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The Wool Fibre






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 (p. 002)
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 (p. 003)
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 (p. 004)
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 (p. 005)
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 (p. 006)
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 deg. 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 deg. 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 (p. 007)
well shown in the following analyses which, however, do not by any
means represent extreme cases.

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

No. 1. No. 2.
Moisture 6.26 10.4
Yolk 47.30 27.0
Pure Wool 30.31 59.5
Dirt 11.13 3.1
------ ------
100.00 100.00

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:--

Mulder. Bowman.
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
----- -----
100.0 100.0

It is sometimes stated that wool fibre consists of a definite (p. 008)
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 (p. 009)
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 NH{4}Cl, 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 (p. 010)
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 deg. 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 (p. 011)
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 (p. 012)
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 (p. 013)
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 deg. 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 deg. F., while at about 260 deg. F.
the fibre begins to disintegrate. According to the researches of Persoz,
however, temperatures ranging from 260 deg. F. to 380 deg. 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 deg. F. (100 deg. Cent.) it becomes (p. 014)
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.





Next: Processes Preparatory To Dyeing Scouring And Bleaching Of Wool




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