Natural animal textile fibres: structure, characteristics and identification

3 Natural animal textile fibres: structure, characteristics and identification S R TRIDICO, Australian Federal Police, Australia

Abstract: The use of natural animal fibres in textile materials began before recorded history. Animal fibres of the most significant economic value in the textile market today are those made from wool, mohair, Angora rabbit, cashmere, camel, alpaca and cultivated silk. Natural fibres sourced from the pelage of animals exhibit a variety of morphological features which may be used to identify the particular family the hair originated from, which contrasts to the processes involved in the identification of silk. This chapter details the growth, structure and properties of animal fibres which affords each animal fibre type different and unique properties enabling industry to manufacture a plethora of textiles destined for a variety of end-uses. Key words: natural animal textile fibres, growth, structure, composition and properties of animal hairs and silk fibres, morphology of animal hairs and silk fibres, identification of animal hairs and silk fibres, silk production.

3.1

Introduction

The use of natural fibres of animal origin for textile materials began before recorded history. Textile fibres can be classified into two main categories, natural and man-made (see Fig. 3.1). Wool is generally accepted by the textile industry as a term referring to animal fibres originating from sheep; accordingly, this convention is used in the remainder of the chapter. Fibres from wool, mohair, angora, cashmere, camel and alpaca have the most significant economic value in the textile market today. The other significant animal fibre is cultivated silk originating from the silkworm Bombyx mori (B.mori). Accordingly, the following sections will predominantly focus on these seven fibre types. Section 3.2 details the growth, structure and properties of animal fibres. All mammalian hairs grow from follicles embedded in the skin, in contrast to the silks which are extruded from silk moth larvae. The chemical composition of all animal hairs is the same; they are made from the protein keratin, whereas silk fibres consist of the protein fibroin. This difference affords each animal fibre type different and unique properties which result in a plethora of textiles destined for a variety of end-uses. 27

28

Identification of textile fibers Fibre Natural

Animal (protein fibres)

Plant

Mineral

Man-made

Regenerated

Synthetic

Wool sheep Hair

Goat family (Bovidae) (mohair, cashmere)

Camel family (Camelidae) (camel, alpaca, vicuna) Other fur-bearing animals, in particular the rabbit family (Leporidae) (angora) Silk

3.1 Classification of textile fibres.

Sections 3.3 and 3.4 detail the different types of hairs found on the pelage of animals, the types of silk fibres produced and the range of morphological characteristics exhibited by these fibres. Section 3.5 focuses on the manner in which animal hairs may be identified as originating from a particular animal family or species and the contrast to the processes involved in the identification of silk. Sections 3.6 and 3.7 recommend sources of further information and future trends. The most significant future trend is the production of OptimTM fibres created from stretched wool which produces a thinner more luxuriant fibre akin to silk. The impact this fibre will have regarding identification is that the OptimTM fibres microscopically look like silk fibres and as a result may be erroneously identified.

3.2

Animal fibre growth, structure, composition and properties

3.2.1 Hair growth All mammalian hair fibres are of similar structure, chemistry and physical behaviour differing only in fine detail between the species; as Wildman stated,1 ‘it will help the reader to an understanding of the unique structure of animal fibres, their reactions to reagents, and the principles employed in

Natural animal textile fibres

29

their identification if he follows. . . . how they are developed in the skin. . .’ If a piece of skin was to be sectioned at right angles to the skin surface to produce a thin vertical section, a microscopic examination would reveal that the skin consists of two main portions; an ‘underskin’ or dermis and overlying this a thinner ‘outerskin’ or epidermis. These two major components of the skin will retain their separate identities throughout the growth of the animal. When a hair fibre is going to develop, a series of changes begins in the skin which results in the formation of a little plug or follicle. Hair follicles develop in utero as a downgrowth or invagination of the epidermis into the dermis and it is from the bottom of this structure that a new hair fibre starts its growth. The hair follicle is a dynamic organ in which division, differentiation and migration of cells occur in the various tissues of which it is composed. The mature hair fibre contains at least two cell types, the surface layer or cuticle, consisting of flattened overlapping cells, whose free margins point towards the tip of the hair fibre, and the main central cortex, or inner ‘body’, made up of spindle-shaped cortical cells. The hair may also possess a third and central structure consisting of an open meshwork of condensed cells called a medulla (or air space). The main cellular features and processes of a mature, growing hair follicle are illustrated in Fig. 3.2. As the cells of the immature forming hair fibre are pushed upward from the base of the follicle, their central nuclear bodies become reduced in size; whilst this is happening there is deposited in the cell a material which is an intermediate product in the formation of the protein keratin of the mature hair fibre. This keratinisation, or hardening, process of follicular structures proceeds faster and lower down, particularly in the cells forming the outermost layer of the inner root sheath, than it does in the more central layers which form the main ‘body’ of the hair fibre. The presence of this comparatively rigid inner root structure around the soft young hair fibre cells and the direction of growth are important factors in determining the shape of the hair fibre when it later emerges from the skin. Attached to the hair follicle is the arrector pili muscle which upon contraction causes the hair to ‘stand on end’, and to some follicles one or more sebaceous glands. The wax or sebum produced from the sebaceous gland facilitates the movement of the growing hair fibre as it pushes its way through the cells in the follicle and ultimately, through the horny dermis. Thus, the mammalian hair fibre is the product of a delicately adjusted living organism and results from a series of intricate growth structures genetically designed to produce a variety of hairs of finite lengths. The type of mammal and its hereditary or genetic constitution can have an effect on the appearance of these three main parts of the hair fibre which will ultimately dictate the end-use of that fibre.

30

Identification of textile fibers

Epidermis Dermis Mature hair

Arrector pili muscle Sebaceous gland

Medulla

Zone of hardening of hair fibre Disulphide bonding, resorption and dehydration

Inner root sheath Cuticle Cortex Cortical cells Outer root sheath Dermal sheath Basement membrane Follicle bulb Dermal papilla

Keratin gene expression

Cell proliferation and differentiation

3.2 Schematic diagram of a hair follicle showing the various features and major areas of cell proliferation and keratinisation (courtesy of Thomson publishers2).

3.2.2 Silk production Silk is an animal fibre but instead of being grown from a follicle embedded beneath the skin in the form of hair, it is produced by insects during the construction of their webs, cocoons or climbing ropes. Two types of silk fibres are utilised in the textile industry: cultivated silk (produced through the process of sericulture), from the mulberry silkworm B. mori, the mainstay of the silk industry comprising 95% of the world’s silk production; and wild or tussah (tasar) silk mainly produced by various species of the silkworm of the Antheraea genus which live in the wild, feeding mainly on oak leaves. As a result of their varied diets B. mori produces silk filaments which are usually white in colour and highly lustrous in contrast to the tussah silkworms which produce silk filaments ranging green or tan in color and lack the high lustre of the cultivated silk.

Natural animal textile fibres

31

Irrespective of the type of genus of silkworm used to produce cultivated or wild silk, the mode in which the animal produces the fibre is the same. The silkworm larva possesses a pair of modified salivary glands (sericiteries) which produce a clear, viscous, proteinaceous fluid that is extruded through openings or spinnerets on its mouthparts. As this fluid is exposed to the air it hardens; this hardened silk filament is then used by the larva to wrap the fibre around itself in the form of a cocoon.

3.2.3 Animal fibre structure and composition The composition of all the animal fibres are the same in that they are made up amino acid chains joined together through condensation to form the polymeric molecule protein. However, the protein constituting wool and hair fibres (keratin) varies enormously to the protein comprising the silk (fibroin). The major difference being that keratin fibre proteins are highly cross-linked by disulphide bonds, whereas the secreted silk fibroin fibres tend to have no cross-links and a more limited array of less complex amino acids. Irrespective of this significance difference in their composition and structure all fibres of animal origin are all fibres of character; each one exhibiting unique properties which ensures it a position of special significance as a textile fibre. All animal fibres consist exclusively of proteins and, with the exception of silk, constitute the fur or hair of animals. Proteins are nitrogen-containing substances which are essentially chain-like molecules formed by the union of α-amino acids joined together by peptide linkages which retain one terminal amino (NH+) group and one terminal carboxylic acid (CO−) group resulting in the elimination of water (condensation). Although amino acids may have other formulae, those in proteins invariably have a general formula as illustrated in Fig. 3.3. Each amino acid consists of a single carbon atom to which is attached a carboxyl function (–COOH), an amino function (–NH2), a hydrogen atom and a side-chain (R) which defines each particular amino acid and its chemical character. Over 20 amino acids with different side groups (R) are known; the difference between proteins arises from the differences between these side groups attached to the main chain illustrated in Fig. 3.4.

H H2NCCOOH R

3.3 Amino acid structure.

32

Identification of textile fibers O CHCNH R

n

3.4 Protein structure.

Two major classes of natural protein fibres exist and include keratin, found in hair and fur, and fibroin secreted (insect) fibres. In general, the keratin fibres are proteins highly cross-linked by disulfide bonds from the cystine, (–CH2SSCH2–), residues in the protein chain which comprises some 10–15% of wool fibres. Although the exotic fibres alpaca, cashmere, Mohair, angora and camel are chemically similar in their composition to wool, their cystine content in their protein chains differ, which can be up to 24% in some fibres. The keratin fibres tend to have helical, intermittent helical sections within the protein sequence and are extremely complex in structure with the inclusion of a cortical cell matrix surrounded by a cuticle sheath laid on the surface as overlapping scales. The cell matrix, or cortex, of some hair fibres may contain a central cavity or medulla. The keratin fibres tend to be round in cross-section with an irregular crimp along the longitudinal fibre axis. Raw silk, whether cultivated or wild, contains about 75% fibre and 25% of a globular protein called sericin. The sericin is usually left on the silk filaments to protect them from mechanical damage during processing. The silk yarn or fabric is degummed to remove the sericin, resulting in a silk fibre which is essentially pure fibroin. The protein fibroin, as illustrated in Table 3.1, has a markedly different amino acid composition from that of keratin. Fibroin, unlike the keratin fibres, has a more limited array of less complex amino acids; glycine (H–) and alanine (CH3–) constitute in total some 60% of the amino acids comprising this protein. Literature citations regarding glycine, alanine, tyrosine and serine as the major amino acids comprising fibroin accord with the notable of exception of the amino acid cystine. Needles3 and Cook4 report that fibroin lacks the amino acid cystine whereas Kushal and Murugesh5 report that fibroin contains negligible amounts of this amino acid. Thus, with the virtual absence of cross-links and with limited bulky side chains present in the amino acids, fibroin molecules align themselves parallel to each other and hydrogen bond to form a highly crystalline and oriented ‘pleated-sheet’ or ‘beta’ structure. Because of its high cost, silk finds a limited use in textiles; a minor amount of wild or tussah silk is produced for specialty items. The silk fibres comprising the ‘wild’ silks differ from those of cultivated silk in colour and texture; however, the wild silk and

Natural animal textile fibres

33

Table 3.1 The content of the α-amino acid side-groups (R) in wool and silk protein fibres (grams of amino acid per 100 g protein). Data from ‘Textile Fibres, Dyes, Finishes and Process – A Concise Guide’3 with the exception of * value which is taken from ‘Studies on Indian Silk’5 α-Amino acid

Wool keratin

Silk fibroin (cultivated)

Inert Glycine Alanine Valine Leucine Isoleucine Phenylalanine

5–7 3–5 5–6 7–9 3–5 3–5

36–43 29–36 2–4 0–1 0–1 1–2

Acidic Aspartic acid Glutamic acid

6–8 12–17

1–3 1–2

Basic Lysine Arginine Histidine

0–2 8–11 2–4

0–1 0–2 0–1

Hydroxyl Serine Threonine Tyrosine

7–10 6–7 4–7

13–17 1–2 10–13

Miscellaneous Proline Cystine Methionine Tryptophan

5–9 10–15 0–1 1–3

0–1 0.00 / 0.13* 0.0 0–1

cultivated silk fibres are sufficiently similar in chemical composition and structure as to be considered as homogeneous fibre types.

3.2.4 Physical and chemical properties of protein fibres Hair fibres are related to wool in their chemical structure; they all comprise keratin. But they all differ from wool, and from each other, in their physical (and morphological) characteristics; they are of different length and fineness and have different shapes and internal structures. With silk fibres, on the other hand, despite several species of silkworms being used in their production, the construction of their cocoons are sufficiently alike for the silk to be regarded as a fairly homogeneous material. The configuration and orientation of the individual molecular chains within each protein fibre, in conjunction with its the overall shape, will affect the fibre properties. In protein fibres, like other natural fibres, the

34

Identification of textile fibers

orientation of the molecules within the fibre is determined by the biological source during the growth or production, and the maturity process of the fibre. According to Needles3 there are several essential ‘primary’ properties that any polymeric material must possess in order to produce a fibre adequate enough for its intended final product. These properties are fibre length to width ratio, fibre uniformity, fibre strength and flexibility, fibre extension and elasiticity and fibre cohesiveness. However, the polymeric material should also exhibit additional characteristics in order to increase their desirability and value in its intended end uses. Such properties include moisture absorption characteristics, fibre resilience, density, lustre and chemical resistance. Man-made fibres are specifically manufactured in order to meet these essential critera; however, nature ensures that the protein fibres it produces are ‘ready made’ to fulfil these requirements. In relation to fibre lengths, hairs are grown to genetically determined finite lengths and as such, they are regarded as staple fibres and treated as such in the production of wool products; silk fibres, however, are extruded as extremely long continuous filaments and therefore regarded as a filamentous fibre in the textile industry. Protein fibres are generally fibres of moderate strength, resilience and elasticity and at moderate humidities do not build up significant static charge. Wool, like other hair fibres, contains a substantial amount of the amino acid cystine. Cystine residues play a very important role in the stabilisation of the fibre structure due to the cross-linking action of their disulphide bonds, which holds the polymer chains together not only in wool, but also in other animal fibres. The disulphide bonds are responsible for relatively good wet strength of wool. Wool is resistant to attack by acids but is readily attacked by weak bases even at low dilutions and is irreversibly damaged and coloured by dilute oxidising bleaches such as hypochlorite. Reducing agents will cause reductive severance of the disulphide bonds within the wool, evetually causing it to dissolve. However, this property is exploited in the textle industry as under controlled conditions, reducing agents can be used to partially reduce the wool and flat set or set permanent pleats in the wool. Wool, unless chemically treated, is susceptible to attack by several species of moths which are able to to dissolve and digest wool fibres. However, it is reasonably resistant to attack by other biological agents such as mildew. Wool fibres have excellent resiliency and recovery rate from deformation except under high humidity, it is insoluble in all solvents with the exception of those capable of breaking the disulphide cross-links. Wool is a good heat insulator due to its low heat conductivity and bulkiness, which permits the air to become trapped in the fibres comprising the textile constructions.

Natural animal textile fibres

35

Wool and other hair fibres are unique amongst natural fibres for their possession of overlapping cuticular scales present on the outermost surface of the fibre, their position akin to overlapping tiles on a roof. This characteristic scaly outer surface is vital in the process of felting; a process which is unique to wool and other animal fibres. Felting is the consolidation of these fibrous materials by the application of heat, moisture and mechanical action, causing the scales on the wool and hair fibres to interlock and mat together. The fabric shrinks and undergoes characteristic changes in its structure. The fabric becomes thicker and the fibres are matted into closely packed masses. The outline and character of the yarn pattern in the fibre becomes indistinct and the fabric loses much of its elasticity; the surface of the fabric is covered by fibres, and its appearance is altered. The outer scales on the wool or hair fibre are aligned such that their edges point towards the tip end of the fibre. Felting appears to most amenable with wool or hair fibres which bear prominent cuticular scales, probably due to the fact that the felting treatment tends to bend the scale edges and fibres into loops, which during the mechanical action with repeated fabric compression, causes the loops and scales to ‘travel’ and become interlocked and entangled; unlike bonded fabrics, felts do not require adhesive for their production. Felted fabrics are used in the hat industry, apparel and drapery, in industry for insulation, packing and polishing materials and felt padding is used in apparel and furniture. Cashmere fibres are almost identical to wool in relation to their chemical composition; however, owing to fineness and better wetting properties, cashmere fibres are more susceptible to chemical damage, especially with respect to alkalis. Silk, due to the virtual absence of cystine, is not as resistant to acids as wool, but is more resistant to alkalis. Silk is very resistant to organic solvents but soluble in hydrogen bond breaking solvents such as cupammonium hydroxide. Unlike other natural fibres silk is more resistant to biological attack. Strong oxidising agents such as hypochlorite will cause silk to rapidly discolour and dissolve, whereas reducing agents have negligible effect except under extreme conditions. Silk fibres are relatively stiff and show good to excellent resiliency and recovery from deformation depending on the temperature and humidity conditions. These fibres exhibit favourable heat-insulating properties but owing to their moderate electrical resistivity, tend to build up static charge.

3.3

Types of natural animal fibres

3.3.1 Mammalian hairs The pelage, or coat, of animals comprises various hair types which are illustrated in Fig. 3.5. Close examination of the pelage will reveal that some

36

Identification of textile fibers

(a)

(b)

(c)

3.5 Examples of hair types which may be found on the pelage of animals (a) over hair, (b) guard hair and (c) under hairs (courtesy of Dr Hans Brunner).

sparsely distributed hairs are distinctly longer than the hairs comprising the bulk of the coat; these longer hairs are called overhairs. The larger or coarser hairs forming the bulk of the pelage are termed guard hairs. These hairs generally exhibit a variety of sizes in one pelage, ranging from the coarse and long to those that cannot be distinguished from the underhairs. The guard hairs may be of uniform diameter along the hair shaft, tapering to a tip. However, some guard hairs are specialised into a type described as shield hairs. In shield hairs the distal (tip region) of the hair is noticeably wider and flattened, forming a shield. The underhairs are shorter and much finer than the overhairs and guard hairs, these hairs are usually found close to the body and serve to insulate the animal. In general, underhairs are wavy and retain a uniform diameter along the length of the hair with the exception of the tip which tapers to a point. The classification of hair types, as outlined above, predominantly relies upon the appreciation of the profile or general outline of the hair, e.g. straight or wavy. Some examples of the types of profiles which may be seen on the pelage of animals are illustrated in Fig. 3.6.

Natural animal textile fibres

(a)

(b)

37

(c) (d)

3.6 Examples of hair profiles (a and b) guard hairs (c) under hair, (d) magnified view of a constriction in an under hair (courtesy of Dr Hans Brunner).

Carpets Bedding Upholstery Blankets Woollen fabrics Worsted fabrics 17–24

25–27

28–30

31–33

34–40

41+

Fibre diameter in microns

3.7 Wool fibres and their uses in the textile industry based on their diameters (modified from FAO Agricultural Bulletin 1227).

The coarseness or fineness of the animal hairs determines their end use in the textile industry and as such animal hair fibres need to be graded according to their fibre diameter. For example, the major end uses of wool are apparel products, bedding and carpets. Figure 3.7 illustrates how the fibre diameters determine the end product, with the coarser fibres being used

38

Identification of textile fibers

for carpets and the finer fibres being used for apparel fabrics which need to be softer against the skin. In general wool fibres coarser than 21 microns in diameter cannot normally be processed into yarns destined to produce lighter, softer fabrics that are both functional as well as aesthetically pleasing.

3.3.2 Cultivated and wild silk fibres Silkworm is a common name for the silk-producing larvae several species of moths; however, the mulberry silkworm B. mori is the most common moth used in the commercial production of silk. B. mori feeds exclusively on the leaves of the mulberry tree and has flourished only where conditions are suitable for large numbers of leaf-bearing mulberry trees. However, this moth has been cultivated over many centuries and is no longer found in the wild, and today is totally dependent on humans for its existence and as such the silk it produces is known as cultivated silk. Unlike silk produced from silkworms living in the wild, cultivated silk is harvested from a cocoon as a continuous silk filament approximately 1000 m in length. This is achieved by killing the pupa prior to its emergence as a moth during which the pupa secretes an alkali which dissolves the cocoon threads thereby ruining the silk. Silk is a continuous filament around each cocoon and is freed by softening the cocoon in water, locating the free end and harvesting the silk thread. Wild or tussah silk, on the other hand, is produced by silkworms which live in an environment free to feed on a variety of leaves and complete their life cycles, with the pupa contained in the cocoon being allowed to live and emerge as a moth. As a consequence of this ‘wild’ existence the integrity of the single silk filament produced by some species of larvae to build its cocoon is broken, resulting in silk which consists of numerous strands. This silk is generally coarser than the cultivated silk because the wild silk consists of numerous strands, rather than a single seamless one, and is also variously coloured due to the uncontrolled diet of the insects.

3.4

Natural animal fibre characteristics

3.4.1 Animal hairs Morphological characteristics exhibited by animal hairs are usually examined using the optical microscope, which is excellent for examining the interior of the hair shaft; however, the exterior of the hair, i.e. the cuticle, is best examined using a scanning electron microscope (SEM) as the resolution of the image is far higher than that attainable with the optical microscope, resulting in a much more detailed image. The use of these two

Natural animal textile fibres

39

Scale

Cuticle Medulla Cortex

3.8 Diagrammatic representation of the major structural components which may be exhibited by hairs (courtesy of Dr Hans Brunner).

microscopes in the examination of animal hair is detailed in the identification section of the chapter and unless otherwise stated, any references to microscopy will be in relation to the use of the optical microscope. As seen in preceding sections, animal hair fibres have a unique structure consisting of the outermost scale cuticle, an inner cortex and, in some hair fibres a central medulla, as diagrammatically illustrated in Fig. 3.8. All mammalian hairs bear morphological characteristics typical to the family of the particular species. These morphological characteristics may be seen on the outside of the hair shaft as cuticular scale patterns, inside the cortex as medullary patterns or at the root end which, in some animals, bears a characteristic shape; however, for the majority of animal fibres used in the textile industry the root will be absent, with exception of the coarse kemp fibres, which may be found in some fabrics; these fibres are medullated and exhibit a brush-like root. The works of Wildman1 and Brunner and Coman6 are considered as seminal works and standard references in relation to the idenification of animal hairs based on their classification of morphological features and characteristics on the basis of their microscopical appearances. Wildman, and Brunner and Coman classified the hair characteristics on the basis of their cuticular scale patterns and medullae; Brunner and Coman further classified hair characteristics on the basis of their cross-sectional shapes. As illustrated in Fig. 3.8 the cuticle or outer layer of the hair shaft comprises a single layer of overlapping cells, arranged like tiles on a roof, with the free edges pointing towards the tip. Unlike human hairs, animal hairs exhibit a variety of cuticular scale arrangements to form distinct patterns. In relation to the cuticular scale patterns exhibited by animal hairs, Brunner and

40

Identification of textile fibers Form of scale margins

(a) Smooth

(b) Crenate

(c) Rippled (d) Scalloped (e) Dentate

Distance between scale margins

(f) Distant

(g) Near

(h) Close

Scale patterns

(i) Simple coronal

(o) Regular wave

(j) Diamond petal

(p) Irregular wave

(k) Narrow diamond petal

(l) Broad petal

(m) Regular mosaic

(n) Flattened irregular mosaic

(q) Single chevron

(r) Double chevron

(s) Streaked

(t) Transitional

3.9 Cuticular scale patterns as classified by Brunner and Coman (courtesy of Dr Hans Brunner).

Coman divided the classification into three criteria, each based on the following main features: terms which describe the form of the scale margin, terms which describe the distance between the external scale margins and terms which are descriptive of the general scale patterns as illustrated in Fig. 3.9. As depicted in Figs 3.10 and 3.11 Wildman similarly classified the cuticular scale patterns with the exception that the dentate scale margin, coronal

Natural animal textile fibres

Mosaic (regular)

Interrupted regular wave

Single chevron (a form of regular wave)

Mosaic (irregular)

Mosaic (irregular wave)

Double chevron

41

Simple regular wave

Wave (medium depth)

Streaked wave (a variety of interrupted wave)

3.10 Cuticular scale patterns as classified by Wildman (courtesy of BTTG Ltd).

and transitional scale patterns are not represented. Owing to the differences in the terminologies of these two main works it is recommended to use one reference or the other when identifying animal textile fibres in order that consistency of terms and descriptors is maintained and to provide the source used in the identification process.

42

Identification of textile fibers

Irregular petal (a form of interrupted irregular wave)

Lanceolate (a form of fine pectinate and also of regular wave)

Coarse pectinate (a form of regular wave)

Diamond petal

Narrow diamond petal

3.11 Cuticular scale patterns as classified by Wildman (courtesy of BTTG Ltd).

Brunner and Coman defined four major structural groups of animal hair medullae: unbroken, broken, ladder and miscellaneous. They further subdivided each of these groups into a total of 12 distinct types as depicted in Fig. 3.12. The top half of each type details the structure seen in a hair in which the medulla is filled with air; the lower half illustrates animal hairs which have been treated in order to facilitate the observation of the medulla. The medulla consists of shrunken cells, the spaces between these shrunken cells are usually filled with air. Under the microscope these appear as obvious black, opaque structures which can obscure the structure or pattern of the medulla. If these hairs are treated in such a way as to allow mounting medium to enter the cortex and infiltrate these air spaces, viewing the medulla shape and form is facilitated. Wildman, on the other hand, classified the medulla types into the following four broad categories: (a) unbroken (wide) lattice, (b) and (c) simple

Natural animal textile fibres

(a) Narrow medulla lattice

(g)

(b)

(c)

Wide medulla lattice

Narrow aeriform lattice

(h)

Fragmental

Uniserial ladder

(i) Multiserial ladder

(d) Wide aeriform lattice

43

(e)

(f)

Simple

Interrupted

(j)

(k)

(l)

Globular

Stellate

Intruding

3.12 Medulla types as classified by Brunner and Coman (courtesy of Dr Hans Brunner).

unbroken, (d) interrupted and (e) fragmental as illustrated in Fig. 3.13. The following medulla types by Brunner and Coman are illustrated in Fig. 3.12: narrow medulla lattice, uniserial ladder, multiserial ladder and globular, stellate and intruding being absent. The latter three comprise the ‘miscellaneous’ major structural group being found in animals such as seals, wombats and platypus. The different classifications of medulla types and scale patterns may reflect the different aims of each of the works. Wildman’s work details morphological characteristics of animal fibres of importance in the textile industry; whereas Brunner and Coman deal with the morphological characteristics of a variety of mammalian hairs for use by animal ecologists and in the examination of animal hairs found as contaminants in food. Brunner and Coman noted that in addition to the differences in medullae types and cuticular scale patterns exhibited by animal hairs, significant

44

Identification of textile fibers

(a)

(b)

(c)

(d)

(e)

3.13 Medulla types as classified by Wildman (courtesy of BTTG Ltd).

differences are also apparent in the cross-sectional shapes of animal hairs. In Fig. 3.14, Brunner and Coman depict the most common cross-sectional shapes enountered in animal hairs, the dark, central features representing the medulla.

3.4.2 Silk fibres Silk, although produced by an animal and a protein-based fibre, is not generally regarded as a true animal fibre since it comprises fibroin and not keratin, nor does it grow from a follicle embedded beneath the skin but is extruded from modified salivary glands from a larva. As such it does not bear any of the morphological characteristics exhibited by the true keratin animal fibres. Under the microscope silk has the appearance of a glass-like filament of uniform diameter which may bear striations along its length. In the raw state the colours of the fibres may reveal if the fibre has been produced as cultivated silk or as the product of wild silk; cultivated silk, once degummed, has a high natural lustre and sheen white in colour. Wild silks vary in colours such as, but limited to white, cream, green, brown, and amber. The variety of colours is attributable to the variety of leaves consumed by the various wild silk moth species.

3.5

Identification of natural animal fibres

3.5.1 Keratin-based animal fibres All animal hairs bear morphological characteristics and features which not only allow differentiation between them but also their identification to a species or family level. Animal hairs used in the textile industry, despite

Natural animal textile fibres

Circular medium size medulla

45

Circular large medulla

Oval large medulla

Oval medium size medulla

Oval medulla absent

Eye-shaped

Oblong large medulla

Oblong medium size medulla

Cigar-shaped

Concavo-convex divided medulla

Concavo-convex bilobed medulla

Concavo-convex large medulla

Reniform

Dumb-bell shaped

3.14 Cross-sectional shapes as classified by Brunner and Coman (courtesy of Dr Hans Brunner).

being processed and dyed, may retain sufficient characteristics to determine the possible source. The identification of animal hair fibres begins with the determination that the hair is animal not human in origin. This is generally easy to achieve and Table 3.2 details the morphological characteristics human and animal hairs show, which enable their differentiation. Clearly, in animal hairs used to manufacture textile products, the banding characteristic may not be commonly seen. Once a hair has been identified as animal in origin, a further more detailed macroscopic examination is performed to determine a number of

46

Identification of textile fibers

Table 3.2 Characteristics which may be used to differentiate between human and animal hairs Feature

Human hair

Animal hair

Colour

Relatively consistent along hair shaft

Medulla

Less than 1/3 of the width of the hair shaft Amorphous, mostly not continuous when present

Pigment distribution Cuticular scales

Even, slightly more towards the cuticle Imbricate, similar along the shaft from root (proximal end) to tip (distal end) Usually bulbous (club shaped) and indistinct

Often showing abrupt, profound colour changes known as banding Usually greater than 1/3 of the width of the hair shaft Continuous, often varying in appearance along the shaft, defined structure Central or denser towards the medulla A variety of patterns often showing variation in structure from root to tip Variety of shapes and forms, usually distinct

Root

features such as the profile of the hair, e.g. the length and appearance of the hair, i.e. straight or wavy, and determine whether the hair is likely to be a guard hair or an underhair. A preliminary examination of the animal fibre with a stereomicroscope (up to 100× magnification) may reveal the width and gross morphology of the medulla characteristic to a particular hair type and possible animal or origin, e.g. a hair with a brush-like root and an obvious wide unbroken medulla would strongly indicate the presence of a kemp fibre. In general the largest of the guard hairs (primary guard hairs) are of paramount importance in the identification of animal hairs, for it is these hairs which generally exhibit the most diagnostically useful characteristics. The underhairs are generally of little diagnostic value in determining the identification of animal fibres. The identification of animal hair fibres predominantly relies upon the morphological features present inside the cortex, such as the medulla and on the outside of the hair from the cuticular scale patterns, as the size and shape of these scales and their pattern of arrangement around the hair are useful criteria for identification purposes. In relation to observing the cuticular scale patterns present on animal fibres for identification purposes, a number of methods can be employed, each with their own merits and limitations. The scale patterns may be visualised by mounting the hairs in a semipermanent mounting medium with a refractive index (RI) lower than that of keratin (RI 1.55) which facilitates the viewing of the external scale patterns but slightly masks the internal features. Figure 3.15 illustrates the efficacy of three semi-permanent mounting media each with varying RIs.

Natural animal textile fibres

(a)

(b)

47

(c)

3.15 Photomicrographs of the same woollen fibre mounted in cedar oil RI 1.513 (a), liquid paraffin RI 1.470 (b) and glycerine and water RI 1.403 (c) (courtesy of BTTG Ltd).

The hair will need to be dried and/or cleaned of the semi-permanent medium following the examination. Scale casts may also be made by coating a cover slip with a medium such as clear nail polish, laying the hair in the polish, once the polish is dry, the hair is removed, the cover slip inverted and placed on a microscope slide; cuticle scale cast, embedded in the nail polish, can be viewed under a compound microscope. This method does not require the hair to be dried or cleaned and the cast is a permanent record. Examination of the external features of animal hair fibres may be achieved with the use of a scanning electron microscope (SEM). The SEM uses secondary electrons to ‘view’ the surface characteristics of a fibre, the surface of which is usually coated with a thin layer of gold to assist the speedy display of the scanned surface. The major advantage of the SEM, over the optical microscope, is its very high resolution (down to 2 nanometres) and the relatively large depth of field. This enables the complete surface of a fibre to be seen in high detail thus enabling a better discrimination of the

48

Identification of textile fibers

surface characteristics. However, unlike the optical microscope, no internal features are visible and due to the thin gold coating, the hair cannot be examined further. If the textile of origin is unknown, determining the width or diameter of the animal fibre may assist in determining its textile provenance, e.g. a coarse wool fibre greater than 40 microns may indicate that it originated from carpet; or if a garment is a blend of wool and cashmere, determining the diameters of the fibres may assist in their differentiation and identification. Although Brunner and Coman regard the cross-sectional shape of a hair as ‘undoubtedly the most single important criterion used in . . . hair identification’ it is a destructive technique and as such should be used with caution. Wildman does not illustrate cross-sectional shapes of animal hairs; he does, however, discuss their value and use for animal hair identification and for studies on the micro-structure of the hairs. Seta8 makes the following comments in relation to the use of cross-sectioning of hairs regarding their identification: The variable shape of the shaft gives a clue to the identification of species. . . . Many hair examiners have adopted the cross-sectional shape for characterising hairs. . . . For this examination some investigators used the longitudinal mount without preparing a cross section. . . . This may be justifiable from the following points: 1. The consumption of the . . . hair would be minimal 2. The cross-sectional shape is variable from hair to hair and from point to point on the same hair. 3. The cross-sectional shape does not have as much validity as has been thought. 4. The production of suitable cross-sections depends completely on the experience and ability of the examiner.

The preceding sections detail a plethora of medullae types and the cuticular scale patterns which may be seen on the animal fibre. The medulla types and scale patterns not only vary from hairs of different species but these characteristics may vary along the length of the same hair fibre and between hairs comprising the pelage of the one animal. The widest point of the hair fibre provides the most diagnostic medulla type. Tables 3.3 and 3.4 illustrate the most significant, general features and characteristics present in fine and coarse animal fibres for identification purposes. Wool, like all other fibres of animal origin, consists of a cuticle of scales, a cortex and in some instances a medulla. Very fine fibres, for example those produced by merino sheep, have no detectable medullae but consist of cuticle and cortex as depicted in Fig. 3.16. The images of representative types of wool, illustrated in Figs 3.17 and 3.18 show that there is a variation in thickness not only between the fibres, i.e. inter-species variation but also

None Circular to elliptical 15–24 μm

Medulla Cross-section

Simple broken Circular to oval

Irregular waved mosaic

Mohair

Uniserial ladder Oval to rectangular

Single or double chevron

Angora rabbit Waved mosaic near to distant scale margins Not observed Circular to oval

Camel Coronal, distant scale margins smooth Non medullated Circular to oval

Cashmere

Varies Almost circular

Irregular waved mosaic

Alpaca

Fibre diameter

Cross-section

Medulla

Cuticular scale pattern

30–36 μm

Up to 40+ μm

Fragmental or unbroken to wide unbroken lattice Circular to oval

Irregular mosaic and simple waved pattern

Mohair (Angora goat)

Regular mosaic and irregular mosaic; smooth, near margins Wide lattice Simple unbroken narrow or fragmented narrow Round to elliptical

Wool

Dumb-bell ovoid TBA

Wide unbroken multi-serial ladder

Double chevron

Angora rabbit

Up to 120 μm

Circular to oval

Irregular waved mosaic; near and smooth margins Simple unbroken (fine lattice)

Camel

mfd 80–86 μm

Simple broken or unbroken medulla (medium diameter) Circular to oval

Irregular waved mosaic; near margins

Cashmere goat

40–60 μm

Varies

Irregular waved mosaic; smooth; near margins Varies

Alpaca

Table 3.4 Morphological features present on coarse wool, mohair, angora rabbit, camel, cashmere and alpaca animal fibres

Fibre diameter

Simple, coronal

Cuticular scale pattern

Wool

Table 3.3 Morphological features present on the finer wool, mohair, angora rabbit, camel, cashmere and alpaca animal fibres

50

Identification of textile fibers

3.16 Photographs showing the scale structure of fine woollen fibres (courtesy of BTTG Ltd).

3.17 Photographs showing the structure of coarse woollen fibres (courtesy of BTTG Ltd).

3.18 Photographs showing the structure of coarse woollen fibres (courtesy of BTTG Ltd).

Natural animal textile fibres

(a)

51

(b)

3.19 Diagrammatic representations illustrating the extremes of uniformity and irregularity of woollen fibre diameters. (a) Crosssectional shapes of fine, high quality woollen fibres; (b) crosssectional shapes of the coarser, lower quality woollen fibres (courtesy of BTTG Ltd).

along the length of each fibre. The inter-species variation is mainly due to genetic or inherited characteristics, i.e. the intra-species variation is generally due to nutritional feasts and famines during the period of fibre growth. The lower quality and coarser wool fibres tend to become medullated with the wide lattice type occuring in the very coarse fibres, including the kemps; this medullation is commonly seen in carpet wool fibres. The simple unbroken medium to narrow type of medulla is seen in many fibres from longwools and cross bred wools. The fragmental type of medulla often occurs in wool fibres, but it is often significantly smaller in relation to the rest of the fibre. Fibre diameters of wool fibres can vary from uniform to irregular fibre diameters, as illustrated in Fig. 3.19. The higher qualities of wool fibres exhibit a smaller mean fibre diameter but also less variation in the fibre thickness; the lower quality and coarser fibres have an increased variation in fibre diameter. The coarser wool fibres as well as being medullate, exhibit a regular mosaic-type scale pattern illustrated in Fig. 3.20, which may alternate with short lengths of irregular waved pattern; in contrast to the fine wool fibres which exhibit the same scale pattern type irrespective of the breed of sheep they originated from (see Figs 3.21 and 3.22). Alpaca is from the fleece of the alpaca Lama pacos which belongs to the llama family, so that alpaca fibres and llama fibres have many morphological features and characteristics in common which may be seen in the preceding figures. Alpaca fibres produced, range in diameter from 24–26 μm. These fine alpaca fibres bear scales which are smooth-margined

52

Identification of textile fibers

3.20 Cuticular scale patterns which may be exhibited by coarser woollen fibres, all of which are of the regular mosaic pattern with the exception of (d) which shows the regular mosaic pattern merging into an irregular waved mosaic form (courtesy of BTTG Ltd).

Natural animal textile fibres

53

3.21 Scale pattern exhibited by fine woollen fibres all of which have the same type of scale pattern (irregular-waved mosaic), with scale margins which are smooth and distant (courtesy of BTTG Ltd).

18 μm

3.22 SEM image of a scale pattern of a merino fibre (courtesy of CSIRO Textile and Fibre Technology).

54

Identification of textile fibers

25 μm

3.23 SEM image of the scale pattern of an alpaca fibre (courtesy of CSIRO Textile and Fibre Technology).

as illustrated in Fig. 3.23. The coarser fibres (fibres 50–60 μm or over) have scales which form an interrupted irregular wave pattern as illustrated in Fig. 3.24. Regarding alpaca fibre cross-sections, these animals produce a spectacular array of shapes and forms which can be seen in Figs 3.25 and 3.26 and are characteristic of not only the alpaca but also the llama. The pelage of the angora rabbit, like that of other animals, has two major fibre types, the outer guard hair and the shorter fur or underhair. In the textile industry, angora rabbit fibres may be used alone or blended with wool or nylon. The angora rabbit fibre bears a medulla which is characteristic of the lagomorph family to which it belongs (which includes ‘domestic’ rabbits and hares). The coarser angora rabbit hairs have a wide, unbroken, mulitserial ladder medulla; the finer hairs, in general, bear a uniserial ladder medulla. The cuticle shows a single or double chevron scale pattern as seen in Fig. 3.27. The dumb-bell cross-sectional shape is typical for rabbit fibres as seen in Fig. 3.28. Mohair comes from the angora goat Capra hircus aegragus; these fibres are very regular in thickness along their lengths and have smooth outlines, which cause the scale margins to be difficult to detect in profile. The outlines of mohair are, in this respect, sharply distinct from those of wool fibres with

Natural animal textile fibres

55

3.24 Scale pattern found on a coarse alpaca fibre (courtesy of BTTG Ltd).

which they may be mixed. The cuticular scale pattern of coarse mohair is illustrated in Fig. 3.29. Cashmere originates from the cashmere goat, Capra hircus laniger; the outstanding characteristic of the very fine cashmere fibres is that the majority of scale margins are distant, as illustrated in Figs 3.30 and 3.31. This characteristic of distant smooth margined scales, together with the even fibre outline and thickness makes them easily recoginisable by the experienced examiner. In contrast, very coarse cashmere fibres in the basal half of the hair exhibit irregular waved mosaic with near and crenate rippled margins as seen in Fig. 3.32. Like cashmere only the soft underhair (or underwool) or down hair of the camel Camelus bactrianas is used in the production of yarn. For the

56

Identification of textile fibers

Fibres of fine to medium thickness Outline of fibre section almost circular, outline of medulla section almost circular and relatively narrow. Type A

Medium to coarse fibres (first type) (45–50m diameter) Outline of fibre section approaching circularity, outline of medulla section almost circular and relatively narrow. Type B

Medium to coarse fibres (second type) Outline of fibre section ovoid, medulla elongated in section and in a direction along the major axis of the section. Type C

Rather coarse fibres Outline of fibre section ovoid to angular. Medulla section characteristically dumb-bell shaped in outline. This type is frequently seen in the coarser grades of brown, white and black alpaca. Type D

Coarse to very coarse fibres A coarse fibre whose fibre section outline is approximately triangular, but with two of the sides almost equal to each other in length, i.e. almost the shape of an isosceles triangle. Found in samples of white alpaca. Medulla section appoximately T-shaped. Type E

3.25 Excerpt from ‘The Microscopy of Animal Textile Fibres’1 showing the various cross-sectional shapes exhibited by alpaca fibres (courtesy of BTTG Ltd).

Natural animal textile fibres

57

3.26 Photomicrographs showing the cross-sectional shapes exhibited by alpaca fibres (courtesy of BTTG Ltd).

camel, the colour of the hairs collected or harvested range from reddish to light brown with variants from brown to grey (white hairs may occur but these are extremely rare). The camel fibres possess certain features which help in their identification. With the use of low power microscopy camel hairs, unlike wool fibres, are seen to be very regular in outline or profile and to exhibit a uniform diameter along their lengths; the cuticular scale edges project so very slightly from the hair shaft that its profile appears

58

Identification of textile fibers

3.27 Scale pattern characteristic of rabbit fibres (courtesy of Dr Hans Brunner).

3.28 Photomicrograph depicting cross-sectional shapes exhibited by rabbit fibres (courtesy of BTTG Ltd).

Natural animal textile fibres

59

3.29 Photomicrographs depicting scale patterns present on mohair fibres (courtesy of BTTG Ltd).

almost a straight line (see Figs 3.33 and 3.34). This particular feature is extremely useful in distinguishing camel fibres from wool fibres with which they may be mixed or blended. According to Wildman1 the medulla of the coarse camel hair fibre is of the unbroken type and is quite narrow in relation to the diameter of the fibre. This tendency to have a relatively narrow medulla is characteristic of the most coarse camel fibres and is a useful characteristic for identification.

3.5.2 Non-keratin silk fibres In contrast to the animal hair fibres, the identification of silk is generally easy to achieve. Degummed silk filaments are smooth-surfaced and semi transparent. Kushal and Murugesh5 found that the mulberry and nonmulberry silks exhibit an entirely different cross- and longitudinal sectional shapes and varieties which are illustrated in Figs 3.35, 3.36 and 3.37; the mulberry silks show a more or less triangular cross-section and a smooth surface, which markedly differ from the the non-mulberry (wild silk) varieties. Until recent times, cultivated silk was easily distinguished from all other fibres by its narrow diameter, but the advent of microfibres has changed this. The identification of silk must now be approached with care as nylon

60

Identification of textile fibers

3.30 Photomicrograph showing the smooth distant scale margins present on cashmere (goat) fibres (courtesy of BTTG Ltd).

Natural animal textile fibres

61

10 μm

3.31 SEM image of the scale pattern of a cashmere fibre (courtesy of CSIRO Textile and Fibre Technology).

microfibres and silk can be confused because of the similarities in their diameters and their infrared spectra. Silk, however, is normally less regular in appearance along its length than a microfibre. An easy way to view this irregularity is between crossed polars using the interference colours in the same way that one views a topographic map. The most definitive difference, if difficulties are encountered, is to place a short segment or cross-section from the fibre in question in the hot stage. Nylon will melt while silk will not. Colour is the principal point of comparison once it has been established with certainty that the fibre is silk and what type of silk it is. Fluorescence microscopy may provide additional features based on any fluorescence of the dyes.

3.6

Future trends

The most significant breakthrough in wool technology since the development of shrink-resistant technology in the 1960s is touted to be the OptimTM fibres created by the Textile and Fibre Technology Division of the Commonwealth Industrial Research Organisation (CSIRO) in Australia. Woollen fibres are stretched and then set, these ‘parent’ woollen fibres are then processed which causes them to rearrange themselves, resulting in fibres which possess a structure akin to that found in silk fibres. A wool fibre with a diameter of 19 microns will be reduced to a fibre with a diameter of

62

Identification of textile fibers

3.32 Photomicrograph showing the scale pattern on a coarse cashmere fibre (courtesy of BTTG Ltd).

15–16 microns. OptimTM fine is smoother than wools as in the stretching process become elongated along the wool fibre which gives the fibre a smoother appearance and feel; the cross-sectional also changes from round to oval found in untreated wools to triangular which resemble those of silk fibres as illustrated in Figs 3.38 and 3.39.

3.7

Sources of further information and advice

Australia and New Zealand represent the major contributors of wool used in the textile industry. The following internet sites may be accessed for further

Natural animal textile fibres

63

3.33 Photomicrographs showing the cuticular scale arrangement on a camel fibre (courtesy of BTTG Ltd).

10 μm

3.34 SEM image of a camel fibre (courtesy of CSIRO Textile and Fibre Technology).

64

Identification of textile fibers

3.35 SEM images showing the various longitudinal scale patterns exhibited by mulberry silk fibre and wild silk fibres (courtesy of Wiley Publishers).

3.36 SEM image showing cross-sectional shapes of mulberry silk fibres (courtesy of Wiley Publishers).

Natural animal textile fibres

65

3.37 SEM image showing the cross-sectional appearance of wild silk (tussah/tasar) fibres (courtesy of Wiley Publishers).

information: CSIRO at http://www.csiro.au and the Wool Research Organisation of New Zealand at http://www.woolresearch.com. There are several publications which deal with the identification of animal fibres. The publication produced by Wildman1 is a very comprehensive and significant reference guide solely in relation to animal textile fibres featuring numerous photographs depicting the various morphological characertistics exhibited by many animal hairs used in the textile industry in the 1950s and describes various methodologies employed for their examination; despite being published over 50 years ago the principles of the examination of animal textile fibres and the basis of their identification apply today. In 1978 H.M. Appleyard9 produced a ‘Guide to the Identification of Animal Fibres’ which was a concise version of ‘The Microscopy of Animal Textile Fibres’ by A.B. Wildman;8 the purpose of the publication being to act more as a laboratory manual to assist practitioners who may not require all the details given in ‘The Microscopy of Animal Textile Fibres’ in relation to laboratory techniques. The book describes the morphological features exhibited by 49 different animal species, which includes those described by Wildman.

3.8

Acknowledgements

I am indebted to the following people for their invaluable assistance: Ms Tahnee Dewhurst (VPFSC document section) for her patience in the production of images, Ms Tracey Archer (VPFSC Librarian) for her tenacity and skills in obtaining reprints, my dear friend Dr Hans Brunner for his

66

Identification of textile fibers

Merino wool

36 μm

(a)

Optim wool

36 μm

(b) 3.38 SEM images of the cross-sectional shapes of unprocessed wool fibres (a) and the processed OptimTM wool fibres (b) (courtesy of CSIRO Textile and Fibre Technology).

Natural animal textile fibres

Silk

67

18 μm

3.39 SEM image of the cross-sectional shapes of mulberry silk fibres (courtesy of CSIRO Textile and Fibre Technology).

‘carte-blanche’ approach to my republication requests. I would also like to thank Ms Heather Forward and Margaret Pate (CSIRO-Textile and Fibre Technology Division, Melbourne), Lyndon Arnold (RMIT Melbourne) and Dr Matthew Fleet (SARDI, South Australia) who were generous in devoting their time and efforts in assisting a person whom they had never met.

3.9

References

1. Wildman A B (1954), The Microscopy of Animal Textile Fibres, Leeds, Wool Industries Research Association (WIRA). 2. Harding H and Rogers G (1999), ‘Physiology and Growth of Human Hair’, in Robertson J (Ed), Forensic Examination of Hair, London, Taylor and Francis, 6. 3. Needles H L (1986), Textile Fibres, Dyes, Finishes, and Processes A Concise Guide, New Jersey, Noyes. 4. Cook J G (1984), Handbook of Textile Fibres Natural Fibres, Durham, Merrow. 5. Kushal S and Murugesh B K (2004),‘Studies on Indian Silk. I. Macrocharacterization and Analysis of Amino Acid Composition’, Journal of Applied Polymer Science, 92, 1080–1097. 6. Brunner H and Coman K (1974), The Identification of Mammalian Hairs, Melbourne, Inkata. 7. Petrie O J (1995), ‘Harvesting of textile animal fibres’. Food and Agriculture Organisation (FAO) of the United Nations, Bulletin 122. 8. Seta S, Sato H and Miyuke B (1988), ‘Forensic Hair Investigation’, Forensic Science in Progress, 2. 9. Appleyard H M (1978), Guide to the identification of Animal Fibres, Leeds, Wool Industries Research Association (WIRA).