BLEACHING
217. The beauty and the commercial value of uncolored fabrics depend
upon the purity and perfection of their whiteness; a man's white
collar and a woman's white waist must be pure white, without the
slightest tinge of color. But all natural fabrics, whether they come
from plants, like cotton and linen, or from animals, like wool and
silk, contain more or less coloring matter, which impairs the
whiteness. This coloring not only detracts from the appearance of
fabrics which are to be worn uncolored, but it seriously interferes
with the action of dyes, and at times plays the dyer strange tricks.
Natural fibers, moreover, are difficult to spin and weave unless some
softening material such as wax or resin is rubbed lightly over them.
The matter added to facilitate spinning and weaving generally detracts
from the appearance of the uncolored fabric, and also interferes with
successful dyeing. Thus it is easy to see that the natural coloring
matter and the added foreign matter must be entirely removed from
fabrics destined for commercial use. Exceptions to this general fact
are sometimes made, because unbleached material is cheaper and more
durable than the bleached product, and for some purposes is entirely
satisfactory; unbleached cheesecloth and sheeting are frequently
purchased in place of the more expensive bleached material. Formerly,
the only bleaching agent known was the sun's rays, and linen and
cotton were put out to sun for a week; that is, the unbleached
fabrics were spread on the grass and exposed to the bleaching action
of sun and dew.
[Illustration: FIG. 158.--Preparing chlorine from hydrochloric acid
and manganese dioxide.]
218. An Artificial Bleaching Agent. While the sun's rays are
effective as a bleaching agent, the process is slow; moreover, it
would be impossible to expose to the sun's rays the vast quantity of
fabrics used in the civilized world of to-day, and the huge and
numerous bolts of material which daily come from our looms and
factories must therefore be whitened by artificial means. The
substance almost universally used as a rapid artificial bleaching
agent is chlorine, best known to us as a constituent of common salt.
Chlorine is never free in nature, but is found in combination with
other substances, as, for example, in combination with sodium in salt,
or with hydrogen in hydrochloric acid.
The best laboratory method of securing free chlorine is to heat in a
water bath a mixture of hydrochloric acid and manganese dioxide, a
compound containing one part of manganese and two parts of oxygen. The
heat causes the manganese dioxide to give up its oxygen, which
immediately combines with the hydrogen of the hydrochloric acid and
forms water. The manganese itself combines with part of the chlorine
originally in the acid, but not with all. There is thus some free
chlorine left over from the acid, and this passes off as a gas and can
be collected, as in Figure 158. Free chlorine is heavier than air, and
hence when it leaves the exit tube it settles at the bottom of the
jar, displacing the air, and finally filling the bottle.
Chlorine is a very active substance and combines readily with most
substances, but especially with hydrogen; if chlorine comes in contact
with steam, it abstracts the hydrogen and unites with it to form
hydrochloric acid, but it leaves the oxygen free and uncombined. This
tendency of chlorine to combine with hydrogen makes it valuable as a
bleaching agent. In order to test the efficiency of chlorine as a
bleaching agent, drop a wet piece of colored gingham or calico into
the bottle of chlorine, and notice the rapid disappearance of color
from the sample. If unbleached muslin is used, the moist strip loses
its natural yellowish hue and becomes a clear, pure white. The
explanation of the bleaching power of chlorine is that the chlorine
combines with the hydrogen of the water and sets oxygen free; the
uncombined free oxygen oxidizes the coloring matter in the cloth and
destroys it.
Chlorine has no effect on dry material, as may be seen if we put dry
gingham into the jar; in this case there is no water to furnish
hydrogen for combination with the chlorine, and no oxygen to be set
free.
219. Bleaching Powder. Chlorine gas has a very injurious effect on
the human body, and hence cannot be used directly as a bleaching
agent. It attacks the mucous membrane of the nose and lungs, and
produces the effect of a severe cold or catarrh, and when inhaled,
causes death. But certain compounds of chlorine are harmless, and can
be used instead of chlorine for destroying either natural or
artificial dyes. One of these compounds, namely, chloride of lime, is
the almost universal bleaching agent of commerce. It comes in the form
of powder, which can be dissolved in water to form the bleaching
solution in which the colored fabrics are immersed. But fabrics
immersed in a bleaching powder solution do not lose their color as
would naturally be expected. The reason for this is that the chlorine
gas is not free to do its work, but is restricted by its combination
with the other substances. By experiment it has been found that the
addition to the bleaching solution of an acid, such as vinegar or
lemon juice or sulphuric acid, causes the liberation of the chlorine.
The chlorine thus set free reacts with the water and liberates oxygen;
this in turn destroys the coloring matter in the fibers, and
transforms the material into a bleached product.
The acid used to liberate the chlorine from the bleaching powder, and
the chlorine also, rot materials with which they remain in contact for
any length of time. For this reason, fabrics should be removed from
the bleaching solution as soon as possible, and should then be rinsed
in some solution, such as ammonia, which is capable of neutralizing
the harmful substances; finally the fabric should be thoroughly rinsed
in water in order that all foreign matter may be removed. The reason
home bleaching is so seldom satisfactory is that most amateurs fail to
realize the necessity of immediate neutralization and rinsing, and
allow the fabric to remain too long in the bleaching solution, and
allow it to dry with traces of the bleaching substances present in the
fibers. Material treated in this way is thoroughly bleached, but is at
the same time rotten and worthless. Chloride of lime is frequently
used in laundry work; the clothes are whiter than when cleaned with
soap and simple washing powders, but they soon wear out unless the
precaution has been taken to add an "antichlor" or neutralizer to the
bleaching solution.
220. Commercial Bleaching. In commercial bleaching the material to
be bleached is first moistened with a very weak solution of sulphuric
acid or hydrochloric acid, and is then immersed in the bleaching
powder solution. As the moist material is drawn through the bleaching
solution, the acid on the fabric acts upon the solution and releases
chlorine. The chlorine liberates oxygen from the water. The oxygen in
turn attacks the coloring matter and destroys it.
[Illustration: FIG. 159.--The material to be bleached is drawn through
an acid _a_, then through a bleaching solution _b_, and finally
through a neutralizing solution _c_.]
The bleached material is then immersed in a neutralizing bath and is
finally rinsed thoroughly in water. Strips of cotton or linen many
miles long are drawn by machinery into and out of the various
solutions (Fig. 159), are then passed over pressing rollers, and
emerge snow white, ready to be dyed or to be used as white fabric.
221. Wool and Silk Bleaching. Animal fibers like silk, wool, and
feathers, and some vegetable fibers like straw, cannot be bleached by
means of chlorine, because it attacks not only the coloring matter but
the fiber itself, and leaves it shrunken and inferior. Cotton and
linen fibers, apart from the small amount of coloring matter present
in them, contain nothing but carbon, oxygen, and hydrogen, while
animal fibers contain in addition to these elements some compounds of
nitrogen. The presence of these nitrogen compounds influences the
action of the chlorine and produces unsatisfactory results. For animal
fibers it is therefore necessary to discard chlorine as a bleaching
agent, and to substitute a substance which will have a less disastrous
action upon the fibers. Such a substance is to be had in sulphurous
acid. When sulphur burns, as in a match, it gives off disagreeable
fumes, and if these are made to bubble into a vessel containing water,
they dissolve and form with the water a substance known as sulphurous
acid. That this solution has bleaching properties is shown by the fact
that a colored cloth dipped into it loses its color, and unbleached
fabrics immersed in it are whitened. The harmless nature of sulphurous
acid makes it very desirable as a bleaching agent, especially in the
home.
Silk, lace, and wool when bleached with chlorine become hard and
brittle, but when whitened with sulphurous acid, they retain their
natural characteristics.
This mild form of a bleaching substance has been put to uses which are
now prohibited by the pure food laws. In some canneries common corn is
whitened with sulphurous acid, and is then sold under false
representations. Cherries are sometimes bleached and then colored with
the bright shades which under natural conditions indicate freshness.
Bleaching with chlorine is permanent, the dyestuff being destroyed by
the chlorine; but bleaching with sulphurous acid is temporary, because
the milder bleach does not actually destroy the dyestuff, but merely
modifies it, and in time the natural yellow color of straw, cotton,
and linen reappears. The yellowing of straw hats during the summer is
familiar to everyone; the straw is merely resuming its natural color
which had been modified by the sulphurous acid solution applied to the
straw when woven.
222. Why the Color Returns. Some of the compounds formed by the
sulphurous acid bleaching process are gradually decomposed by
sunlight, and in consequence the original color is in time partially
restored. The portion of a hat protected by the band retains its
fresh appearance because the light has not had access to it. Silks and
other fine fabrics bleached in this way fade with age, and assume an
unnatural color. One reason for this is that the dye used to color the
fabric requires a clear white background, and loses its characteristic
hues when its foundation is yellow instead of white. Then, too,
dyestuffs are themselves more or less affected by light, and fade
slowly under a strong illumination.
Materials which are not exposed directly to an intense and prolonged
illumination retain their whiteness for a long time, and hence dress
materials and hats which have been bleached with sulphurous acid
should be protected from the sun's glare when not in use.
223. The Removal of Stains. Bleaching powder is very useful in the
removal of stains from white fabrics. Ink spots rubbed with lemon
juice and dipped in bleaching solution fade away and leave on the
cloth no trace of discoloration. Sometimes these stains can be removed
by soaking in milk, and where this is possible, it is the better
method.
Bleaching solution, however, while valuable in the removal of some
stains, is unable to remove paint stains, because paints owe their
color to mineral matter, and on this chlorine is powerless to act.
Paint stains are best removed by the application of gasoline followed
by soap and water.