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Processes Preparatory To Dyeing Scouring And Bleaching Of Wool






Wool scouring takes place at two stages in the process of manufacture
into cloth. First, in the raw state, to free the wool from the large
amount of grease and dirt it naturally contains; second, after being
manufactured into cloth, it is again scoured to free it from the oil
that has been added to the scoured raw wool to enable it to spin
easily. This oiling is generally known as wool batching, and before
the spun yarns or woven fabrics can be dyed it is necessary to remove
it.

Raw wool is a very impure substance, containing comparatively little
wool fibre, rarely more than 50 to 60 per cent. in the cleanest
fleeces, while it may be as low as 25 per cent. in the dirtiest.

First there is a small quantity of dirt; there is what is called the
suint, a kind of soapy matter, which can be removed by washing in hot
water. This soap has for its base potash, while its acids are numerous
and complex. The wool contains a fatty-like substance of the nature of
wax, called cholesterine, and this imparts to the fatty matter, which
be extracted from the wool fibre, very peculiar properties. Besides
these there are several other bodies of minor importance, all of which
have to be removed from the wool before it can be manufactured into
cloth.

Marker and Schulz give the following analysis of a good sample of (p. 016)
raw wool:--

Moisture 23.48 per cent.
Wool fat 7.17
Wool soap (suint), soluble in water 21.13
Soluble in alcohol 0.35
Soluble in ether 0.29
Soluble in dilute hydrochloric acid 1.45
Wool fibre 43.20
Dirt 2.93
------
100.00

Two principles underlie the methods which are in use for this purpose.
The first principle and the one on which the oldest method is based is
the abstraction of the whole of the grease, etc., from the wool by
means of an alkaline or soapy liquor at one operation. This cannot
nowadays be considered a scientific method. Although it extracts the
grease, etc., from the wool, and leaves the latter in a good condition
for after processes, yet with it one might almost say that the whole
of the soap or alkali used, as well as the wool grease itself, is lost
as a waste product; whereas any good process should aim at obtaining
the wool grease for use in some form or another. The second principle
which underlies all the most recent methods for extracting the grease
from the wool, consists in treating the fibre with some solvent like
benzol, carbon bisulphide, petroleum spirit, carbon tetrachloride,
etc., which dissolves out the cholesterine and any other free
fatty matter which is in the wool fibre, leaving the latter in such a
condition that by washing with water the rest of the impurities in the
wool can be extracted. By distilling off and recondensing the solvent
can be recovered for future use, while the wool fat can also be
obtained in a condition to use for various purposes. This is rather a
more scientific method than the old one, but it has not as yet come
into extensive use.

#Wool Scouring. Old Methods.#--In the early days of wool scouring (p. 017)
this operation was done in a very primitive fashion, generally in a
few tubs, which could be heated by steam or otherwise, and in which
wool was worked by means of hand forks. These primitive processes are
still in use in some small works, especially where the wool is dyed in
the loose condition, but in all the large works machinery has been
adopted, which machinery has been brought to a high state of
perfection, and does its work very well, and without much attendant
manual labour.

The alkaline substances used in this process of scouring demand some
notice. These comprise soda ash, soda crystals, caustic soda, silicate
of soda, potash, caustic potash, soaps of various kinds, stale urine,
ammoniacal compounds. Which of these may be used in any particular
case depends upon a variety of reasons. Potash is the best alkaline
agent to use. It agrees better with the fibre than any other, leaving
it soft and elastic. Ammonia is the next best, but it does not take
out the grease as well as the potash. Soda does not suit as well as
potash, as it has a tendency to leave the fibre harsh in feel and
somewhat brittle, yet on account of its being so much cheaper it is
the most largely used. The use of silicate of soda cannot be
recommended, as it has a great tendency to leave the fibre hard, which
may be ascribed to the deposition of silica on the fibre.

The caustic alkalies cannot be used as they have too solvent an action
on the fibre. The carbonates, therefore, in the form of soda ash or
potash, or pearl ash, are used, or better still, soap is used as it
has a greater solvent action on the fatty matter of the wool than have
the alkalies, and in this respect a potash soap is better than a soda
soap.

The character of the wool determines the alkali to be used; fine,
long-stapled wools, which are usually very free from grease in excess,
should always be treated with potash, or a potash soap, which will (p. 018)
remove the whole of the grease from the wool, leaving the latter in a
fine, soft, silky condition.

Short-stapled wools can be treated with soap and a little soda ash,
but too much of the latter is to be avoided. Coarse and greasy wools
may be scoured with soap and soda ash, or other alkali which is almost
necessary to remove the large amount of grease these wools contain.

Practically the only alkaline products now in use are the various hard
and soft soaps, and the carbonates of soda and potash in their various
forms of soda ash, soda crystals, potashes, pearl ash, etc. Ammonia
and its compounds are rarely used, while stale urine, which acts in
virtue of the ammonia it contains has practically gone out of use.

#Hand-Scouring.#--Wool scouring by hand is easily done and requires few
appliances, simple tubs or vats of sufficient capacity in which steam
pipes are placed, so that the scouring liquors can be heated up. The
best temperatures to use are about 130 deg. to 140 deg. F., and it is
not advisable to exceed the latter, as there is then some risk that the
alkali may act on the fibre too strongly.

The strength of the scouring liquor necessarily varies with the kind
of wool being treated, and with the kind of alkaline product used; if
soft, fine wools are being treated, then the liquor may be made with 1
to 2 lb. of soap to 10 gallons of water (if a mixture of soap and
alkali is used, then it may contain from 1/4 to 1/2 lb. soda ash, and
1/2 lb. to 1 lb. of soap). For coarse, greasy wools these quantities
may be increased by about one-half. The best plan of scouring by hand
is to treat the wool in a tub with a scouring liquor for about half an
hour, then to squeeze out the surplus liquor and to treat again in a
new liquor for half an hour; this liquor may be used for a new batch
of wool. The wool is often put into nets, and these are lifted up and
down in the liquor so as to cause it to penetrate to every part of the
wool.

It is not advisable to work the wool about too much, otherwise (p. 019)
felting might ensue and this must be avoided. The felting of the wool
is one of the troubles of the wool-scourer and is often difficult to
avoid, it is mostly brought about by excessive working of the wool
during the process, and by the use of too high a temperature in the
scouring bath. The remedies are obvious to the practical man, as
little handling of the wool as possible, and at as low a temperature
as possible. Still it is necessary to see that the scouring liquor
penetrates to every part of the wool which is being treated.

To ensure this, care must be taken not to scour too much at one time,
so that the wool is loosely placed in the scouring tub, if placed
loose in the latter, the workmen can by means of forks work it to and
fro while in process of treatment. After the wool has been through
these scouring liquors it is thrown on a scray to drain, and is next
placed in cisterns which have perforated false bottoms. In these
cisterns it is washed with cold water two or three times, the water
being run off from the wool between each washing; it is then spread
out in a room to dry. As a rule, a man can wash from 500 lb. to
600 lb. of wool in a day by this method. Another plan which is
sometimes adopted so as to avoid handling the wool as much as
possible, and thus prevent felting, is to place the wool in cages
having perforated sides which will hold about 1 cwt. of wool. They are
lowered by means of cranes into the washing liquors, and the wool in
them is then worked for a quarter of an hour, when the cages and their
contents are lifted out and the surplus liquor allowed to drain off.
They are then lowered into the next bath, treated or worked in this,
again lifted out and dropped into the wash waters.

There is by this plan a saving of handling, and more wool can be got
through in the same time, but it requires two men to work it. These
hand processes are only in use in small works, having been (p. 020)
replaced in all large works by mechanical methods described below.

#Machine Scouring.#--Wool-scouring machinery has been brought to a high
state of perfection by the successive efforts of many inventors, and
by their means wool washing has been much simplified and improved.
Wool-washing machinery is made by several firms, among whom may be
mentioned Messrs. J. & W. McNaught, and John Petrie, Junior, Limited,
both of Rochdale.


Fig. 6 shows one form of wool-washing machine. It consists of a long
trough which contains the scouring liquor. In this machine the wool
enters at the left-hand end, and is seized by a fork or rake and
carried forward by it a short distance, then another rake seizes it
and carries it further forward to another rake, and this to the last
rake of the machine, which draws it out of the machine to a pair of
squeezing rollers which press out the surplus liquor, and from these
rollers the scoured wool passes to a travelling band for delivery from
the machine. Sometimes the wool is not entered into the trough direct,
but is put on a travelling apron which opens it and delivers it in a
more open form into the trough. The movement of the forks causes some
degree of agitation in the scouring liquor which facilitates the
penetration of the liquor through the wool, and thus brings about a
better scouring.

After the wool has passed through the machine it is taken and run once
more through the machine. Some scourers use the same liquor, but it is
better to use fresh liquors, after which it is washed in the same
machine with water two or three times. With a single machine there is
some time and labour lost in transferring the wool from one end to the
other between the separate treatments, and in large works where a
great deal of wool is scoured it is usual to place three or four of
these machines end to end.

The first is filled with strong scouring liquor, the second with (p. 022)
a weaker liquor, while the third and fourth contains wash waters, and
the wool is gradually passed by the action of the machine through the
series without requiring any manual aid. Between each machine it is
passed through squeezing rollers as before, and finally emerges
thoroughly scoured. A good plan of working in connection with such a
series of machines is to have four as above, two washing machines and
two soaping machines, the soap liquor is run through these in a
continuous stream, entering in at the delivery end of the second
soaper and passing out at the entering end of the first soaper. The
wool as it first enters the machine comes into contact with rather
dirty soap liquor, but this suffices to rid it of a good deal of loose
dirt; as it passes along the machine it comes in contact with cleaner
and fresher soap liquor, which gradually takes all grease and dirt out
of it, and, finally, when it passes out it comes in contact with fresh
liquor, which removes out the last traces of dirt and grease. In the
same way it passes through the washers, being treated at the last with
clean water. By this plan the scouring is better done, while there is
some saving of soap liquor and wash water, for of these rather less is
required than by the usual system. These are matters of consideration
for wool scourers. The wool-washing liquors after using should be
stored in tanks to be treated for recovery of the grease which they
contain.

The temperature of the scouring liquors should be about 100 deg. F.,
certainly not more than 120 deg. F., high temperatures are very liable to
bring about felting, while tending to increase the harshness of the
wool, particularly when soda is the agent used. By this method all the
wool fat, suint, etc., of the wool find their way into the soap
liquors. These were formerly thrown away, but they are generally
treated with acid and the fat of the soap and wool recovered, under
the name of wool grease or Yorkshire grease. (Vide G. H. Hurst, (p. 023)
Yorkshire Grease, Jour. Soc. Chem. Ind., February, 1889.)

The wool fat consists largely of a peculiar fat-like body known as
cholesterine. This, however, is unsaponifiable, and cannot be made
into soap; at the same time when it gets into, as it does, the
recovered wool grease it spoils the latter for soap-making purposes.

Cholesterine has some properties which make it valuable for other
purposes; it is a stable body not prone to decomposition, it is
capable of absorbing a large quantity of water, and it is on these
accounts useful for medicinal purposes in the production of ointments,
and it might be useful in candle-making. When it gets into recovered
grease it cannot be extracted from it in an economical manner. The
wool suint consists largely of the potash soaps of oleic and stearic
acids. These two fatty acids find their way into the recovered wool
grease but the potash salts are lost, while they would be valuable for
various purposes if they could be recovered.

Another form of wool-washing machine has a frame carrying a number of
forks arranged transversely to the machine. The forks are by suitable
gearing given a motion which consists of the following cycle of
movements. The forks are driven forwards in the trough of the machine,
carrying the wool along with them, they are then lifted out, carried
back, and again allowed to drop into the machine, when they are ready
to go forward again. Thus the forks continually push the wool from one
end of the machine to the other.

It is a common plan to have three machines placed end to end, so that
the wool passes from one to the other; in a set of this kind the first
machine should have a capacity of 1,500 gallons or thereabouts, the
second 1,000 gallons, and the third 750 gallons.

#Wool Scouring by Solvents.#--Of late years processes have been (p. 024)
invented for the scouring of wool, either raw or spun by means of
solvents, like carbon bisulphide, benzol, petroleum spirit, etc. Such
processes are in a sense rather more scientific than the alkali
processes, for whereas in the latter the grease, etc., of the wool and
the oil used in batching it are practically lost for further use, and
therefore wasted, being thrown away very often, although they may be
partially recovered from the used scouring liquors, in the solvent
processes the grease and oil may be recovered for future use for some
purpose or other.

The great objection to these processes is the danger that attends
their use, owing to the inflammable character of the solvents. Several
other objections may be raised, some of which are mechanical, and due
to the want of proper machinery for carrying out the processes. There
are many ways in which solvents may be applied, some are the subject
of patents. It is not possible to describe the details of all these,
but two of the most recent will be mentioned.

In Singer's process, which was described in detail by Mr. Watson Smith
some time ago before the Society of Dyers and Colourists, carbon
bisulphide is used. The raw wool is placed between two endless bands
of wire, and it is carried through a series of troughs containing
bisulphide of carbon; during its passage through the troughs the
solvent takes out the grease, and loosens the other constituents of
the wool. After going through the bisulphide the wool is dried and
passed through water which completes the process. The carbon
bisulphide that has been used is placed in steam-heated stills,
distilled off from the grease, condensed in suitable condensers, and
used over again. In this process, with care, there is very little loss
of solvent. The grease which is recovered can be used for various
purposes, one of which is the manufacture of ointments, pomades, etc.
The disadvantages of bisulphide are: (1) It tends after some time (p. 025)
to cause the wool to acquire a yellow cast, due to the free
sulphur which it contains, and which being left in the wool gradually
causes it to turn yellow. By using redistilled bisulphide this defect
may be avoided. (2) Another defect is the evil odour of the solvent.
This, however, is less with redistilled bisulphide than with the
ordinary quality, and with suitable apparatus is not insuperable. (3)
Another defect is the volatility and inflammability of carbon
bisulphide. On the other hand, bisulphide possesses the very great
advantage of being at once heavier than, and insoluble in, water, and
it can be, therefore, stored under water very much more safely than
can any of the other solvents which are used.

Burnell's machine has two troughs filled with benzoline. In these are
arranged a large central roller round which are some smaller rollers.
The wool passes round the large roller and is subjected to a number of
squeezings in passing the smaller rollers. A current of the benzoline
is continually passing through the machine. The whole is enclosed in a
hood to avoid loss of solvent as far as possible. After passing
through the benzoline trough the wool passes through a similar trough
filled with water. Benzoline is better than carbon bisulphide in that
there is no tendency on the part of the wool to turn yellow after its
use, on the other hand it is more inflammable, and when it does take
fire is more dangerous, and being lighter than water is not so readily
and safely stored. Another feature is that it is not so completely
volatile at steam temperatures, so that a little may be left in the
grease and thus tend to deteriorate it. Coal-tar benzol, the quality
known as 90's, would be better to use.

The solvent processes are well worth the attention of wool scourers,
all that is required for their proper development being the production
and use of suitable machinery.

After the raw wool has been scoured it is batched, i.e., it is (p. 026)
mixed with a quantity of oil for the purpose of lubricating the
wool to enable it more easily to stand the friction to which it is
subjected in the subsequent processes of spinning and weaving by
giving it greater pliability.

For this purpose various kinds of oil are used. Olive oil is the
principal favourite, the variety mostly used being Gallipoli oil.
Ground-nut oil is also extensively employed, and is cheaper than
olive. Oleic acid a by-product of the candle industry, is extensively
used under the name of cloth oil, there is also used oleine, or wool
oil, obtained by the distillation of Yorkshire grease.

So far as merely oiling the wool is concerned there is not much to
choose between these different oils, olive perhaps works the best and
agrees best with the wool. Mineral oils have been and can be used
either alone or mixed with the oils above mentioned, and so far as
lubricating the wool is concerned do very well and are much cheaper
than the fatty oils named above.

The following are some analyses of various oils used as cloth oils
which the author has had occasion to analyse.

1. 2. 3. 4.
Specific gravity at 60 deg. F. 0.9031 0.9091 0.6909 0.8904
Free fatty acid 55.02 64.42 51.52 68.05
Unsaponifiable oil 34.56 9.95 32.80 9.52
Saponifiable oil 10.32 25.32 15.68 12.43
------ ------ ------ ------
100.00 100.00 100.00 100.00

Nos. 1 and 2 are prepared from Yorkshire grease. The unsaponifiable
matter in these is purely natural, it will be seen it varies within
wide limits. Nos. 3 and 4 are made from the oleic acid of the candle
factories, and the unsaponifiable matter is due to their containing
mineral oil which has been added to them.

So far as regards the object for which the wool is oiled, the mineral
oils will answer almost as well as the fatty oils and with most (p. 027)
satisfactory results from an economical point of view, for they are
much cheaper. But this is not the only point to be considered. The oil
has to be got out of the wool before the latter can be dyed. Now while
the fatty oils can be easily removed, by treatment with soap, and they
can be recovered along with the fat of the soap, mineral oils cannot
be entirely removed from the wool, what remains in will interfere very
much with the satisfactory dyeing of the wool, and what is got out
finding its way into the covered wool grease, spoils this for soap
making and other uses, so that on the whole what is gained in lessened
cost of oiling is lost by the increased liability to defects in dyeing
and consequently depreciation in value of the wool, and to decrease in
value of the recovered grease.

The amount of oil used varies from 7 per cent. with the best wools to
15 per cent. with shoddy wools. The scouring agents generally used are
the same as those used in loose wool scouring, namely, carbonate of
soda for coarse woollen yarns, soap and soda for medium yarns, and
soap and ammonia for fine yarns. Prior to treating the yarns it is
best to allow them to steep in hot water at about 170 deg. F. for twenty
minutes, then to allow them to cool. The actual scouring is often done
in large wooden tubs, across which rods can be put on which to hang
the hanks of yarn, and in which are placed steam pipes for heating up
the bath. The best temperature to treat the yarn at is about 150 deg. F.;
too high a temperature must be avoided, as with increased heat the
tendency to felt is materially augmented, and felting must be avoided.
The hanks are treated for about twenty minutes in the liquor, and are
then wrung out, drained, and again treated in new scouring liquor for
the same length of time. After rinsing in cold water they are dried
and finished.

When the oiled wool has been spun into yarns, whether worsted or (p. 028)
woollen, and passes into the hands of the dyer, it is necessary to
remove from it all the oil before any dyeing operations can be
satisfactorily carried out. This oil is removed by the use of soap and
weak alkaline liquors, using these at about 110 deg. to 120 deg. F. The
most common way is to have the liquor in a rectangular wooden tank, and
hang the hank of yarn in by sticks resting on the edges of the tank;
from time to time the hanks are turned over until all the oil has been
washed out, then they are wrung out and passed into a tank of clean
water to wash out the soap, after which the yarn is ready for dyeing.

When the yarn is of such a character that it is liable to curl up,
shrink and become entangled, it is necessary that it be stretched
while it is being treated with the soap liquor; this is effected by a
stretching apparatus consisting of two sets of rollers connected
together by a screw attachment, so that the distance between the two
sets of rollers can be varied. The hanks are hung between each pair of
rollers, and can be stretched tightly as may be required.

For pressing out the surplus liquor from the hanks of yarn a pair of
squeezing rollers is used.

#Scouring Woollen Piece Goods.#--Very often before weaving the yarns are
not scoured to remove the oil they contain, as the weaving is more
conveniently done with oily yarn than with a scoured yarn. Before
dyeing the oil must be taken out of the pieces, and this can be
conveniently done by scouring in a washing machine such as is shown in
figures 7 and 8, using soap and soda liquors as before, and following
up with a good rinse with water.

The soap liquors used in scouring yarns and pieces become charged with
oil, and they should be kept, and the oil recovered from them together
with the fatty matter of the soap, by treatment with sulphuric acid.
By subjecting the grease or fatty matter so obtained to a boil with
caustic soda soap is obtained which may be again used in scouring (p. 029)
wool.

#Bleaching Wool.#--The wool fibre has to be treated very differently
from cotton fibre. It will not stand the action of as powerful
bleaching agents, and, consequently, weaker ones must be used. This is
a decided disadvantage, for whereas with cotton the colouring matter
is effectually destroyed, so that the bleached cotton never regains
its original colour, the same is not the case with wool, especially
with sulphur-bleached wool, here the colouring matter of the fibre is,
as it were, only hidden, and will under certain circumstances return.
The two materials chiefly used for bleaching wool are sulphur and
peroxide of hydrogen.

#Sulphur Bleaching.#--Bleaching wool by sulphur is a comparatively (p. 030)
simple process. A sulphur house is built, the usual size being 12 feet
high by 12 feet broad, and about 17 feet long. Brick is the most suitable
material. The house should have well-fitting windows on two sides,
and good tight doors at the ends (see fig. 9). Some houses have a (p. 031)
small furnace at each corner for burning the sulphur, two of these
furnaces are fitted with hoods, so that the sulphur gases can be
conveyed to the upper part of the chamber, but a better plan, and one
mostly adopted where the chamber is used for bleaching pieces, is to
construct a false perforated bottom above the real bottom of the chamber,
the sulphur being burnt in the space between the two floors. If yarn
is being bleached the hanks are hung on wooden rods or poles in (p. 032)
the chamber, while with pieces an arrangement is constructed so that the
pieces which are stitched together are passed in a continuous manner
through the chamber.

When all is ready the chamber doors are closed, and the furnaces are
heated, some sulphur thrown upon them, which burning evolves sulphur
dioxide gas, sulphurous acid, and this acting upon the wool bleaches
it. The great thing is to cause a thorough circulation of the gas
through every part of the chamber, so that the yarn or pieces are
entirely exposed in every part to the bleaching action of the gas.
This is effected by causing the gas to pass into the chamber at
several points, and, seeing that it passes upwards, to the ventilator
in the roof of the chamber. Generally speaking, a certain quantity of
sulphur depending upon the quantity of goods being treated is placed
in the chamber and allowed to burn itself out; the quantity used being
about 6 to 8 per cent. of the weight of the goods. After the
sulphuring the goods are simply rinsed in water and dried.

Sulphur bleaching is not an effective process, the colouring matter is
not actually destroyed, having only entered into a chemical
combination with the sulphur dioxide to form a colourless compound,
and it only requires that the wool be treated with some material which
will destroy this combination to bring the colour back again in all
its original strength; washing in weak alkalies or in soap and water
will do this. Another defect of the process lies in sulphur being
volatilised in the free form, and settling upon the wool causes it to
turn yellow, and this yellow colour cannot be got rid of.

The goods must be thoroughly rinsed with water after the bleaching,
the object being to rid the wool of traces of sulphuric acid, which it
often contains, and which if left in would in time cause the
disintegration of the wool.

Sometimes the wool is washed in a little weak ammonia or soda (p. 033)
liquor, but this is not advisable, as there is too much tendency for
the colour of the wool to come back again, owing to the neutralising
of the sulphur dioxide by the alkali.

Instead of using the gas, the sulphur dioxide may be applied in the
form of a solution in water. The goods are then simply steeped for
some hours in a solution of the gas in water until they are bleached,
then they are rinsed in water and dried. In this method it is
important that the solution of the gas be freshly made, otherwise it
is liable to contain but little sulphurous acid, and plenty of
sulphuric acid which has no bleaching properties, but, on the other
hand, is liable to lead to damage of the goods if it be not washed out
afterwards.

A better method of utilising the bleaching action of sulphur in a
liquid form is to prepare a bath of bisulphite of soda, and acidify it
with hydrochloric acid, then to enter the wool, stirring well for some
time, and allowing it to steep for some hours, next to expose to the
air for a while, and rinse as before.

It is better to allow the wool to steep for about an hour in a simple
bath of bisulphite, then enter into a weak hydrochloric acid bath for
a few hours. The acid liberates sulphur dioxide in a nascent
condition, which then exerts a more powerful bleaching action than if
it were already free.

Even with liquid bleaching the bleach is not any more perfect than it
is with the gas bleaching; the colour is liable to come back again on
being washed with soap or alkali, although there is a freedom from the
defect of yellow stains being produced.

Goods properly bleached will stand exposure to air for some
considerable time, but those imperfectly bleached exhibit a tendency
to regain their yellow colour on exposure to air. One fault which is
sometimes met with in sulphur bleaching is a want of softness in (p. 034)
the wool, the process seeming to render the fibre harsh.

Washing in a little weak soft soap or in weak soda will remedy this
and restore the suppleness of the wool; at the same time care must be
taken that the alkaline treatment is not too strong, or otherwise the
bleaching effect of the sulphur will be neutralised as pointed out
above.

#Bleaching Wool by Peroxide of Hydrogen.#--During recent years there has
come into use for bleaching the animal fibres peroxide of hydrogen,
or, as the French call it, oxygenated water. This body is a near
relation to water, being composed of the same two elements, oxygen and
hydrogen; in different proportions in water these elements are
combined in the proportion of 1 part of hydrogen to 8 parts of oxygen,
while in the peroxide the proportions are 1 of hydrogen to 16 of
oxygen. These proportions are by weight, and are expressed by the
chemical formulae for water H{2}O, and for hydrogen peroxide
H{2}O{2}. Water, as is well known, is a very stable body, and
although it can be decomposed, yet it requires some considerable power
to effect it. Now the extra quantity of oxygen which may be considered
to have been introduced into water to convert it into peroxide has
also introduced an element of instability, the extra quantity of
oxygen being ever ready to combine with some other body for which it
has a greater affinity than for the water. This property can be
utilised in the bleaching industry with great advantage, true
bleaching being essentially a process of oxidation. The colouring
matter of the fibre, which has to be destroyed so that the fibre shall
appear white, is best destroyed by oxidation, but the process must not
be carried out too strongly, otherwise the oxidation will not be
confined to the colouring matter, but will extend to the fibre itself
and disintegrate it, with the result that the fibre will become
tendered and be rendered useless.

Peroxide of hydrogen is a weak oxidiser, and therefore, although (p. 035)
strong enough to destroy the colouring matter of the fibre is not
strong enough to decompose the fibre itself. Hydrogen peroxide is sold
as a water-white liquid, without any odour or taste. Its strength is
measured by the quantity of oxygen which is evolved when one volume of
the liquid is treated with potassium permanganate; the most common
strength is 10 volume peroxide, but 30 and 40 volume peroxide is made.
On keeping it loses its oxygen, so that it is always advisable to use
a supply up as quickly as possible.

Articles of all kinds can be bleached by simply placing them in a weak
solution of the peroxide, leaving them there for a short time, then
taking out and exposing to the air for some time. The best plan of
applying peroxide of hydrogen is the following: Prepare the bleaching
bath by mixing 1 part of peroxide with 4 parts of water. The strength
can be varied; for those goods that only require a very slight bleach
the proportions may be 1 to 12, while for dark goods the proportions
first given may be used. This bath must be used in either a wooden or
earthenware vessel. Metals of all kinds must be avoided, as they lead
to a decomposition of the peroxide, and therefore a loss of material.
To the bath so prepared just enough ammonia should be added to make it
alkaline, a condition that may be ascertained by using a red litmus
paper, which must just turn blue. Into the bath so prepared the
well-scoured goods are entered and worked well, so that they become
thoroughly saturated. They are then lightly wrung and exposed to the
air for some hours, but must not be allowed to get dry, because only
so long as they are moist is the bleaching going on; if they get dry
the goods should be re-entered into the bath and again exposed to the
air.

If one treatment is not sufficient the process should be repeated. The
peroxide bath is not exhausted, and only requires new material to (p. 036)
be added to it in sufficient quantity to enable the goods to be
readily and easily worked in the liquor. Any degree of whiteness may
be obtained with a sufficient number of workings. No further treatment
is necessary. It is found in practice that an alkaline bath gives the
best results.

Another plan of preparing the bleaching bath is to prepare a bath with
peroxide and water as before, then add to a sufficient quantity of a
solution of silicate of soda 4 parts of water to 1 of silicate of soda
at 100 deg. Tw., to make the bath alkaline. Into this bath the goods are
entered and are then exposed to the air as before, after which they
may be passed through a weak bath of sulphurous acid, being next well
washed in water and dried.

The advantage of bleaching with peroxide is that, as it leaves only
water in the goods as the result of action, there is no danger of
their becoming tendered by an after development of acid due to
defective washing, as is the case with the sulphur bleach. The goods
never alter in colour afterwards, because there is nothing left in
that will change colour. Some bleachers add a little magnesia to the
bath, but this is not at all necessary.

#Bleaching with Peroxide of Soda.#--Peroxide of soda has come to the
front of late for bleaching wool. With it a stronger bleaching bath
can be made, while the product itself is more stable than peroxide of
hydrogen, only it is needful to keep it in tightly closed metal
vessels, free from any possibility of coming in contact with water or
organic matter of any kind, or accidents may happen. In order to
bleach 100 lb. of wool, a bath of water is prepared, and to this is
added 6 lb. of sulphuric acid and then slowly 4 lb. of peroxide of
sodium in small quantities at a time. Make the bath slightly alkaline
by adding ammonia. Heat the bath to 150 deg. F., enter the wool and allow
to remain five to six hours, then rinse well and dry. If the (p. 037)
colour does not come out sufficiently white repeat the process.





Next: The Chlorination Of Wool

Previous: The Wool Fibre



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