SILK
=Silk.= The silk of commerce is obtained from the cocoons of several
species of insects. These insects resemble strongly the ordinary
caterpillars. At a certain period of its existence the silkworm gives
off a secretion of jelly-like substance. This hardens on exposure to
the air as the worm forces it out and winds it about its body.
[Illustration: MOTH, SILKWORM, AND COCOONS]
It takes about three days for the worm to form the cocoon. After the
cocoon has been formed the silkworm passes from the form of a
caterpillar into a moth which cuts an opening through the cocoon and
flies away. It is very important that the moth should not be allowed
to escape from the cocoon; the mere breaking of the cocoon greatly
decreases the value of the thread. The cocoon is preserved by killing
the chrysalis by heat.
There are a great many varieties of caterpillars, but few of them
secrete a sufficient quantity of silk to render them of commercial
value. The principal species is the mulberry silkworm which produces
most of the silk in commerce. It is cultivated and fed on mulberry
leaves. There are other varieties of silkworms that are not capable of
being cultivated and are called wild silkworms. The silk produced by
the wild worms of China and India is called "tussah" (or "tussur").
The silk is inferior to that produced by the cultivated worms and is
used for making pile fabrics, such as velvet, plush, etc.
The color of the cocoons varies greatly. Most of the European cocoons
are bright yellow, though some are white. The Eastern cocoons, on the
other hand, are mostly white, while a few are yellow. The wild silks
are for the most part écru color, though some are pale green. The
color, except in the wild silks, is derived from the gum which is
secreted by the worm, and with which the fibers are stuck together.
This gum comprises from 15 to 30 per cent of the weight and is removed
by boiling in soap and water before the silk is dyed. All silks except
the wild silks, after the gum is removed, are from white to cream in
color. The tussah, or wild silks, remain an écru color.
[Illustration: REELING RAW SILK]
The greatest care has to be exercised throughout in the care of the
moths, eggs, worms, and cocoons--this being the succession of
changes. That is, the moth lays eggs which are collected and kept cool
till the proper season for incubation. They are then kept warm during
the time occupied in hatching, sometimes about the person of the
raiser. After a time these eggs hatch out worms, tiny things hardly
larger than the head of a pin. After the worms are hatched they
require constant care and feeding with chopped mulberry leaves till
they reach maturity. They are then about three inches in length, and
spin their cocoons from a fiber and gum which they secrete. When the
cocoons are spun the worms become chrysalises inside of them. The
cocoons are then collected and the chrysalises killed, generally by
heat, before they can again become moths.
=Raw Silk.= The cocoons are next sent to the reelers or filatures. A
number of cocoons, greater or less, according to the size of thread
desired, are placed in a basin of hot water, which softens the gum.
After the outside fibers are removed so that the ends run free, the
ends are collected through a guide and are wound upon a reel. As the
silk cools and dries, the gum hardens, sticking the fibers from the
different cocoons together in one smooth thread varying in size
according to the number of cocoons used. After the silk has been
reeled and dried it is twisted into hanks and sent to America and
other countries as raw silk.
Most of the raw silk of commerce is produced in China, Japan, and
Italy. It is also produced to a large extent in Italy, Turkey, and
Greece, also France and Portugal. The cultivation of silk is not only
carried on by private firms, but is encouraged by the government to
the extent of granting money to the manufacturers.
Various attempts have been made to raise silkworms in the United
States. All have failed on account of the high price of labor
necessary to feed the worms.
=Throwing.= The manufacture in the United States begins with raw silk.
We import our raw silk chiefly from Italy, China, and Japan. It is
handled here first by the "throwster," who winds it from the skein and
makes various kinds of thread for different purposes.
Raw silk wound on spools in a single thread, and called singles is
often used to make warps (that is, the threads running lengthwise of a
piece of cloth) for piece-dyed goods, or cloth which is woven with the
gum in the silk, and afterward boiled out and dyed. Singles are also
sometimes used for filling (that is, cross threads) in very thin
fabrics.
Silk yarn that is used for weaving is divided into two kinds, "tram"
and "organzine." Tram silk is made by twisting two or more loosely
twisted threads. It is heavier than organzine and is used for filling.
Organzine silk is produced by uniting a number of strongly twisted
threads. It is used for warp. Crêpe yarn is used in making crêpe,
chiffon, and for other purposes. It is very hard twisted thread,
generally tram, from forty to eighty turns per inch.
Embroidery silk is made by winding the raw silk, putting a large
number of ends together, giving them a slack twist, then doubling and
twisting in the reverse direction with a slack twist.
Sewing silk is made by winding and doubling the raw product, then
twisting into tram, giving it a slack twist, doubling and twisting in
the reverse direction under tension. Machine twist is similar, but
three ply.
The principal fabrics made of silk are: silk, satin, plush, chenille,
crêpe, crepon, gauze, damask, brocade, pongee, and ribbons. Silk
thread and cord are also extensively used. The United States is among
the leaders in the manufacture of silk fabrics.
=Silk Waste.= When the cocoons are softened for reeling a certain
portion of the silk is found to consist of waste and broken threads.
The tangled silk on the outside of the cocoon is called floss. The
residue after reeling, and other wastes in reeling, are known as
frisonnets. Floss silk is not used for weaving. It is a slack twisted
tram, generally composed of a large number of threads of singles.
=Spun Silk.= There is another class of threads made from waste silk by
spinning and known as spun silk. Waste silks include the pierced
cocoons, that is, those from which the moth has come out by making the
hole and breaking the fibers in one end of the cocoon; the waste made
in the filatures in producing raw or reeled silk, chiefly the outside
fiber of the cocoon and the inside next the chrysalis; and also the
waste made in manufacture. The waste silk is ungummed; that is, the
gum is removed from the fibers by boiling with soap, by macerating or
retting, or by chemical reagents.
After the gum is removed from the cocoons, they are opened and combed,
most of the chrysalis shell being removed. The remainder, with other
foreign matter, is picked out by hand from the combed silk. The silk
is put through a number of drawing frames to get the fibers even on
the roving frames, where it first takes the form of thread, then on
the spinning frames, where it is twisted. If it is to be used as
singles, the manufacture ends here. In two- or three-ply yarns, the
singles are doubled, twisted again, singed by running through a gas
flame, cleaned by friction, controlled, that is, the knots and lumps
taken out, and then reeled into skeins for dyeing or put on spools.
=Spun Numbers.= There are two methods in general use for numbering
spun silk. In the French system, the number is based on the singles,
by the meters per kilogram; two and three cord yarns have one-half,
one-third, etc., the length the numbers indicate. Thus--
No. 100 singles has 100,000 meters per kilogram.
No. 2-100 has 50,000 meters per kilogram.
No. 3-100 has 33,333 meters per kilogram.
The other system which is more generally used in this country, is the
English system. The hank is 840 yards, and the number of hanks in one
pound avoirdupois is the count of the yarn. It is based on the
finished yarn, and singles, two or three cord yarns of the same number
all have the same yards per pound. Thus--
No. 50 singles has 42,000 yards per pound.
No. 50-2 has 42,000 yards per pound.
No. 50-3 has 42,000 yards per pound.
=Dyeing Yarns.= Generally speaking there are two large classes into
which silk goods may be divided, those in which the threads are
colored before weaving and called yarn-dyed goods, and those dyed or
printed after weaving and called piece-dyed or printed goods. In
dyeing yarns, the silk is first ungummed and cleaned by boiling in
soap and water, then washed in cold water. If the thread is to be
weighted, as is frequently done, tin salts, iron, or other heavy
material is deposited on the fiber. If carried far, this is injurious,
making the silk tender and weak. Sometimes there is more weighting
than silk. Yarns are usually dyed in hot liquors, aniline colors
being the ones in most common use to-day, though other dyes are used
for special purposes. Some yarns are dyed in the gum, and some with a
part of the gum left in. After dyeing, they are washed in cold water,
dried, and wound on spools.
=Silk Dyeing.= Silk occupies in several respects an intermediate
position between the animal and vegetable fibers. Like wool, it is a
highly nitrogenous body, but contains no sulphur. It readily takes up
many of the colors which can be worked upon vegetable fiber by the aid
of the mordants. This is particularly the case with reference to a
large number of aniline colors, which require merely to be dissolved
and mixed with perfectly clear water in the dye vessel. The great
attraction of silk for these colors simplifies silk dyeing
exceedingly. The sad colors, on the other hand, and especially black,
are in many cases exceedingly complex, the main object of the dyer
being not so much to color the silk as to increase its weight.
Dyeing black on silk is unquestionably the most important branch of
silk dyeing, and it has probably received more attention than any
other branch, in consequence of which it has been brought to a high
degree of perfection. Blacks on silks are produced both from natural
and artificial coloring matters, the former having, so far, retained
their pre-eminence despite the recent discoveries of chemists. For
various reasons coal-tar colors have never proved successful in dyeing
black on silk. Since the discovery of America, logwood blacks have
formed the staple of the black-silk dyer, who has carried their
production to a high degree of excellence. But unfortunately, besides
aiming at a high state of perfection in the actual dyeing operation,
the black-silk dyer has also aimed at increasing the weight of the
dyed silk, so that nowadays it is possible for him to receive ten
pounds of raw silk and to send out fifty pounds of black silk, the
extra forty pounds being additions made in the process of dyeing.
[Illustration: WINDING SILK ON SWIFTS]
Logwood black-silk dyeing consists essentially of alternate dippings
in separate baths with the mordant and dyestuffs suitable for
producing the required color and weight. The number of dippings and
the length of time taken in each operation depend on the intensity of
the black wanted and the amount of weighting which is desired. The
chief substances used for weighting are lead salts, catechu, iron, and
nut-galls, with soap and oil to soften in some degree the harshness of
the fabric which these minerals cause. As the details of the
operations are practically the same for all kinds of logwood blacks
(raven, jet, crape, dead black, etc.), the method for producing one
will suffice for all. The process involves several distinct
operations, as follows:
1. =The Boiling Off.= This is the removal of the gum and natural
coloring matter in the silk. It is accomplished by boiling the skeins
of silk in water and good olive oil soap for about one hour. This
dissolves the gum and leaves the fiber clean and glossy.
2. =Mordanting.= This is done in a bath of nitrate of iron, in which
the skeins of silk are allowed to remain one hour. The silk gains some
in weight in this operation by absorbing a quantity of the iron in the
bath. After having been dipped in the first bath three or four times,
it is ready for the soap and iron bath, in which it is repeatedly
immersed, the operation causing a deposit of iron-soap on the fiber
which adds to its weight, but at the same time does not lessen its
flexibility and softness. Eight dippings in the iron and soap bath
increase the weight of the silk about 100 per cent.
3. =Blue Bottoming.= The next operation is to dye the silk blue, which
is done by immersing it in a solution of potash. In this it is worked
for half an hour, when it acquires a deep blue color. It is then
taken out, and after rinsing is ready for the "weighting" operations.
4. ="Weighting" Bath.= A catechu bath is now prepared, in which the
silk is entered and worked for an hour, and then allowed to steep over
night. The result is that the blue on the silk is decomposed, and the
goods by absorbing the tannin in the catechu increase in weight from
35 to 40 per cent. This bath is the most important one in the dyeing
of "weighted" black silks, as the dyer can regulate the strength of
the bath by the addition of tin crystals so as to increase the weight
of the silk to an astonishing degree. The proportion of tin crystals
used is regulated by the number of iron baths that have previously
been given the silk; if two baths of iron have been given, 5 per cent
of tin crystals are used; if four baths, 10 per cent, and so on. The
action of these chemicals is somewhat complex. All that is known is
that by reason of some peculiar quality possessed by silk it is
enabled to combine with iron and tin, and that exposure to the air
after the baths fixes these chemicals permanently upon the fibers,
thus increasing their weight to almost any desired extent. Silk,
according to its quality and weight, will take up of these substances
from 50 to 200 per cent without creating much suspicion. Instances
have been known in which silk has been increased nine times its own
weight. All the operations thus far have had for their object the
weighting of the silk, although the blueing and the catechu baths have
some influence on the finished result. After these come the dyeing
operations proper, two in number, mordanting and dyeing.
5. =Mordanting.= A bath of iron liquor heated to 130 degrees F. is
provided. The silk is entered, worked well for one hour, then wrung
out and hung up to "age" for two hours, after which it is ready for
the logwood dye.
6. =Dyeing.= A bath of logwood liquor is prepared to which is added 10
per cent of fustic, and the solution is brought to a temperature of
150 degrees F. In this the silk is entered and worked for an hour,
then taken out and wrung dry. Sometimes the black does not come up
full enough, and in such cases the bath is repeated.
7. The final operation has for its object the restoration of the
luster and suppleness of the silk, which has to some extent
deteriorated from the many operations through which it has passed. The
brightening and softening of the fiber are effected by immersing the
silk in a bath of olive oil in the form of an emulsion. In this the
silk is worked until it is thoroughly impregnated with the oil, when
it is taken out and wrung dry, after which it is ready for the loom.
Practically the same process is followed in piece dyeing, though only
inferior grades of silk are dyed in the web.
=Colored Silks.= This class of silks is generally purer than black and
sad-colored silks. It is not nearly so easy to weight the former as
the latter, for the reason that there are but few substances capable
of giving weight which do not interfere with the effect of light
colored dyes. The weighting agents most generally used are sugar and
acetate of lead. The weighting by sugar is done after the silk is
dyed. A solution is made of pure lump sugar by placing it in a large
copper pan with water and heating until dissolved. In this bath the
silk is thoroughly saturated, and then dried and finished; or, the
dipping process may be repeated several times if desired. One dipping
will weight the silk about 12 per cent, two about 20 per cent, and
three about 30 per cent. In a solution of acetate of lead, each
dipping will weight the silk about 8 per cent, and these may be
repeated as often as it is wished. In this case the weighting is
generally done on the undyed, boiled-off silk, although it may be done
on the dyed silk if the color is such as will stand the acid.
=Mixed Silk Fabrics.= Until lately silk was invariably dyed in the
state of yarn. When the silk was to be woven into mixed fabrics, such
as satin, gloria, etc., it was impossible to dye both fibers exactly
the same shade. Formerly such fabrics were woven with the cotton and
silk yarns dyed separately, care being taken to match them as closely
as possible. The weaving of dyed yarns of different fibers is open to
the objection that when the fabric comes to be finished there is a
wide difference in the color, no matter how closely they may have
matched in the beginning.
=Ribbons.= Ribbons are woven several pieces in one loom, with a
separate shuttle for each piece. The shuttle is carried through the
shed or warp by a rack and pinion, instead of being thrown through as
in broad goods; otherwise the weaving is the same.
=Velvets.= Velvets and other pile fabrics are woven in two pieces,
one over the other, with the pile threads woven back and forth between
them. A knife travels between the two pieces cutting the pile threads
so as to leave the ends standing up straight. Velvets used to be woven
over wires and cut by hand, but this method is practically obsolete.
=Piece Dyeing.= If the goods are woven with the gum still in the silk,
it must be taken out afterward, and the goods either dyed in the piece
or prepared for printing.
=Printing.= The most primitive method of printing is by the use of
stencils. It is the method employed by the Japanese and Chinese. Next
came block printing, which is still extensively used in Europe. The
pattern is raised in felt on wooden blocks, the color taken up from
pads, one block for each color. The results are good, but the work is
very slow. Most silk goods are to-day machine printed. The design is
engraved or etched on copper cylinders, one cylinder for each color;
the color thickened with gum is supplied by rolls running against the
cylinders, and the surplus is scraped off by a knife blade, leaving
only that in the engraving which is taken up by the cloth. After
printing, the cloth is steamed to set the colors, and then washed in
order to remove the gum used to thicken the colors for printing.
[Illustration: JACQUARD SILK LOOM]
=Finishing.= All silk goods, whether yarn dyed or piece dyed or
printed, are given some kind of finish; sometimes it is no more than
is necessary to smooth out the wrinkles. There are many finishing
processes by which goods may be treated. They are run through gas
flames to singe off loose fiber, and over steam cylinders to dry and
straighten them, over a great variety of sizing machines to stiffen
them with starch or glue. There are calenders or heavy rolls to smooth
and iron them, steam presses of great power to press them out,
breaking and rubbing machines to soften them, and tentering machines
to stretch them to uniform width. There are also moireing or watering,
embossing, and various other machines for special purposes.
=Waterproofing.= One of the worst difficulties with which the
manufacturer of piece-dyed and printed silk goods has to contend is
the ease with which they become spotted with water, and for a number
of years many people have tried to prevent this by various processes.
There are no less than two hundred such processes patented. None of
them have met with much success, as they injured the feel or strength
of the goods. After goods are finished they are carefully inspected
for imperfections, measured, and wrapped in paper and packed in cases
for shipment. The complexity and number of processes for treating silk
goods may be realized when we know that a piece-dyed or printed fabric
is handled its entire length between fifty and one hundred times after
it comes from the loom, sometimes even more.