tories. Magnifications of 2000X or thereabouts do lio\vever, bring out character- istics of fibres \vliicli cannot be seen by other methods and which often have important diagnostic value. Unfortunately the method has its disadvantages, one of which is the damage caused to the fibres by the electron beams. This shows itself principally by tlic production of gases which distend the fibre walls.

<. 1 o a great extent this difficulty has been overcome by making life-size solid reproductions of the fibres in metal. This is achieved by taking a mould or matrix of tlle dry fibre prepared in cellulose acetate which has been softened with acetone. On evaporation of tlle acetone the mould hardens, and it then may be coated with a layer of silver about 0.001 mm. thick by evaporation of the metal on to it in a high vacuum. This thin silver layer serves as a base on which to build up the solid metal replica by the electrodeposition of copper. The plastic matrices used above have also proved useful in visible light microscopical work because they eliminate tlie interference which normally results from the scattering of light a t liigll magnifications by structural features above or below the plane of focus. This does not occur with a plastic reproduction to the same extent because of its llornogeneous structure.

A further refinement of this technique is to produce a coating on the surface of the matrix obliquely and a t a fixed angle, by evaporation of a metal in a high vacuum. This "metal sliado~v casting" tecl~nique as it is called results in a preferential coating of tlie raised features of tlle structure, since one side of these features intercepts the metal much as snow builds up on the north side of a mountain range. Tllis effect produces an enhanced contrast between the metal-coated portions and tlie non-coated portions. I t will be appreciated that this technique is highly spccialised but it has great advantages in certain prob- lems.

I t is often necessary to measure dimensions as an aid to the identification of the fibres present. Unfortunately, although fibres of one kind do not vary very greatly in width, tlieir lengths will depend on the nature and extent of the processing they have undergone. Consequently in such cases only fibres which are obviously whole sliould be measured, and the average of a large number of measurements sllould be uscd. In the siinplest method a slide on which the fibres are mounted is projected on to a screen so that the fibres are enlarged to a known extent ; if curved they may then be measured with a map mileage indicator. The degree of enlargement may be ascertained by projecting a millimetre-scale, rulecl on a microscope slide, on to the screen under the same conditions, and measuring its new dimensions with a ruler. This method has been perfected within recent years by means of a recorder which enables over 1,000 fibres to be counted per hour. Measurements of this order are essential if the results are to be assessed statistically.

If the microscope has a mechanical stage and a cross-wire in the eye-piece, the former may be moved so that the "cross" traverses the length or breadth of the fibre in question. The distance moved is read on the scale of the mechanical stage. Obviously, fibres which are nearly straight and parallel to the side of the slide are preferable for tliis purpose, ant1 a dissecting needle may be used to straighten them out.

The following method is particularly useful. The sample (1 gram) is disinte- grated by shaking with 100 ~ n l . of water and dilute alkali in the usual way. The resulting fibres are then stained by adding 10 ml. of 2 per cent. Congo red solution. They are then removed on a glass filter and washed. They are then dispersed in 1 litre of water, and a 100 ml. aliquot is diluted to 1 litre successively

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until a 0.001 per cent. suspension results. Of this 100 ml., well mixed, are poured on to a filter paper in a Buchner funnel and sucked through so that the fibres are well distributed individually over the surface of the paper. They may then be measured under the microscope by reflected light, sample fields being taken and the results analysed statistically. In an improvement on this method, the fibres are transferred by contact from the moist paper to the half-dry gelatine layer of a fixed and washed photographic plate. The fibres may then be enlarged by projection and measured in the usual way. A semi-automatic recorder enables 600 fibres to be measured and classified into length groups in 30 minutes.

A classical example of the use of fibre identification is the work of Carter and Pollard on the detection of forgeries of paper. They found esparto grass (first used in paper manufacture in about 1861) in the paper of a supposed first edition of a copy of Tennyson's "Morte dJArthur" dated 1842.

This pioneer work has subsequently been developed very effectively by paying due attention not only to the nature of the fibres present, but also to the methods by which they have been treated. I t is thus possible (with the aid of other indications) to date many papers made since about 1800 with a fair degree of accuracy. This is because from 1800 onwards a large number of different fibres have been used in paper for the first time, whereas prior to that date paper was made entirely from rag pulp. Since the dates on which these new raw materials were used for the first time are well-known, their presence in the paper indicates that the paper must have been made subsequent to that date. By an extension of the method to features and constituents other than fibres (e.g., coatings, sizing) its range of time has been extended considerably.

Chemical Methods

In general these methods require relatively larger amounts of sample than do microscopical methods, and they are of course destructive. Man-made fibres are relatively pure chemical substances and therefore they usually have the distinctive chemical reactions which may be used to identify them. The most useful single test of this nature is the burning test, and by working on a micro-scale under a low-power binocular microscope the method can be applied to single fibres. Tlie same applies to other chemical tests most of which depend on solubility and swelling characteristics. These are easily demonstrated on a microscope slide. Glass and asbestos fibres are stable towards chemical tests, including heating if this is carried out carefully ; the presence or absence of nitrogen and/or chlorine is also a useful sorting test but one with which it is difficult to obtain reliable results on single fibres. On the other hand the Schiff or Jorissen test for formaldehyde can be made very sensitive on a small scale, and can be used to detect an amino-formaldehyde finish. An interesting case of this nature arose in connection with a piece of dress fabric which had a fishy odour. There was a suggestion that this was an indication of its origin, and for this reason it was of forensic importance. However, the odour was shown to be due to the decomposition of the amino-formaldehyde surface finish.

Separation of the constituents of mixtures prior to identification is difficult and a t times impossible. Organic fibres may be removed by gentle ashing leaving mineral fibres, but they nevertheless leave some residue as well. Flotation methods may be used in some cases ; thus polythene is noteworthy among textile fibres as being the only one which floats in water. Polytetra- fluoroethylene (Teflon) on the other hand is the densest of these fibres (ex- cluding asbestos and glass). Selective dissolution will remove certain fibres ; thus nylon dissolves in a cold solution of calcium chloride in 90% formic acid. These however, are destructive methods of separation. Where relatively large samples are available the density gradient method may be used. The density of the fibre is in fact a valuable physical diagnostic characteristic, although not

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a chemical method. However, like many of the others it must be interpreted with care because the presence of certain finishes on fibres can give misleading results. This applies particularly to silicones and aluminium compounds used for water-proofing purposes and these therefore, must be removed before the test is carried out.

The method used is based on the well-known principle that if a fibre is placed in a number of liquids of different densities then the one in which it neither floats nor sinks has the same density of the fibre itself. Suitable liquids for most fibres are dried xylene (specific gravity, 0.9) and dried pentachloroethane (specific gravity, 1.7). If these are mixed in various proportions, a series of density gradients between the two above figures may be obtained.

A convenient technique is to fill carefully a 500-ml. glass-stoppered cylinder with the solutions by means of a pipette, so that a series of layers of liquid of gradually changing density is obtained. In this event the actual test should made without undue delay, otherwise difiusion between the solutions of different densities will take place. The fibres are best tested in short lengths and they should first be boiled successively in a small test-tube with pentachloroethane and xylene separately. This also serves to remove any air. The fibres are then dropped into the tube and when they have come to rest, the mixture in which they are suspended has the same density as the fibres. Some typical fibre den- sities are shown in the table below, and this serves to indicate the range of the method.

Approximate Fibre Specific Gravity

Polyethylene . . . . . . . . . . . . . . . . . . . . . . . . 0.9 Nylon . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Orlon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Wool . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Cellulose acetate . . . . . . . . . . . . . . . . . . . . 1.3 Silk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Dacron . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Viscose . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Cotton . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6 Polytetrafluoroethylene (Teflon) .... 2.3 Glass . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Asbestos . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5

Anatomical Methods These are applied chiefly to plant fibres and in particular to woods, and they

are of undeniable forensic importance because of the frequency with which they occur as evidence in such cases. Safe fillings have already been mentioned ; there are also wood particles removed from doors and window frames during entries, and so on. The value of wood identification is however, very limited when the wood concerned is one in common use, such as the coniferous soft- woods or the common British hardwoods. Where however mixtures of woods are involved the results can be more convincing. Since wood is such an ex- tremely complex material anatomically as well as chemically this identification can be correspondingly difficult. Anatomical methods are the more useful but they may be supplemented in certain cases by chemical determinations or tests for starches, etc.

If the sample is large enough a cross-section should first be prepared and this is cut in a sledge microtome to a thickness of about 1 5 ~ , and stained. Sections are cut in transverse, tangential longitudinal and radial longitudinal directions. A preferable stain for routine work is iron haematoxylin followed by safranine, and a low-power microscope having a large flat field is the most useful instru- ment.

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The following features of special importance are the pores, rays, fibres and resin ducts, and the size, position and shape should be noted in each case.

If sufficient data can be accumulated the punched card system of the Forest Products Research Laboratory, Princes Risborough, England, can be utilised. For each species a rectangular card is provided with holes punched around the four edges ; each hole corresponds with a special characteristic of the wood, such as size of rays, pores, etc. The holesin each card which describe the character- istics of the species named on the card are clipped so that the hole joins up with the edge of the card. All the cards in the set of about 500, each card repre- senting a different species, are assembled in a pack so that all the holes in all the cards in the same position are in register. The top card is punched to corres- pond with the characteristics found in the sample. A needle is then inserted through any one of the punched holes in the top card so that it passes through each hole in this position in every card, and the cards which have this hole punched to the edge can then be shaken free of the pack. The cards so separated are again re-treated in the same way using a different punched hole, and so on until by process of elimination, all but one or a few cards are left, from which the final choice can be made. I t will be appreciated that this method cannot be applied unless the sample is sufficiently large to provide most of the infor- mation required by the punched card, and this is quite considerable.

However there are usually some supplementary diagnostic features which may be used as a confirmation. These include the spiral thickenings, which are absent from spruce and present in larch, and also the pits in the cell walls connecting adjacent cells. These differ markedly as between the hardwoods and certain conifers. Crystal cells, distended vessels and storage cells are also worthy of note in special cases. An important chemical test is the Maule re- action which differentiates hardwoods and softwoods because the methoxyl contents of the lignin that they contain are 20 to 22% and 14 to 16%, respec- tively. The sample is treated successively with potassium permanganate solution, hydrochloric acid and ammonia. Hardwoods give a purple colour ; softwoods a brown shade.