1. The matter which prevents the moistening of cotton wool may be
separated by means of alcohol, which, when evaporated, leaves thin
yellowish scales, soluble in alkalies, in acids, and even in a large
quantity of boiling water. For a long time the bleaching process
commenced with the removal of this resinous stuff, by passing the cloth
or the yarn through an alkaline ley. This was called scouring; it is now
nearly laid aside.
2. The colouring matter of cotton seems to be superficial, and to have
no influence on the strength of the fibres; for the yarn is found to be
as strong after it has been stripped by caustic soda of its resinous and
colouring matters, as it was before. The colouring matter is slightly
soluble in water, and perfectly in alkaline leys. When gray calico is
boiled in lime water, it comes out with a tint darker than it had
before; whence it might be supposed that the colouring matter was not
dissolved out, even in part. This, however, is not the case; for if we
filter the liquor, and neutralise it with an acid, we shall perceive
light flocks, formed of the resinous substance, united with the
colouring matter. The dark colour of the cloth is to be ascribed solely
to the property which lime possesses of browning certain vegetable
colours. This action is here exercised upon the remaining colour of the
cloth.
It may be laid down as a principle, that the colouring matter is not
directly soluble by the alkalies; but that it becomes so only after
having been for some time exposed to the joint action of air and light,
or after having been in contact with chlorine. What change does it
thereby experience, which gives it this solubility? Experiments made
upon pieces of cloth placed in humid oxygen, in dry oxygen, in
moist chlorine, and in dry chlorine, tend to show that hydrogen is
abstracted by the atmosphere; for in these experiments proofs of
_dis-hydrogenation_ appeared, and of the production of carbonic acid. In
all cases of bleaching by chlorine, this principle combines immediately
with the hydrogen of the colouring matter, and forms muriatic acid,
while the carbon is eliminated.
Undoubtedly water has an influence upon this phenomenon, since the
bleaching process is quicker with the humid chlorine than with the dry;
but this liquid seems to act here only mechanically, in condensing the
particles of the gas into a solution. We should also take into account
the great affinity of muriatic acid for water.
3. The weaver’s dressing is composed of farinaceous matters, which are
usually allowed to sour before they are employed. It may contain glue,
starch, gluten; which last is very soluble in lime-water.
4. When the dressing gets dry, the hand-weaver occasionally renders his
warp-threads more pliant by rubbing some cheap kind of grease upon them.
Hence it happens, that the cloth which has not been completely freed
from this fatty matter will not readily imbibe water in the different
bleaching operations; and hence, in the subsequent dyeing or dunging,
these greasy spots, under peculiar circumstances, somewhat like
lithographic stones, strongly attract the aluminous and iron mordants,
as well as the dye stuffs, and occasion stains which it is almost
impossible to discharge. The acids act differently upon the fatty
matters, and thence remarkable anomalies in bleaching take place. When
oil is treated with the acetic or muriatic acid, or with aqueous
chlorine, it evolves no gas, as it does with the sulphuric and nitric
acids, but it combines with these substances so as to form a compound
which cannot be dissolved by a strong boiling ley of caustic soda.
Carbonic acid acts in the same way with oil. On the other hand, when the
oils and fats are sufficiently exposed to the air, they seize a portion
of its oxygen, and become thereby capable of saponification, that is,
very soluble in the alkalies.
5. When the hand-weaver’s grease continues in contact for a night with
the copper dents of his reed, a kind of cupreous soap is formed, which
is sometimes very difficult to remove from the web. Lime-water does not
dissolve it; but dilute sulphuric acid carries off the metallic oxide,
and liberates the margaric acid, in a state ready to be acted on by
alkalis.
6. When cloth is boiled with milk of lime, the grease which is
uncombined unites with that alkaline earth; and forms a calcareous soap,
pretty soluble in a great excess of lime-water, and still more so in
caustic soda. But all fats and oils, as well as the soaps of copper and
lime, cease to be soluble in alkaline leys, when they have remained a
considerable time upon the goods, and have been in contact with acetic,
carbonic, muriatic acids, or chlorine. These results have been verified
by experiment.
7. Cotton goods are sometimes much soiled, from being sewed or tamboured
with dirty hands; but they may be easily cleansed from this filth by hot
water.
8. Any ferruginous or earthy matters which get attached to the goods in
the course of bleaching, are readily removable.
We are now prepared to understand the true principles of bleaching
cotton goods, for the most delicate operations of the calico printer.
1. The first process is steeping, or rather boiling, the goods in water,
in order to remove all the substances soluble in that liquid.
2. The next step is to wash or scour the goods by the dash-wheel or the
stocks. This is of great importance in the course of bleaching, and must
be repeated several times; so much so, that in winter, when the water of
the dash-wheel is cold, the bleaching is more tedious and difficult.
Yarn and very open fabrics do not much need the dash-wheel.
By these first two operations, the woven goods lose about sixteen per
cent. of their weight, while they lose only two parts out of five
hundred in all the rest of the bleaching.
3. In the third place the calicoes are boiled with milk of lime, whereby
they are stripped of their gluten, and acquire a portion of calcareous
soap. Formerly, and still in many bleach-works, the gluten was got rid
of by a species of fermentation of the farinaceous dressing; but this
method is liable to several objections in reference to the
calico-printer. 1. The fermentative action extends sometimes to the
goods, and weakens their texture, especially when they are piled up in a
great heap without being previously washed. 2. The spots of grease, or
of the insoluble soaps, become thereby capable of resisting the caustic
alkalies, and are rendered in some measure indelible; an effect due to
the acetic and carbonic acids generated during fermentation, and which
will be easily understood from what has been said concerning the action
of acids on fatty substances. It is not, therefore, without good reason
that many practical men throw some spent leys into the fermenting vats,
to neutralise the acids which are formed. Were it not for the presence
of fat, fermentation, skilfully conducted, would be an excellent means
of carrying off the gluten; and the steep is therefore applicable to
power-loom goods, which are not polluted with grease.
4. The goods are now subjected to a caustic soda ley, which dissolves
out the soaps of lime and copper, as well as that portion of the
colouring matter which is sufficiently dis-hydrogenated to be capable of
combining with it. This bucking with ley, which is repeated several
times upon the goods, in order to purge them completely from the fatty
matter present in the hand-loom webs, and also partially introduced in
the spinning, is almost the only operation to which yarns for turkey red
are subjected. After being boiled in a caustic soda ley, they are passed
through solutions of chloride of lime, and afterwards through the acid
steep.
5. When the goods are sufficiently bucked in the leys, they are either
exposed to chlorine, or laid out on the grass; sometimes both are had
recourse to for delicate work. These different modes of action have the
same influence on the colouring matter, but they give rise to different
effects in reference to greasy stains.
The goods are dipped in a solution of chloride of lime, which should be
kept tepid by means of steam. Alongside of the chlorine cistern, there
is another filled with dilute sulphuric or muriatic acid. When the goods
are taken out of the chlorine, they are drained on the top of its
cistern till no more liquid runs off them, and they are then plunged
into the _sour_. The action of the acid in the present case may be
easily explained. In proportion as a salt of lime is formed, this base
quits the chlorine, and allows it to act freely upon the colouring
matter. Thus we prevent the development of too great a quantity of
chlorine at once, which would be apt to injure the fibres; and we pursue
both a prudent and economical plan. Only so much chlorine as is strictly
necessary is called forth, and hence it excites no smell in the
apartment.
The chlorine serves to acidify the colouring matter, by abstracting a
portion of its hydrogen; but we must take the greatest care that there
is no grease upon the goods before immersion in it, for the consequence
would be, as above shown, very troublesome spots. When the cloth is laid
out upon the grass, it is the oxygen of the air which acidifies the
colouring matter; for which reason, the dew, which contains much air
rich in oxygen, singularly accelerates the bleaching process. It is
likewise, by absorbing oxygen from the atmosphere, that fats or oils
pass to the state of margaric and oleic acids, and become most easily
saponified. Should the goods, however, be left too long on the grass,
the fats absorb carbonic acid, and become insoluble in leys.
6. The goods must now receive a new soda ley, to dissolve out that
portion of the colouring matter which has been dis-hydrogenated in the
chlorine of the air, as well as the grease, if any perchance remained in
the soluble state. These last two operations are to be several times
repeated, because the colouring matter should be removed only by
degrees, for fear of injuring the texture of the goods, by subjecting
them to too much chlorine at a time.
7. We finish with the dilute sulphuric acid, which should be very weak
and tepid. It dissolves out the iron, and some earthy matters
occasionally found upon cotton. The goods must be most carefully washed
at the dash-wheel, or in a stream of water on quitting the sour bath,
for if the acid were allowed to dry in them, it would infallibly injure
their texture by its concentration. In winter, if the goods are allowed
to get frozen with the acid upon them, they may likewise be damaged.
We may here observe, that when the goods are not to remain white, their
bleaching may be completed with a ley; for though it leaves a faint
yellow tint, this is no inconvenience to the dyer. But when they are to
be finished with a starching after the last ley, they must have another
dip of the chlorine to render the white more perfect. An immersion in
the dilute acid has nearly the same effect.
The principles expounded above lead to this important consequence, that
when we wish to bleach goods that are free from greasy stains, as is the
case generally with the better kinds of muslins, or when we wish to
bleach even greasy goods for the starch finish, we may content ourselves
with the following operations:--