Wool is one of the most important textile fibres used in the manufacture of woven fabrics of all kinds. It belongs to the group of animal fibres of which three kinds are met with in nature, and used in the manufacture of textile fibres; two of these are derived from quadruped animals, such as the sheep, goat, etc., while the third class comprises the products of certain insects, e.g., silk.

The skin of all animals is covered with more or less of a fibrous coat, which serves as a sort of protecting coat from the weather to the skin underneath. Two different kinds of fibres are found on animals; one is a stiff kind of fibre varying in length very much and called hairy fibres, these sometimes grow to a great length. The other class of animal fibres are the woolly fibres, short, elastic and soft; they are the most esteemed for the manufacture of textile fabrics, it is only when the hairy fibres are long that they are serviceable for this particular purpose. There is a slight difference in the structure of the two kinds of fibre, woolly fibres having a more scaly structure than hairy fibres; the latter also differ in being more cylindrical in form.

Wool.--By far the most important of the animal fibres is wool, the fibre of the domestic sheep. Other animals, the llama or alpaca, the Angora and Cashmere goats also yield fibres of a similar character, which are imported under the name of wools. There are many varieties of wools Which are yielded by the various breeds of sheep, but they may be roughly divided into two kinds, according to the length of "staple," as it is called. In the long-stapled wools the fibres average from 7-1/2 to 9-1/2 inches in length, while the short-stapled wools vary from 1 to 2 inches long. The diameter varies very considerably from 0.00033 to 0.0018 of an inch.

Two varieties of thread are spun from wool, one is known as "worsted," the other as "woollen" yarns; from these yarns two kinds of cloths are woven, distinguished as worsted and woollen cloths; the former are in general not subjected to any milling or felting process, while the latter invariably are.

Physical Properties.--When seen under the microscope the wool fibres show a rod-like structure covered with broad scales, the edges of which project from the body of the fibre, and all point in one direction.

Fig. 1 shows typical wool fibres as viewed under the microscope; the sketch shows very well the scales.

The shape of the scales varies in different breeds of wool. The outer scales enclose inner medullary cells, which often contain pigment matter. A transversed section of the wool fibre shows the presence of a large number of cells. Sometimes wool fibres are occasionally met with which have a peculiar white horny appearance; these do not felt or dye well. They are known as "kempy" fibres. See figure 2. The microscope shows that they are largely devoid of structure, and are formed of very horny, impenetrable tissue, which is difficult to treat in the milling or dyeing process.

The curly or twisted character of the fibre is caused by the unequal contraction of the outer scales, and depends in a great measure upon the hygroscopic nature of the wool. It may be entirely removed for the time by wetting the wool in hot water, then drying it in a stretched condition, or the curl may be artificially induced by unequal drying, a fact which is turned to practical account in the curling of feathers and of hair.

The amount of curl in different varieties of wool is very variable, being as a rule greatest in the finer qualities, and diminishing as the fibre becomes coarser. The diameter of the wool fibre varies from 1/2000 to 1/5000 of an inch, and the number of curls from about 30 per cent. In fine wool as little as 1 or 2 per cent. in the thicker fibres.

Elasticity and strength are properties which, in common with silk, wool possesses in a greater degree than the vegetable fibres. When submitted to strain the wool fibre exhibits a remarkable strength, and when the breaking point is reached the fracture always takes place at the juncture of two rings of the outer scales, the embedded edges of the lower layer being pulled out of their seat. The scales themselves are never broken.

When first formed the cells are more or less of a spherical shape, and contain a nucleus surrounded by the ultimate photoplasmic substance. Those cells which constitute the core or central portion of the fibre retain to some extent this original globular form and pulpy condition. Surrounding this central portion or medulla, as it has been called (see fig. 3), and forming the main bulk of the fibre, there is a comparatively thick layer of partially flattened cells, which are also elongated in the direction of the length of the fibre, and outside this again there is a thinner stratum which may be compared to the bark of a tree. This outer covering differs materially from the rest of the fibre in its physical structure, but is, probably, nearly identical with it, though possibly not entirely so, in chemical composition. It consists of a series of flattened horny scales, each being probably an aggregation of many cells. The scales, which have been compared to the scales of a fish or to slates on a housetop, overlap each other, the free edges protruding more or less from the fibre, while the lower or covered edges are embedded and held in the inner layer of cells. The free edges always point away from the root of the fibre, just as do the bracts of a fir cone.

When viewing a section of a wool fibre there is, of course, no sharp line of division between the three portions above described, but the change from the central spherical cells to the elongated cellular portion, and from these again to the flattened horny scales, is quite gradual, so that the separation into zones, though well marked, is very indefinite in respect of boundaries.

The scaly structure of wool is of great importance in regard to what is known as felting property. When woollen fabrics are worked in boiling water, especially in the presence of soap, they shrink in length and breadth, but become thicker in substance, while there is a greater amalgamation of the fibres of the fabric together to form a more compact and dense cloth; this is due to the scaly structure of the wool fibres enabling them to become entangled and closely united together. In the manufacture of felt hats this is a property of very great value.

Variations in Physical Structure.--Wool fibres vary somewhat amongst themselves; fibres from different breeds of sheep, or even from different parts of the same animal, vary greatly, not only in thickness, length, etc., but also in actual structure. A typical wool fibre, such as may be obtained a good merino or Southdown fleece, will possess the typical structure described above, but frequently the type is departed from to such an extent that the central core of globular cells is entirely absent. Also the serrated character of the outermost layer of cells reaches a much higher state of development in some samples of wool than in others.

Wool is a much more hygroscopic fibre than cotton or any of the other vegetable fibres, usually it contains about 18 per cent. of water, but much depends upon the atmospheric conditions that prevail. This water is contained in the wool in two forms: (1) as water of hydration amounting to about 81 per cent., and (2) as hygroscopic water.

Experiments have shown that when a piece of dried wool is exposed to an atmosphere saturated with water vapour it will absorb 50 per cent. of its weight; cotton under the same conditions will take up 23 per cent.; flax, 27·5 per cent.; jute, 28·5 per cent., and silk, 36·5 per cent.

Heated to about 100°C. it parts with nearly the whole of its water and becomes hard, horny and brittle, exposed to the air, the dry wool again absorbs water and is restored to its former condition. When heated to 100°C. wool becomes somewhat plastic, so that whatever form is then imparted to it it will retain when it becomes cold, this property is very useful in certain processes of finishing wool fabrics, making hats, etc.

Chemical Composition.--In the natural or raw state each wool fibre is surrounded by a considerable amount of foreign matter, so that in treating of its chemical constitution it is necessary to distinguish between pure wool and the raw fibre. The incrusting substance is technically known as "Yolk," or "Suint," and is principally composed of a kind of natural soap, consisting of the potash salts of certain fatty acids, together with some fats which are incapable of saponification.

The amount of yolk present upon different samples of wool varies greatly, the finer varieties containing, as a rule, a larger proportion than the coarser, and less valuable sorts.

The variation in the relative amount of pure fibres and yolk is well shown in the following analyses which, however, do not by any means represent extreme cases.

ANALYSES OF RAW MERINO WOOL. DRIED AT 100°C.

Yolk consists very largely of two complex substances which have been termed wool perspiration and wool fat. The former is composed of the potash salts of fatty acids, principally oleic and stearic acids; the latter of the neutral carbohydrate, cholesterine, with other similar bodies. The wool perspiration may be removed by a simple washing with water, and on the Continent forms a valuable source of potash salts, since the ash after ignition contains 70 to 90 per cent. of potassium carbonate. The wool fat is insoluble in water, but dissolves readily in ether, benzene, carbon disulphide, etc.

It is also removed from the wool by a treatment with alkali, and it is not easy to explain the action in the case, since the wool fat is not a glyceride, and will not form a soap, but is probably emulsified by the wool perspiration.

Chemical Composition of the Pure Fibre.--The following analyses of purified and dried wool fibre indicate its percentage composition:--

Carbon 50·5 per cent. 50·8 per cent.
Hydrogen 6·8 " 7·2 "
Nitrogen 16·8 " 18·5 "
Oxygen 20·5 " 21·2 "
Sulphur 5·4 " 2·3 "
------- -------
100·00 100·00

It is sometimes stated that wool fibre consists of a definite substance, keratine, but this view cannot now be admitted, since wool appears to be composed of a mixture or combination of several very complex substances. It is possible and even probable that the outer epidermal scales have a somewhat different composition to the bulk of the fibre, but whether that is the case or not is not known with any degree of certainty, this much can be asserted, that wool is not a simple definite chemical compound.

Sulphur is by far the most variable constituent of wool, sometimes as little as 1·5 and occasionally as much as 5 per cent. being found. It appears to be always present in two different forms, one portion being in very feeble combination and easily removed by alkalies, the remainder, which, according to Knecht, amounts to about 30 per cent. of the total sulphur, cannot be removed without complete disintegration of the fibre. This latter portion does not give a black coloration with plumbite of soda.

The amount of ash left on incinerating dry wool varies from 1 to 2 per cent., and some have considered this inorganic matter as an essential constituent. It consists principally of salts of potassium, calcium and aluminum, with, of course, sulphur.

The chemical composition of the wool fibre is evidently of a most complicated nature; judging from its behaviour in dyeing it is evident that it may contain two bodies, one of a basic character which enables it to combine with the azo and acid series of dyes, the other possessing acid characters enabling it to combine with the basic dyes of the magenta and auramine type. Dr. Knecht has isolated from the wool fibre by extraction with alkalies and precipitation with acids a substance to which the name of lanuginic acid has been given. It is soluble in hot water, precipitates both acid and basic colouring matters in the form of coloured lakes. It yields precipitates with alum, stannous chloride, chrome alum, silver nitrate, iron salts, copper sulphate. It appears to be an albuminoid body. From its behaviour with the dyes, and with tannic acid and metallic salts, it would appear that lanuginic acid contains both acidic and basic groups. It contains all the elements, carbon, hydrogen, oxygen, nitrogen and sulphur, found in wool.

If wool is dyed in a dilute solution of Magenta (hydrochloride of rosaniline), the whole of the base (rosaniline) is taken up, and the whole of the acid (HCl) left in the bath, not, however, in the free state, but probably as NH₄Cl, the ammonia being derived from the wool itself. A further proof of the acid nature of lanuginic acid is that wool may be dyed a fine magenta colour in a colourless solution of rosaniline base; for since rosaniline base is colourless, and it only forms a colour when combined with acids, the fibre has evidently acted the part of an acid in the combination.

Chemical Properties. Action of Alkalies.--Alkalies have a powerful action on wool, varying, of course, with the nature of the alkali, strength of solution and temperature at which the action takes place.

An ammoniacal solution of copper hydroxide (Schweizer's reagent), has comparatively little action in the cold, but when hot it dissolves wool fairly readily.

The caustic alkalies; sodium hydroxide, NaOH, or potassium hydroxide KOH, have a most deleterious action on wool. Even when very dilute and used in the cold they act destructively, and leave the fibre with a harsh feel and very tender, they cannot therefore be used for scouring or cleansing wool. Hot solutions, even if weak, have a solvent action on the wool fibre, producing a liquid of a soapy character from which the wool is precipitated out on adding acids.

This action of alkalies has an important bearing on the scouring of wool, for if this operation be not carried out with due care there is in consequence great liability to impair the lustre and strength of this fibre. From microscopical examination this effect of alkalies is seen to be due to the fact that they tend to disintegrate the fibre, loosen and open the scales, this is shown by contrasting the two fibres A and B shown in figure 4, A being a normal wool fibre, B one strongly treated with an alkali.

The alkaline carbonates have but little action on wool, none if used dilute and at temperatures below 120°F.

Soap has practically no action on wool, and is therefore an excellent scouring material for wool. The carbonate of ammonia is the best and has the least action of the alkaline carbonates, those of potash and soda if used too strong or too hot have a tendency to turn the wool yellow, the carbonate of potash leaves the wool softer and more lustrous than the carbonate of soda.

The influence of scouring agents on wool will be discussed in the chapter on cleansing wool fabrics in more detail.

Caustic or quick-lime has a similar injurious action on the wool fibre as the caustic alkalies.

Action of Acids.--Acids when dilute have but little influence on the wool fibre, their tendency is to cause a separation of the scales (see fig. 5) of the wool and so make it feel harsher. Strong acids have a disintegrating action on the wool fibre. There is a very considerable difference between the action of acids on wool and on cotton, and this difference of action is taken advantage of in the woollen industry to separate cotton from wool by the process commonly known as "carbonising," which consists in treating the fabric with a weak solution of hydrochloric acid or some other acid, then drying it; the cotton is disintegrated and falls away in the form of a powder, while the wool is not affected, sulphuric acid is used very largely in dyeing wool with the acid- and azo-colouring matters.

Nitric acid affects wool in a very similar manner to the acids named above when used in a dilute form; if strong it gives a deep yellow colour and acts somewhat destructively on the fibre.

Sulphurous acid (sulphur dioxide) has no effect on the actual fibre, but exercises a bleaching action on the yellow colouring matter which the wool contains, it is therefore largely used for bleaching wool, being applied either in the form of gas or in solution in water; the method will be found described in another chapter. Wool absorbs sulphur dioxide in large amount, and if present is liable to retard any subsequent dyeing processes.

Action of Other Substances.--Chlorine and the hypochlorites have an energetic action on wool, and although they exert a bleaching action they cannot well be used for bleaching wool. Hot solutions bring about a slight oxidation of the fibre, which causes it to have a greater affinity for colouring matters; advantage is taken of this fact in the printing of delaines and woollen fabrics, while the woollen dyer would occasionally find the treatment of service. A paper by Mr. E. Lodge, in the Journal of the Society of Dyers and Colourists, 1892 (p. 41), may be consulted with advantage on this subject. Wool treated with chlorine loses its felting property, and hence becomes unshrinkable, a fact of which advantage is taken in preparing unshrinkable woollen fabrics.

When wool is boiled with solutions of metallic salts, such as the sulphate of iron, chrome, aluminium and copper, the chlorides of tin, copper and iron, the acetates of the same metals, as well as with some other salts, decomposition of the salt occurs and a deposit of the metallic oxide on the wool is obtained with the production of an acid salt which remains in solution. In some cases this action is favourably influenced by the presence of some organic acid or organic salt, as, for examples, oxalic acid and cream of tartar (potassium tartrate), along with the metallic salt.

On this fact depends the process of mordanting wool with potassium bichromate, alum, alumina sulphate, ferrous sulphate, copper sulphate, etc. The exact nature of the action which occurs is not properly understood, but there is reason for thinking that the wool fibre has the capacity of assimilating both the acid and the basic constituents of the salt employed.

Excessive treatment with many metallic salts tends to make the wool harsh to the feel, partly owing to the scales being opened out and partly owing to the feel naturally imparted by the absorbed metallic salt.

The normal salts of the alkaline metals, such as sodium chloride, potassium sulphate, sodium sulphate, etc., have no action whatever on the wool fibre.

Wool has a strong affinity for many colouring matters. For some of the natural colours, turmeric, saffron, anotta, etc., and for the neutral and basic coal-tar colours it has a direct affinity, and will combine with them from their aqueous solutions. Wool is of a very permeable character, so that it is readily penetrated by dye liquors; in the case of wool fabrics much depends, however, upon the amount of felting to which the fabric has been subjected.

If wool be boiled in water for a considerable time it will be observed that it loses much of its beautiful lustre, feels harsher to the touch, and also becomes felted and matted together. This has to be carefully guarded against in all dyeing operations, where the handling or moving of the yarns is apt to produce this unfortunate effect.

After prolonged boiling the fibre shows signs of slight decomposition, from the traces of sulphuretted hydrogen and ammonia gases which it evolves.

When wool is dried at 212°F. it assumes a husky, harsh feel, and its strength is perceptibly impaired. According to Dr. Bowman, the wool fibre really undergoes a slight chemical change at this temperature, which becomes more obvious at 230°F., while at about 260°F. the fibre begins to disintegrate. According to the researches of Persoz, however, temperatures ranging from 260°F. to 380°F. can be employed without any harm to the wool, if it has previously been soaked in a 10 per cent. solution of glycerine.

When wool is heated to 212°F. (100°Cent.) it becomes quite pliant and plastic and may be moulded into almost any shape, which it still retains when cold. This fact is of much interest in the processes of finishing various goods, of embossing velvet where designs are stamped on the woven fabric while hot, and in the crabbing and steaming of woollen goods, making hats, etc.