SCOURING AND BLEACHING OF COTTON.
Preparatory to the actual dyeing operations, it is necessary to treat
cotton in any condition--loose cotton, yarn, or piece--so that the
dyeing shall be properly done. Raw cotton contains many impurities,
mechanical and otherwise; cotton yarns accumulate dirt and impurities of
various kinds during the various spinning operations, while in weaving a
piece of cotton cloth it is practically impossible to keep it clean and
free from dirt, etc. Before the cotton can be dyed a perfectly level and
uniform shade, free from dark spots or light patches, these impurities
must be removed, and therefore the cotton is subjected to various
scouring or cleansing operations with the object of effecting this end.
Then again cotton naturally, especially Egyptian cotton, contains a
small quantity of a brown colouring matter, and this would interfere
with the purity of any pale tints of blue, rose, yellow, green, etc.,
which may be dyed on the cotton, and so it becomes necessary to remove
this colour and render the cotton quite bright. This is commonly called
"bleaching". It is these preparatory processes that will be dealt with
in this chapter.
=Scouring Cotton.=--When dark shades--blacks, browns, olives, sages,
greens, etc., are to be dyed it is not needful to subject the cotton to
a bleaching operation, but simply to a scouring by boiling it with soda
or caustic soda. This is very often-carried out in the same machine as
the goods will be dyed in; thus, for instance, in the case of pieces,
they would be charged in a jigger, this would be filled with a liquor
made from soda or from caustic soda, and the pieces run from end to end,
while the liquor is being heated to the boil--usually half to
three-quarters of an hour is sufficient. Then the alkali liquor is run
out, clean water run into the jigger and the pieces washed, after which
the dyes, etc., are run into the jigger and the dyeing done. There is
usually used 2 lb. to 3 lb. of caustic soda, or 3 lb. to 4 lb. of soda
for each 100 lb. of goods so treated.
If the ordinary dyeing machines are not used for this purpose, then the
ordinary bleachers' kiers may be used. These will be described
presently.
=Bleaching of Cotton.=--Cotton is bleached in the form of yarn, or in
the finished pieces. In the latter case the method depends very largely
on the nature of the fabric; it is obvious that fine fabrics, like
muslins or lace curtains, cannot stand the same rough treatment as a
piece of twilled calico will. Then, again, the bleaching process is
varied according to what is going to be done with the goods after they
are bleached; sometimes they are sent out as they leave the
bleach-house; again, they may have to be dyed or printed. In the first
case the bleach need not be of such a perfect character as in the last
case, which again must be more perfect than the second class of bleach.
There may be recognised:--
(1) Market or white bleach.
(2) Dyers or printers' bleach.
(3) Madder bleach.
As the madder bleach is by far the most perfect of the three, and
practically includes the others, this will be described in detail, and
differences between it and the others will be then pointed out. A piece
is subjected to the madder bleach which has afterwards to be printed
with madder or alizarine. Usually in this kind of work the cloths are
printed with mordant colours, and then dyed in a bath of the dye-stuff.
This stains the whole of the piece, and to rid the cloth of the stain
where it has to be left white, it is subjected to a soap bath. Now,
unless the bleach has been thorough, the whites will be more or less
stained permanently, and to avoid this cloths which are to be printed
with alizarine colours are most thoroughly bleached. The madder bleach
of the present day generally includes the following series of
operations:--
(1) Stitching.
(2) Singeing.
(3) Singeing wash.
(4) Lime boil.
(5) Lime sour.
(6) Lye boil.
(7) Resin boil.
(8) Wash.
(9) Chemicing.
(10) White sour.
(1) =Stitching.=--The pieces are fastened together by stitching into one
long rope, which is passed in a continuous manner through all operations
in which such a proceeding is possible. This stitching is done by
machines, the simplest of which is the donkey machine, whereby the ends
of the pieces, which are to be stitched together, are forced by a pair
of cogwheels working together on to the needle carrying a piece of
thread, this is then pulled through and forms a running stitch, a
considerable length of thread being left on each side so as to prevent
as far as possible the pulling asunder of the pieces by an accidental
drawing out of the thread.
Birch's sewing machine is very largely used in bleach works. It consists
essentially of a Wilcox & Gibb machine fitted on a stand so as to be
driven by power. The pieces are carried under the needle by a large
wheel, the periphery of which contains a number of projecting pins that,
engaging in the cloth, carry it along.
There is also a contrivance by which these pieces to be sewn can be kept
stretched, this takes the form of an arm with clips at the end, which
hold one end of the cloth while it is running through the machine. The
clip arrangement is automatic, and just before the end passes under the
needle it is released, and the arm flies back ready for the next piece;
it is, however, not necessary to use this arm always. This machine gives
a chain stitch sufficiently firm to resist a pull in the direction of
the length of the pieces, but giving readily to a pull at the end of the
thread.
The Rayer & Lincoln machine is an American invention, and is much more
complicated than Birch's. It consists of a sewing machine mounted on the
periphery of a large revolving wheel. This carries a number of pins,
which, engaging in the cloth to be stitched, carry it under the needle
of the machine. Besides sewing the pieces together this machine is
fitted with a pair of revolving cutters which trim the ends of the
pieces as they pass through in a neat clean manner. There is also an
arrangement to mark the pieces as they are being stitched. Like Birch's
it produces a chain stitch.
What is important in sewing the ends of pieces together is to get a firm
uniform stitch that lies level with the cloths without any knots
projecting, which would catch in the bleaching machinery during the
processes of bleaching, and this might lead to much damage being done.
Should it be necessary to mark the pieces so that they can be recognised
after bleaching, the best thing to use is printers' ink. Gas tar is also
much used, and is very good for the purpose. Coloured inks do not resist
the bleaching sufficiently well to be used satisfactory. Vermilion and
Indian red are used for reds, yellow ochre is the fastest of the
yellows, there is no blue which will stand the process, and Guignet's
green is the only green that will at all resist the process, umber will
serve for brown. All these colours are used in the form of printing ink.
The next operation is a very important one, which cannot be too
carefully carried out, that is:--
(2) =Singeing.=--For printing bleaches the cloths are singed. This has
for its object the removal from the surface of the cloth of the fine
fibres with which it is covered, and which would, if allowed to remain,
prevent the designs printed on from coming out with sufficient
clearness, giving them a blurred appearance.
Singeing is done in various ways, by passing the cloth over a red-hot
copper plate, or over a red-hot revolving copper cylinder, or through a
coke flame, or through gas flames, and more recently over a rod of
platinum made red hot by electricity.
Plate singeing is the oldest of these methods and is still largely used.
In this method a semi-cylindrical copper plate is heated in a suitable
furnace to a bright red heat, the cloths are rapidly passed over it, and
the loose fibres thereby burnt off. One great trouble is to keep the
plate at one uniform heat over the whole of its surface, some parts will
get hotter than others, and it is only by careful attention to the
firing of the furnace that this can be obtained. To get over these
difficulties Worral introduced a roller singeing machine in which the
plate was replaced by a revolving copper roller, heated by a suitable
furnace; the roller can be kept at a more uniform temperature than the
plate. The singe obtained by the plate and roller is good, the principal
fault being that if the cloths happen to get pressed down too much on
the hot plate the loose ends are not burnt off as they should be. With
both plate and roller the cloths are singed only on one side, and if
both sides require to be singed a second passage is necessary. Both
systems still retain their hold as the principal methods in use,
notwithstanding the introduction of more modern methods.
Singeing by passing the cloths over a row of Bunsen burners has come
largely into use. This has the great advantage of being very cleanly,
and of doing the work very effectually, much more thoroughly than any
other method, which is due to the fact that while in the methods
described above only the loose fibres on the surface are burnt off; with
gas all the loose fibres are burnt off. This is brought about by the gas
flame passing straight through the cloth. It is not necessary to
describe the gas singeing machine in detail. Singeing machines should be
kept scrupulously clean and free from fluff, which is liable to collect
round them, and very liable to fire. Some machines are fitted with a
flue having a powerful draught which carries off this fluff, away from
any source of danger.
(3) =Singeing Wash.=--After being singed the cloths are run through a
washing machine to remove by water as much of the loose charred fibres
as possible. The construction of a washing machine is well known. It
consists of a pair of large wooden rollers set above a trough containing
water and into which a constant stream of water flows. In the trough is
also fixed another wooden roller and the pieces are passed round this
bottom roller and between the top rollers. The cloth is passed through
and round the rollers several times in a spiral form so that it passes
through the water in the trough frequently, which is a great advantage,
as the wash is thus much more effectual. The pressure between the two
top rollers presses out any surplus water. The operation scarcely needs
any further description.
(4) =Lime Boil.=--After the cloth leaves the singeing or grey wash, as
it is often called, it passes through the liming machine, which is made
very similar to the washing machine. In this it passes through milk of
lime, which should be made from freshly slaked lime. The latter maybe
prepared in a pasty form in a stone cistern. The lime used should be of
good quality, free from stones, badly burnt pieces or any other
insoluble material, so that when slaked it should give a fine smooth
pasty mass.
Lime should not be slaked too long before using, as it absorbs carbonic
acid from the atmosphere, whereby carbonate of lime is formed, and this
is useless for liming cloth. The pasty slaked lime may be mixed with
water to form the milk of lime, and this can be run from the cistern in
which it is prepared into the liming machine as it is required; the
supply pipe should be run into the bottom of the trough of the liming
machine and not over the top, in which latter case it may splash on to
the cloths and lead to overliming, which is not to be desired on account
of its liability to rot the cloth. The amount of lime used varies in
different bleachworks, and there is no rule on the subject; about 5 lb.
to 7 lb. of dry lime to 100 lb. of cloth may be taken as a fair quantity
to use.
The lime boil has for its object the removal or rather the
saponification of the resinous and fatty matters present in the grey
cloth, either naturally or which have been added in the process of
weaving, or have got upon the cloth accidentally during the processes of
spinning and weaving. With these bodies the lime forms insoluble lime
soaps; these remain in the cloth, but in a form easily decomposable and
removable by treatment with acids and washing. Soda or potash is not
nearly so good for this first boiling as lime--for what reason is
somewhat uncertain, but probably because they form with the grease in
the cloths soluble soaps, which might float about the kier and
accumulate in places where they are not required and thus lead to
stains, whereas the insoluble lime soap remains where it was formed. The
lime also seems to attack the natural colouring matter of the cotton,
and although the colour of the limed cloth is darker than before
boiling, yet the nature of the colour is so altered that it is more
easily removed in the after processes. Besides these changes the
starchy matters put into the cloth in the sizing are dissolved away.
Great care should be taken to see that the goods are evenly laid in the
kiers, not too tight, or the liquor will not penetrate properly; and not
too slack, or they will float about and get entangled and more or less
damaged. Then again care should be taken, especially when using
low-pressure kiers, to see that the supply of liquor does not get too
low, in which case the goods in the upper part of the kier are liable to
get dry and are tendered thereby. So long as the goods in the kiers are
not allowed to get dry there is no risk of damage; this trouble rarely
arises with the Barlow and injector kiers. The inside of the kiers
should be kept well limed, so that the goods shall not come in contact
with the bare iron or metal of which the kier is constructed, as this
would be very likely to lead to stains being produced which are by no
means easy to remove. It is usual, and it is a good plan with almost all
kinds of kiers, except the Mather and Edmeston kiers, to put a number of
large pebbles or small stones at the bottom of the kier, which serves to
make a false bottom on which the goods rest and through which the liquor
penetrates and flows away. Before using, the stones should be well
washed to free them from dirt and grit.
[Illustration: Fig. 4.--Mather & Platt's Low-pressure Bleaching Kier.]
The lime boil is carried out in what are called "kiers". Many forms of
kiers have been devised, but the one in most general use is that known
as the "injector kier," of which a drawing is given in Fig. 4, of the
form made by Messrs. Mather & Platt of Salford. Injector kiers are made
to work either under a pressure of 40 lb. to 50 lb. of steam per square
inch, when they are called high-pressure kiers, or at a pressure of 15
lb. to 20 lb., when they are called low-pressure kiers. The one shown in
the drawing is intended for low-pressure kiers. The principle of
construction is the same in all, the details varying somewhat with
different makers. Injector kiers consist of a hollow, upright iron
cylinder made of plates riveted together; the top is made to lift off,
but can be fastened down tightly by means of bolts and nuts as shown in
the drawing. From the bottom, and placed centrally, rises a pipe, known
as the puffer pipe; this terminates at the top in a rose arrangement.
The lower end of the pipe is perforated. A jet of steam is sent in at
the bottom of this pipe, and by its force any liquor at the bottom of
the kier is forced up the puffer pipe and distributed in a spray over
any goods which may be in the kier. The liquor ultimately finds its way
to the bottom of the kier ready to be blown up again. This circulation
of the liquor can be maintained for any length of time and through its
agency every part of the goods gets thorough and effectual treatment.
The length of the boil depends upon the kiers; with the open kier about
ten hours are usually given, with the Barlow and injector kiers, working
at a pressure of 40 lb. to 50 lb., six to seven hours are given.
(5) =Lime or Grey Sour.=--After the lime boil, the next operation is
that of the lime sour or grey sour as it is often called to distinguish
it from a subsequent souring. The souring is done in a machine
constructed in the same way as a washing machine; the trough of the
machine is filled with hydrochloric acid at 2° Tw., which is kept ready
prepared in a stone cistern and run into the machine as required (it is
not advisable to use acid stronger than this). After passing through the
sour the goods are piled in a heap on the stillage for a few hours. The
acid attacks the lime soap which was formed during the lime boil,
decomposes it and dissolves out the lime with the formation of calcium
chloride, while the fat of the soap is liberated, the former is washed
away in the subsequent washing, while the latter remains to a large
extent on the goods, and is removed by the lye boil that follows.
Sulphuric acid is not so satisfactory to use for the lime sour as
hydrochloric acid, because it forms with the lime the insoluble sulphate
of calcium, which is difficult to entirely remove from the goods,
whereas the chloride is very soluble and is entirely eliminated from the
goods by the washing that follows.
It is advisable to keep the acid at a uniform strength in the machine.
The Twaddell is here of no use as an indicator of the actual strength,
because the lime which the acid dissolves, while it neutralises and
reduces the strength of the acid, actually raises the Twaddell, under
which circumstance the only safe method is a chemical test. This can be
carried out very simply and with a sufficient degree of accuracy by the
workmen, and if it be done at regular intervals during the souring, and
the supply of the fresh acid be regulated, the sour will be kept at a
more uniform strength and more uniform results will be obtained than if
the souring were done in a more empirical fashion. The test is best and
most easily done as follows:--
Prepare a solution of 1 oz. of the powdered high strength 98 per cent.
caustic soda in 1 pint of water, weighing and measuring these quantities
very carefully. Now take a tall, narrow, white bottle of about 5 oz.
capacity and make a mark on the neck. Fill this bottle with the test
solution.
Now take exactly 5 ozs. of freshly prepared sour of 2° Tw., pour into a
jar, and add carefully some of the soda-test solution until a piece of
cloth dyed with turmeric is turned brown, when the acid is neutralised.
Now make a mark on the bottle of soda to show how much has been used. In
all subsequent tests of the sour 5 ozs. should always take the same
quantity of soda solution; if it takes less it is too weak, if more it
is too strong; the remedy in each case is obvious. It is worth while to
graduate the test bottle for 1°, 3°, 4°, 5° Twaddell, as well as for 2°
Tw. acid.
After the souring it is often the custom to pile the goods on to a
wooden stillage, but the goods should not be left too long so piled up
for they may become dry, either entirely or in parts. In any case, as
the goods dry the acid becomes concentrated and attacks them and makes
them tender, which is not at all desirable. Therefore, if it is not
convenient to proceed with them for some time after souring, they
should be moistened with water from time to time, but it is best to wash
them off at once, whereby they are made ready for the next operation.
(6) =Lye Boil.=--This is, perhaps, the most important operation in the
whole process of bleaching, especially if the cloths are going to be
printed in the so-called madder style with alizarine colours, or
otherwise stains are liable to occur in the final stage, and it is then
sometimes difficult to put the blame for these upon the right shoulders.
In principle the lye boil is simple, consisting in boiling the goods
with a solution of soda ash, or caustic soda. The quantity of ash used
varies in different works, as might naturally be expected; from 170 lb.
to 200 lb. of ash to 10,000 lb. of cloth is a fair proportion to use.
The length of boil averages about four hours, certainly not less than
three should be given, and it is not necessary to give more than five
hours in either ordinary kiers, with central puffer pipe, or in injector
kiers.
Care should be taken to see that the goods are well packed into the
kiers, not too tightly or the lye will fail to penetrate equally all
through, and this is important if a uniform bleach is desired; neither
should they be too loose, or they will float about and get torn. It is
not necessary to be particular about the quantity of water used, except
that it must be sufficient to keep the goods well covered, and still
have enough to keep the circulation energetic. When the water is not
sufficient in amount the goods get somewhat dry; there is then a
liability to tendering, but with plenty of water there is no fear of any
damage being done during a boil with alkali. Some works use caustic soda
instead of soda ash in which case less is required, from 120 lb. to 150
lb. to 10,000 lb. of cloth, otherwise no alteration is made in the mode
of boiling.
This lye boil clears away the fatty and waxy matter left in the goods
after the lime sour, and thus prepares the way for the next boil. There
is no advantage in using caustic soda in this preliminary boil, soda ash
being just as effective and cheaper.
(7) =Resin Boil.=--Following the lye boil is the resin boil which
consists in boiling the goods in a resin soap liquor. This is made as
follows: a soda ash liquor of about 15° to 20° Tw. is prepared, and into
this is thrown resin, broken up into small pieces.
The whole is boiled up until the resin is dissolved, and then as much
more is added in small quantities as the alkali will take up. The soda
liquor should not be much weaker than 20° Tw., it will then be heavier
than the resin which will float on the top, it is found to dissolve
quicker and better than when the liquor is weak, in which case, the
resin would sink to the bottom of the boiler and would there melt into a
single mass difficult to dissolve. The resin soap liquor when made is
ready to be used. The proportions of resin and alkali used in the boil
vary in different works, but, as a rule, the quantities for 10,000 lb.
of goods are 430 lb. of 58 per cent. soda ash, 180 lb. of resin, and 80
lb. of 70 per cent. caustic soda. Too much resin should be avoided, as
it is found that with an excess the whites obtained are not nearly so
good as when the right quantity is used; on the other hand, too little
acts much in the same way. It may be taken that from 1-1/2 to 1-3/4 per
cent. of the weight of the goods is about the right proportion; 1 per
cent. being too little, and 2 per cent. too much. The quantity of soda
used should be rather more than twice that of the resin, from 3-1/2 to 4
per cent. The length of boil is usually about twelve hours in a
low-pressure kier; in a high-pressure kier about seven hours is
sufficient.
What the special function of the resin is in this boil is not
definitely known; but experience, both on a large and small scale,
proves that it is essential to obtaining a good white for alizarine
printing; without it, when the goods are dyed with alizarine after the
mordants have been printed on, they frequently take a brown stain--with
the resin this never or but rarely happens.
(8) =Wash.=--After the lye boils the goods must be washed, and it is
important that this be done in as thorough a manner as possible. With
the object of accomplishing this most thoroughly many washing machines
have been invented, the main idea in all being to bring every part of
the goods into contact with as much water as possible. Bridson's is an
old form, and a very good one, the principle of this machine is to cause
the cloth to pass to and fro, and to flap upon the surface of the water
in the trough of the machine.
Furnival's square beater works on much the same principle, and does its
work effectively. More modern washing machines are those of Birch,
Farmer, Mather & Platt, and Hawthorne, where by the peculiar
construction of the rollers and the use of beaters the cloth is very
effectually washed. These machines are much more economical in the use
of water than the older forms, and yet they do their work as well, if
not better.
(9) =Chemicing.=--This is the actual bleaching operation, familiarly
known as "chemicing," that is, the treatment of the goods with bleaching
powder. The previous operations have resulted in obtaining a cloth free
from grease, natural or acquired, and from other impurities, but it
still has a slight brownish colour. This has to be removed before the
goods can be considered a good white, which it is the aim of every
bleacher they should be.
To get rid of this colour they are subjected to some final operations,
the first of which is now to be considered. The chemicing consists in
running the goods through a weak solution of bleaching powder (chloride
of lime), piling the goods up into heaps, and allowing them to lie
overnight, the next day they are finished. As the cloth has received, or
ought to have received, a thorough bottoming, only a weak bath of chemic
is required, about 1/2 to 1° Tw. is quite sufficient. The solution is
prepared in a stone cistern. There is very little difficulty in making
it, the only precaution necessary is to have the solution quite clear
and free from undissolved particles, for if these get upon the cloth
they will either lead to the production of minute holes, or they may
overbleach the fibre, which in such case will have the power of
attracting excess of colour in any subsequent dyeing process and thus
lead to stains, the origin of which may not be readily grasped at the
first sight.
It is best, therefore, either to allow the solution to settle in the
cistern till quite clear, which is the simplest way, or to filter
through cloth.
The chemicing is best done cold and with weak solution, at 1/2° Tw.
rather than 1° Tw. Warming the liquor increases the rapidity of the
bleaching action, but there is a risk of over-chloring, which must be
avoided as far as possible, because there is then danger of tendering
the fibre, moreover, such over-chlored cloth has an affinity for
colouring matters that is not at all desirable, as it leads to the
production of stains and patches in the dyeing operations. It is much
better, when a single chemicing does not bleach the cloth sufficiently
and give a white, to run the cloth twice through a weak liquor rather
than once through a strong liquor.
Although the chemicing is followed by a sour, which, acting on the
bleaching powder, liberates chlorine that bleaches the fabric, yet the
greatest proportion of the bleaching effect is brought about while the
pieces are being piled up into heaps between the chemicing and the sour.
In this state they should be left for some hours, covered over with a
damp sheet, care being taken that they are not left piled so long as to
become dry, as in this event there is a great risk of tendering the
cloth or fabric; it is, therefore, a good plan to moisten them with a
little water from time to time. They should not be tightly piled up, but
be as loose as possible, so that the air can get to them, as it is the
carbonic acid and other acid vapours in the air, that by acting on the
chemic causes slow liberation of chlorine, which effects the bleaching
of the goods.
(10) =White Sour.=--After the chemicing the goods are treated to a sour,
for which purpose either hydrochloric acid or sulphuric acid may be
used.
Hydrochloric acid possesses the advantage of forming a more soluble salt
of lime (calcium chloride) than does sulphuric acid (calcium sulphate),
and it has a more solvent action upon any traces of iron and other
metallic oxide stains which may be present in the goods. On the other
hand, on account of its fuming properties, it is unpleasant to work
with. The souring is done by passing the goods through an acid liquor at
2° Tw. strong and piling for two or three hours, after which it is
washed. This final washing must be thorough, so that all traces of acid
and chemic are washed out, otherwise there is a tendency for the goods
to acquire a yellowish colouration.
So far the routine has been described of the so-called madder bleach,
the most perfect kind of bleach applied to cotton cloths. Besides this
two other kinds of bleach are distinguished in the trade. Turkey red and
market bleach. The former is used when the cloth or yarn is to be dyed
plain or self-coloured with delicate shades with Alizarine; the latter
is used for cloth sold in the white. As the operations involved in
producing these are identical in their method of manipulation to those
already described, it will only be necessary to give an outline of the
process for each one.
=Turkey Red Bleach=--(1) Rinse through water into a kier and boil for
two hours. (2) Lime boil for three to four hours. The amount of lime
required is rather less than what is used with the madder bleach, from
2-1/2 lb. to 3 lb, lime to each 1 cwt. of goods being quite sufficient.
(3) Souring as in the madder bleach. (4) Lye boil, using about 100 lb.
caustic soda to a ton of goods, and giving ten hours' boil. (5) Second
lye boil using about 50 lb. soda ash to a ton of goods, after which the
goods are well washed. (6) Chemicing as with the madder bleach. (7)
Souring as with the madder bleach, then washing well.
This represents an average process, but almost every bleacher has his
own methods, differing from the above in some of the details and this
applies to all bleaching processes. It is obvious that the details may
be varied to a great extent without changing the principles on which the
process depends.
=Market Bleach=--Here all that requires to be done is to get the cloth
of a sufficient degree of whiteness to please the eye of the customer.
Market bleachers have, however, to deal with a wider range of goods than
is dealt with in the former kinds of bleaches, from very fine muslins to
very heavy sheetings. Now it is obvious from a merely mechanical point
of view, that the former could not stand as rough a process as the
latter, therefore there must be some differences in the details of
muslin bleaching and sheeting bleaching. Then again with goods sold in
the white, it is customary to weave coloured headings or markings, and
as these have to be preserved, to do so will cause some slight
alteration of the details of the bleach with this object. On all these
points it is difficult to lay down general rules because of the very
varying feature of the conditions which are met with by the market
bleacher.
The resin boil may be omitted, only two lye boils being required, and
these need not be so long or of such a searching character as the
corresponding boils of the madder bleach. Outlines of two or three such
processes, which are now in use in bleach works, will serve to show the
general routine of a market bleach. The proportions given are calculated
for 10,000 lb. of goods:--
(1) Lime boil, using 500 lb. of lime, and giving a twelve-hours' boil.
(2) Grey sour, hydrochloric acid of 2° Tw., then wash well.
(3) Lye boil, 100 lb. caustic soda, 70 per cent. solid, ten to twelve
hours' boil; wash.
(4) Second lye boil, 100 lb., 58 per cent. soda ash, twelve-hours' boil.
(5) Chemic, bleaching powder liquor at 1° Tw., boil for three hours;
wash.
(6) White sour, sulphuric acid at 2° Tw.; wash well.
The length of boil with the lime and lyes will depend upon the quality
of the goods, heavy goods will require from two to three hours longer
than will light goods, such as cambrics, the time given above being that
for heavy goods, sheetings, etc.
Another process is the following:--
(1) Lime boil, using 480 lb. lime, and boiling for ten hours.
(2) Grey sour, hydrochloric acid at 2° Tw.; wash.
(3) Lye boil, 300 lb. soda ash, 58 per cent.; 50 lb. caustic soda, 70
per cent., and 30 lb. soft soap, giving ten hours' boil; wash.
(4) Chemic as above.
(5) White sour as above; wash well.
A smaller quantity of lime is used here, but on the other hand the lye
boil is a stronger one. This process gives good results. Some bleachers
do not use lime in their market bleaches, but give two lye boils, in
which case the process becomes:--
(1) Lye boil, using 140 lb. caustic soda, of 70 per cent., giving ten
hours' boil and washing well.
(2) Second lye boil, using 120 lb. soda ash, 58 per cent., and giving
ten hours' boil; wash.
(3) Chemic as above.
(4) White sour as above; wash well.
Light fabrics, such as laces, lace curtains, muslins, etc., cannot stand
the strain of the continuous process, and they are therefore subjected
to a different bleaching process, which varies much at different bleach
works. One method is to lime by steeping for an hour in a weak lime
liquor, using about 2 lb. of lime to 100 lb. of goods; they are then
boiled in the kier for eight hours, after which they are washed. This
washing is done in what are called dash wheels, large hollow wheels, the
interior of each being divided into four compartments. Into these the
goods are put, and the wheel is caused to revolve, while at the same
time a current of water flows with some force into the interior of the
wheel and washes the goods.
The wheels do their work well, and the action being gentle the finest
fabrics can be washed without fear of any damage. It is necessary that
the speed at which they are driven should be such that as the wheel
revolves the goods are thrown from side to side of each compartment; if
the speed be too slow they will simply slide down, and then they do not
get properly washed; on the other hand, if the speed be too great then
centrifugal action comes into play and the goods remain in a stationary
position in the wheels with the same result. As to the amount of
washing, it should be as before. After this washing they are boiled
again in the kier with soda ash, using about 8 lb. ash for 100 lb. goods
and giving seven hours' boil, which, after washing, is followed by a
second boil with 5 lb. ash and 4 lb. soft soap for each 100 lb. of
goods, giving eight hours' boil. They are then washed and entered into
the chemic. The chemicing is done in stone cisterns, which are fitted
with false bottoms, on which the goods can rest, and which allow any
insoluble particles of bleaching powder to settle out and prevent them
from getting on the goods. The liquor is used at the strength of about
1/2° Tw., and the goods are allowed to steep about two hours; they are
then placed in a hydro-extractor and the surplus chemic is whizzed out,
after which they are steeped in sour of hydrochloric acid at 1° Tw.,
kept in a stone cistern, the goods being allowed to steep for two hours.
Next they are washed, well whizzed, passed through a blueing water,
whizzed again, and dried. The remarks made when describing similar
operations of the madder bleach as to the action, testing, etc., of the
chemicals, are equally applicable here.
Another plan of bleaching fine fabrics is shown in outline in the
following scheme:--
(1) Wash; boil in water for two hours.
(2) Boil in soda for five hours, using 80 lb. soda ash of 58 per cent.,
and 30 lb. soft soap for 1,000 lb. of goods.
(3) Second soda boil, using from 40 lb. to 50 lb. soda ash, and 15 lb.
to 20 lb. soft soap, giving four hours' boil; after each soda boil the
goods should be washed.
(4) Chemic, using bleaching powder liquor at 1/2° Tw., allowing them to
steep for two hours, then wash and whiz.
(5) White sour, using hydrochloric acid at 2° Tw., steeping two hours;
wash.
A further extension of the same process is sometimes given for the best
goods, which consists, after the above, in giving:--
(6) A third soda boil, using 25 lb. to 30 lb. soda ash and 15 lb. to 20
lb. soft soap, giving one hour's boil; washing.
(7) Chemic as before.
(8) Sour as before, after which the goods are well washed.
In the bleaching of Nottingham lace curtains for the soda boils there
is used what is called the "dolly," which consists of a large round
wooden tub about 5 feet to 6 feet in diameter and about 2 feet 6 inches
to 3 feet deep; this is made to revolve slowly at about one revolution
per minute. Above the tub on a strong frame are arranged four stampers
or beaters, which are caused to rise and fall by means of cams. The
goods are placed in the tub with the scouring liquors and the dolly is
set in motion, the beaters force the liquor into the goods, and the
revolution of the tub causes the beaters to work on a fresh portion of
the goods at every fall.
This is rather an old-fashioned form of machine, and is being replaced
by more modern forms of boiling kiers. In bleaching certain kinds of
muslins in which the warp threads are double, and in the case of lace
curtains, it is necessary to endeavour to keep the threads as open and
prominent as possible. This cannot be done with the continuous process,
which puts a strain on the threads and thus effaces their individuality.
To avoid this the fabrics have to be dealt with in bundles or lumps, and
on these no strain is put, therefore every thread retains its
individuality. The process above described is applicable.
=Yarn Bleaching.=--Yarn is supplied to the bleacher in two forms: (1)
warps in which the length of the threads may vary from as little as 50
to as much as 5,000 yards; these can be dealt with in much the same
manner as a piece of cloth, that is, a continuous system can be adopted;
(2) hanks, which are too well known to require description. Sometimes
yarn is bleached in the form of cops, but as the results of cop
bleaching are not very satisfactory it is done as little as possible.
=Warp Bleaching.=--The warp, if very long, is doubled two, three or four
times upon itself, so as to reduce its length. Care should be taken that
the ends of the warp are tied together to prevent any chance of
entangling, which would very likely happen if the ends were left loose
to float about. As a rule, warps are not limed, but the adoption of the
liming would assist the bleaching. In outline warp bleaching consists of
the following operations:--
(1) Lye boil, using 30 lb. caustic soda, 70 per cent., and 50 lb. soda
ash, 58 per cent., giving six hours' boil, and washing.
(2) Sweeting, boil with 80 lb. soda ash, 58 per cent., for two hours.
(3) Washing.
(4) Chemicing, bleaching powder liquor at 1° Tw., washing.
(5) Sour, sulphuric acid at 2° Tw,. washing well.
(6) Hydro extracting and drying.
About 2,000 to 3,000 lb. of warps are usually treated at one time.
The machinery used may be the same as that used in the cloth bleach, and
each operation may be conducted in the same manner. In some warp
bleachworks, while the kiers are made in the same way, the other
machines are made differently. The chemicing and souring is done in
strong cisterns provided with a false bottom; in these the warps are
allowed to remain for about two hours. A more complicated form of
chemicing cistern is also in use. This is made of stone, and is provided
with a false bottom. Above is a tank or sieve, as it is called, having a
perforated bottom through which the liquor flows on the warp in the
cistern below.
Under the chemicing cistern is a tank into which the liquor flows, and
from which it is pumped up into the sieve above. A circulation of liquor
is thus kept up during the whole of the operation. Owing to the action
of the chemic or acid on the metal work of the pump there is great wear
and tear of the latter, necessitating frequent repairs. This is a defect
in this form of chemicing machine. For drying the warps a
hydro-extractor is first used to get the surplus liquor from the goods.
This machine is now well known, and is in use in every bleachworks,
where it is familiarly known as the "whiz," and the operation is
generally called whizzing. Hydro-extractors are described under the head
of "Dyeing Machinery".
The actual drying of the warps is done over the "tins" as they are
called. These are a number of large cylinders measuring about 20 inches
in diameter and about--for warp drying--5 feet long. Usually they are
arranged vertically in two tiers, each tier consisting of about five
cylinders, not arranged directly one above another but in a zig-zag
manner, the centres of the first, third and fifth being in one line, and
the centres of the others in another line. The cylinders are made to
revolve by suitable driving mechanism, and into them is sent steam at
about 5 lb. to 10 lb. pressure, which heats up the cylinders, whereby
the warp passing over them is dried. This drying may be partial or
complete, being regulated by the speed at which the warps pass over the
cylinders and by the quantity of steam passed into the same. The quicker
the speed and the smaller the amount of steam, the less the warps are
dried; while, on the other hand, the slower the speed and the larger the
amount and greater the pressure of the steam, the quicker and more
thoroughly are the warps dried. As there is a great deal of water formed
in the cylinders by the condensation of the steam, means are always
provided for carrying off this water, as its retention in the cylinders
often leads to serious results and damage to the machine.
=Hank Bleaching.=--So far as the chemical part of hank bleaching is
concerned it does not differ from that of warp bleaching; the same
operations and proportions of chemicals may be used and in the same
order, but there is some difference in the machinery which is used. The
hanks may be manipulated in two ways: they may be either kept in
separate hanks, which is the method mostly in vogue in modern
bleach-houses, or they may be linked together in the form of a chain.
In the latter case the operations and the machinery may be the same as
used in the madder bleach, with a few unimportant minor differences. In
the final washing the dumping machine is used, which consists of two
wooden bowls set over a wooden trough containing the wash waters. The
top bowl is covered with a thick layer of rope and merely rests on the
bottom bowl by its own weight, and is driven by friction from the
latter. The chain of hanks passing through between the two bowls has the
surplus liquor squeezed out of it, and as there is considerable increase
in the thickness at the points of linkage between the hanks, when these
pass through the bowls they lift up the top bowl, which, when the thick
places have passed through, falls down with a sudden bump upon the thin
places, and this bumping drives out all the surplus liquor and drives
the liquor itself into the very centre of the hanks, which is sometimes
an advantage.
In modern bleach-houses the chain form is gradually giving place to the
method of bleaching separate hanks, partly because so many improvements
have been made in the hank-bleaching machinery of late years, which
enables bleachers to handle the yarn in the form of separate hanks
better than they could do formerly; and as bleaching in separate hanks
means that the cotton is kept in a more open form, and is thus more
easily penetrated by the various liquors which are used, it follows that
the bleach will be better and more thorough, which is what the bleacher
aims at. At the same time weaker liquors or, what is the same thing,
less material can be used, which means a saving in the cost of the
process. For bleaching yarn in the hank the following process may be
followed with good results:--
(1) Lye boil, using 1,000 lb. yarn, 40 lb. caustic soda of 70 per cent.,
and 50 lb. of soda ash of 58 per cent., giving five to six hours' boil
at low pressure.
(2) Wash through washing machine.
(3) Second lye boil, using 40 lb. soda ash of 58 per cent., and giving
two to three hours' boil, wash again through a washing machine.
(4) Chemic as in warp bleaching.
(5) Sour as in warp bleaching.
(6) Wash well.
(7) Hydro extract and dry.
Sometimes, if the yarn is to be sold in thread form, before the last
operation it passes through another, _viz._, treating with soap and blue
liquors, which will be dealt with presently.
The lye boils are done in the ordinary kiers, and do not call for
further notice, except that in filling the goods into the kiers care
should be taken that while sufficiently loose to permit of the alkaline
liquors penetrating through the hanks properly, yet they should be so
packed that they will not float about and thus become entangled and
damaged.
The washing is nowadays done in a special form of washing machine,
designed to wash the hanks quickly and well with as little expenditure
of labour and washing liquor as possible. There are now several makes of
these washing machines on the market, most of them do their work well,
and it is difficult to say which is the best. Some machines are made to
wash only one bundle at once, while others will do several bundles.
Generally the principle on which they are constructed is the same in
all, a trough containing the ash liquor, over which is suspended a
revolving reel or bobbin, usually made of wood or enamelled iron, the
bobbin being polygonal in form so that it will overcome readily any
resistance the yarn may offer and carry the hank round as it revolves.
The hank dips into the wash liquor in the trough, and as it is drawn
through by the revolution of the bobbin it is washed very effectually.
The moving of the hank opens out the threads, and thus the wash liquor
thoroughly penetrates to every part of the hank, so that a few minutes'
run through this machine thoroughly washes the yarn. A constant stream
of clean water is passed through the trough. This machine may also be
used for soaping and sizing the hanks if required. By extending the
trough in a horizontal direction, and increasing the number of reels or
bobbins, the quantity of material that can be washed at one time can be
extended, although not to an indefinite extent. The workman can start at
one end of the machine and fill all the bobbins with yarn, by the time
he has finished this the first bobbinful will have been washed
sufficiently and can be taken off and replaced with another quantity of
yarn, and thus one by one the bobbins may be emptied and refilled, which
means that a considerable amount of material can be got through in the
course of a day. To avoid the labour of walking to and fro to fill and
refill the bobbins, washing machines are constructed in which the trough
is made in a circular form. The bobbins are placed at the ends of radial
arms which are caused to revolve round over the trough, the workman is
stationed constantly at one part of the circle, and as the arms pass him
in their motion round the trough he takes off the washed hanks and puts
on the unwashed hanks. By this machine he is saved a very considerable
amount of labour, and is able to do his work in a more convenient
manner. The yarn is well washed in such a machine. The trough may be
entire or it may be divided into a number of compartments, each of which
may contain a different kind of wash liquor if necessary. Of course it
goes almost without saying that in all these machines the liquors in
them may be heated up by means of steam pipes if required.
The chemicing and souring of the hanks does not call for special
mention, beyond the fact that these operations are done in the same
manner as warp bleaching. In Fig. 5 is shown Mather & Platt's
yarn-bleaching kier, which is designed to bleach cotton yarn, either in
hanks or in the warp forms, without removing it from the vessel into
which it is first placed. The process is as follows: The hot alkali
solution is circulated by means of a distributing pipe through the
action of an injector or centrifugal pump to scour the yarn; then water
is circulated by means of a centrifugal pump for washing. The chemic and
sour liquors are circulated also by means of pumps, so that without the
slightest disturbance to the yarn it is quickly and economically
bleached.
[Illustration: FIG. 5.--Mather & Platt's Yarn-bleaching Kier.]
STAINS AND DAMAGES IN BLEACHED GOODS.
Some of the stains in bleached goods which are met are beyond the
control of the bleacher to avoid, while others are due to various
defects in the process. Now the subject of stains can only be dealt with
in a very general way, because of the varying manner in which they
arise. The recognition of the particular way in which the stains have
been formed is sometimes difficult to discover. First, there are iron
stains, which are the most common kind of stains that a bleacher is
troubled with. These generally make their appearance in the form of red
spots of greater or less extent. As a rule they are not visible before
the pieces are fully bleached. Their origin is varied. Sometimes they
arise from the machinery; if the kiers are not kept thoroughly
whitewashed out, there is a great liability to produce iron stains.
Every other machine which is used in the process is made of iron, and
should be kept free from rust, or the chances of stains are considerably
increased. The water used in the bleaching must be free from iron. A
small trace will not make much difference, but some waters contain a
great deal of iron, so much so that they are absolutely unusable for
bleaching purposes. Iron stains are often due to a very curious cause:
the dropping of the oil used in the spinning or weaving machinery on to
the cotton during the process of manufacture. This oil is often charged
with iron derived from the wear and tear of the machinery, and which
often gets fixed in the form of red spots of oxide on the fibre. Iron
stains cannot readily be extracted.
Oil stains are also common. These take the form of bright yellow stains
in various shapes, sometimes extending along the piece in streaks, at
other times in patches in various places about the piece. Generally
these oil stains do not make their appearance as soon as the piece is
bleached, and often the bleacher sends out his goods quite white and
apparently all right, and yet soon afterwards comes a complaint that the
goods are stained yellow. One cause of these yellow oil stains can be
traced to the use of paraffin wax in the sizing of the warps. In this
case the stains are more or less streaky in form, and extend along the
length of the piece. They are due to the fact that paraffin wax is not
saponifiable by the action of the alkalies used in the process, and is
therefore not extracted. When the goods are chemiced the chlorine acts
upon the paraffin and forms chlorine compounds, which are acted upon by
light, and turn yellow by exposure to that agent and to the atmosphere.
Paraffin, when used for the sizing of warps, may sometimes be completely
extracted from the fabric, but this depends upon the proportion of
tallow or other fat which is used in the composition of the sizing
grease. If the paraffin is only present in small quantities, and the
grease well mixed, then it may be possible to extract all the paraffin
out of the fabric during the bleaching process, but if the paraffin is
in large proportion, or the grease not well mixed, it is scarcely
possible to extract it all out, and stains must be the result. These
stains can hardly be considered the fault of the bleacher, but are due
to the manufacturer of the cloth using cheap sizing compositions on his
warps. There are no means which can be adopted before bleaching to
ascertain whether paraffin exists in the cloth. If found to be present,
the remedy which is the easiest practically is to saturate the cloth
with a little olive oil, or better, pale oleic acid. Allow the fatty
matter to soak well in, and then boil the goods in a little caustic
soda. Another cause of oil stains is the use of mineral oils in the
lubrication of cotton machinery. These mineral oils partake of the
nature of paraffin in their properties, and therefore they are
unsaponifiable by the action of alkalies. Like paraffin wax, they
resist the bleaching process, and much in the same manner produce
stains. Oil stains show themselves in various forms--sometimes as spots.
These may be due to the splashing of oil from the spindles during the
process of spinning, or they may be in patches of a comparatively large
size over the pieces.
These are perhaps due to the oil dropping on to the piece during the
process of weaving when in the loom. The oils used for the lubrication
of spinning and weaving machinery should contain a fair proportion of
some fatty oil, such as olive or rape or cocoanut oil. Not less than 10
per cent. should be used. More would be better, but the cost of course
would be greater and oil is an item with spinners and manufacturers.
Stains are occasionally due to other causes rather too numerous to be
dealt with in detail, and sometimes these stains only appear once in a
lifetime, and often do not make their appearance during the bleaching
process, but only in after dyeing or calico printing processes in
curious ways the causes of which are very baffling to find out.