SILK—I
So far as we can tell, silk was first discovered and manufactured in
China about 1700 B.C., a date corresponding in Biblical history to
the time of the patriarch Joseph. From China it was exported to the
great and wealthy empire of Persia, and from there was first brought
into Europe by Alexander the Great after his defeat of the Persian
king. Its origin, although known and described by Aristotle, was for
several hundred years a mystery. During the Roman Empire, silken
garments, woven in Europe, from Chinese silk imported by way of
Persia, were important and very highly prized articles of luxury.
About 555 A.D., while commerce with Persia was interrupted by
warfare, two monks in the pay of the Emperor Justinian smuggled
eggs of the silkworm and seeds of mulberry trees from China to
Constantinople. This was the origin of the European silk industry.
It spread rapidly to the various countries bordering on the
Mediterranean, and by the seventeenth century was firmly established
not only in Spain and Italy, but also in France.
Efforts were made to introduce it, at this time, into England, but
without success. In 1622 King James I started the industry, for
the first time, in the colony of Virginia in this country. Since
that time numerous attempts have been made to develop the American
silkworm industry, but with very little success, owing to the large
amount of hand labor necessary to produce the material.
At the present time the very finest raw silk in the world is produced
in the south of France, and next to that come certain brands of
Italian silk. The Japanese silk is more variable in quality, although
steadily improving, while the Chinese silk, as a rule, is less
satisfactory and more apt to be light and fluffy.
With regard to the consumption, it was estimated that in 1907 Europe
used some twenty-five million pounds, and the United States fifteen
million pounds of raw silk, which, at an average price of nearly
$5.50 per pound, amounted to over two hundred and eighteen million
dollars.
=Origin and Varieties of Silk.=—Silk has been defined as a “smooth,
lustrous, elastic fibre of small diameter and of animal origin.” As
is well known, ordinary commercial silk is secreted or “spun” by the
silkworm, the caterpillar form of a moth known as _Bombyx Mori_, the
moth of the mulberry tree. These silkworms have been cultivated for
thousands of years, but there exist in different parts of the world,
notably in India and Japan, wild or uncultivated silkworms, derived
from nearly related, but not identical, families of moths, and whose
silk is collected in the forests by the natives, forming what is
known in commerce as wild or tussah silk.
Of course, the silk from silkworms, cultivated and wild, is the
only one yet produced on a commercial scale. But silk can also be
obtained from other animals, notably from spiders and from a peculiar
shellfish, the pinna, found in the waters of the Mediterranean.
Silk from the silkworm can be divided into two classes, according to
whether the silkworms are the cultivated or the wild varieties. In
each case the silk is produced by the caterpillar spinning a covering
or shroud, the so-called cocoon, around itself to protect it when in
the form of the chrysalis or pupa, awaiting its transformation into
the moth.
The ordinary or cultivated silk of commerce comes from worms fed
almost exclusively upon the leaves of the white mulberry tree, and
cannot be produced successfully without that particular plant.
The somewhat similar worms that produce the wild or tussah silks
live upon the leaves of the oak, elm, ailanthus, castor oil plant,
and others. While the two varieties resemble each other greatly
in their chemical properties, they can always be distinguished,
because cultivated silk is much more lustrous than the other, but is
decidedly less strong.
_Tussah Silk, Pongee, Shantung._—The tussah silks, when woven, are
commonly known under the general name of pongee. Of late years
this name has been applied to imitation goods possessing the
characteristic dull color, and even the feel of the real article,
but far less strong. These are generally made out of spun silk,
derived from “Shappe,” i.e., the by-products of the silk industry,
spoilt cocoons, waste from the spinning machines and the dyehouses,
and the like—silk, to be sure, but silk of very inferior quality.
Accordingly, it is now customary to call real pongee by the name
Shantung, after the Chinese province from which much of the wild silk
is brought.
Shantung, or true pongee, can be readily distinguished from the
imitation by examination of the threads, both warp and filling. These
should be very long, and loosely spun or rather “thrown,” whereas
the imitation threads are spun together tightly, from fibres of many
different lengths, generally quite short.
Preparing Silk for Dyeing.
_Reeling._—All silk, whether cultivated or wild, comes originally
from the cocoons, which are, as a rule, each formed out of a
continuous strand or thread woven by the silkworm round and round its
own body before it passes into the chrysalis state. These cocoons
are collected, carefully dried to kill the quiescent animal inside,
and then, in due course of time, they are placed in basins of warm
water which softens the gum which binds the cocoon threads together,
and the separate fine threads from several cocoons are picked up by
brushing, and are combined into one which is reeled off on machines.
The silk thus obtained is made up into hanks and bundles, and
constitutes the raw silk of commerce.
_Raw Silk._—The raw silk is very different in appearance and texture
to the finished silk that we are accustomed to. It is without lustre,
white, yellow, or even, in the case of some Italian silks, orange in
color, and quite stiff when handled. These qualities are due to the
presence of from 25 to 35 per cent. of gum, which is insoluble in
cold water, but is softened by hot water and dissolves readily in a
hot soap bath.
_Throwing._—The threads of this raw silk are far too fine and
delicate to be fit for the weaving processes or even for dyeing.
So they are combined into coarser and stronger threads by being
“thrown,” a process equivalent to the spinning process of cotton,
linen or wool. In throwing, the raw silk fibres are again softened
in hot water, and are loosely spun or twisted together while still
sticky. Three, four, or five threads of raw silk are usually
combined to form one strand of thrown silk, varying, of course,
with the quality of the original silk and the objects for which the
thrown silk is to be used, when woven. For instance, silk used for
filling—“tram,” as it is called in the trade—is usually thicker and
softer, and less strong than the warp, or “organzine,” and therefore
is usually built up, by the “throwster,” from many threads of less
valuable raw silk, loosely twisted, while the organzine, used for
warp, is generally of the best and strongest available material,
thrown in finer strands out of fewer threads of raw silk, twisted
more tightly.
It must always be remembered that the skein silk is thrown from very
long continuous threads of raw silk, full of gum, whereas spun silk,
which is being used more and more every year, is made from short
lengths of waste and scrap silk, held together not by gum, but by
tight twisting and spinning, just like cotton or linen.
_Stripping or Degumming._—This thrown silk must then be prepared
for the dyeing by getting rid of the gum, which not only makes the
silk stiff and destroys its lustre, but which also would interfere
with the smooth, even dyeing of the fibres themselves. For this
purpose the silk, in skeins, is thoroughly washed, or “stripped,” by
soaking in two or three successive baths of hot, strong, neutral soap
solutions. In the dyehouses Castile (olive oil) soap is invariably
used for this purpose, and, while made of cheap grades of olive
oil, it is always, in good dyehouses, of excellent quality, for the
presence of even minute amounts of free alkali in these baths is
liable to greatly injure and “tender” the silk.
_Boiled-off Liquor._—The soap solution from these stripping baths
is not thrown away in the dyehouses, but is carefully stored as a
valuable reagent. Under the name of “boiled-off liquor” it is almost
exclusively used, by the dyers, for color dyeing. It is not often
used in black dyeing, and therefore, in a dyehouse, the presence of
a large and well-patronized black department is considered of great
importance as providing the color dyer with an abundant supply of
boiled-off liquor.
The stripped or degummed silk is now ready for weaving directly, the
resulting white cloth being sometimes finished and sold as such, and
sometimes “dyed in the piece.” In most cases, however, the stripped
silk is weighted, dyed, and finished “in the skeins,” before weaving.
=Piece Dyeing.=—In dyeing by the piece, the stripped silk is passed
through a weak acid bath, usually acetic, and then woven into goods
of the desired quality. These goods are then dyed in the piece by
being run through the dye-bath until they are of the proper shade.
The dye-bath (for colors) is made by stirring the proper quantity
of Acid dyestuffs into a hot bath of boiled-off liquor (the bath
in which the silk has been stripped), which is faintly acidified,
or “broken,” as the technical phrase goes, by the addition of some
sulphuric acid. This boiled-off liquor has the property of laying
the dyes on the silk evenly and thoroughly, and is better for that
purpose than any other medium. For amateur work, or where boiled-off
liquor cannot be obtained, very fair results can be obtained with a
strong bath of olive oil soap (Castile or Marseilles), “broken” with
weak acid, generally dilute sulphuric acid.
The term “breaking” the soap bath is very significant. The acid
should be added drop by drop to the frothing soap bath until the
bubbles disappear and a thin iridescent film of fatty acid rises to
the top of the liquid.
After the piece goods are brought to the proper shade, they are
finished, usually by carefully rinsing in water to take away all
traces of free acid, then by passing through a cold soap bath, often
with a little olive oil emulsified in it, to increase the lustre;
finally, through a bath of weak organic acid, like acetic acid, to
develop the so-called “scroop” or “feel” of the silk. When silk
is washed in soap, or is dipped in even a weak bath of alkali, it
becomes soft and clammy to the touch, and has no “life” or “snap”
to it when dry. The passage through a bath of weak acid develops the
characteristic stiffness of the silk fibre, and causes it to give its
peculiar rustling sound when pressed.
=Skein Dyeing.=—When weighting or adulteration is not employed,
i.e., in the so-called “pure dye” process, the dyeing of skein silk
resembles the piece dyeing described. The degummed silk is immersed
in a dye-bath containing the dyestuffs (Acid colors) dissolved in
boiled-off liquor, broken with dilute sulphuric acid. The bath
is heated nearly to the boiling point, and the silk turned in it
until the desired shade is produced. It is then taken out, washed
thoroughly in water to remove the last traces of acid, and then
brightened by passing through a soap bath with some oil, and later
through a bath of acetic acid to develop the “scroop.”
_Drying._—An important part of the process is the final drying and
finishing. The drying should be done slowly and carefully, and not
proceed too far, or the silk will be brittle. As is well known to
dyers, silk has the power of absorbing 25% or 30% of its weight
of water without becoming perceptibly damp to the hand, and this
moisture, when not carried too far, is of actual benefit to the
material, making it stronger and more elastic. This property is often
made use of by the honest (?) dyer when, in case some of the silk in
a lot has been spoiled by accident or carelessness, he makes up the
difference in weight by the liberal use of the watering pot.
_Finishing._—This process is perhaps the most difficult and
technical of all, for the value of the finished product depends
very largely on it, and it is almost impossible for an amateur
to accomplish it. The skeins, after drying, are hung on a heavy
polished wooden bar and, with a smooth wooden stick, are shaken out,
straightened, pulled, twisted, and worked until the fibres are all
parallel, the kinks taken out, any weak or injured portion cut out,
and the whole skein has acquired the proper amount of lustre.
Sometimes, for specially brilliant fabrics, the skeins are “lustred”
by machinery; this is the so-called “metallic lustring” when the
silk, generally enveloped in steam so as to be both hot and damp,
is pulled out between two steel arms until it has been stretched a
considerable percentage of its original length. This undoubtedly
lessens the strength of the fibre considerably and diminishes its
elasticity, but under this strain each fibre is stretched out
perfectly smooth and thus becomes much more brilliant and lustrous.
=Dyeing Wild Silks.=—It has been found difficult to handle
satisfactorily the different sorts of wild silks in the factory. The
bleaching of them has been very troublesome, although of late years
the problem has been solved pretty well. And the ordinary process for
dyeing silk with Acid dyes in a broken soap, or boiled-off liquor,
bath is, for full deep shades at any rate, not always satisfactory.
In consequence most of the genuine pongee or Shantung cloth is sold
in the natural unbleached color, a pleasant shade of tan, or else in
light shades.
Perhaps the best results in dyeing pongee silk full, deep, even
shades are obtained by mordanting the material with tannin and
tartar emetic, just as cotton is mordanted before dyeing it with
Basic colors, and then using in the dye-bath one or the other of
the so-called “Janus” colors,—a group of colors on the border line
between Basic and Acid, of which the best are Janus Yellow G, Janus
Yellow R, Janus Red B, and Janus Black 1 (_Metz_).
This process, however, is too complicated for the unprofessional dyer
to use with much success.
For all but the very full shades the craftsman is advised to use the
Acid colors, as, for instance, some of the selected colors of the
different houses, listed in Chapter VII, in a bath acidified with
acetic acid, and without the use of soap.
For dark dull shades the Sulphur colors can be used, especially
if some care is taken to reduce the alkalinity of the bath by
neutralizing or nearly neutralizing the sodium sulphide with a
little acid sodium sulphite. If the desired shade is so dark as to
necessitate heating and dye-bath, it is also advisable to add a
little gelatin.
For full shades of rather brighter quality the Vat dyes may be
employed, also with precautions against the tendering action of the
caustic alkali upon the fibre.
Before, however, starting in to dye a piece of pongee on the
assumption that it is made from tussah silk, it is very advisable
to examine it carefully, picking out the individual threads and
untwisting them, and to make a few dyeing tests upon small samples.
For a large proportion of so-called pongee, which in color, lustre,
feel, and general appearance resembles the genuine Shantung very
closely, is simply made from spun or waste silk, and can be dyed like
ordinary silk.
_Acid Dyes, to be used on Silk._—Any of the dyestuffs mentioned
in the lists on page 127, as suitable for wool, can be used
successfully for silk dyeing. These colors have all been selected as
unusually fast to light and, in this respect, are to be classified
as “practically all of the first class,” i.e., as absolutely
satisfactory against the action of sunlight.
But, for a valuable and comparatively fragile material like silk, it
is quite allowable to use colors for special shades which are less
fast to sunlight, if they possess other valuable qualities. Such,
for instance, are the two red dyestuffs, Fast Acid Eosine G (_Metz_)
and Fast Acid Phloxine (_Metz_), which belong to the group of
so-called Eosine or Fluoresceïn dyestuffs most of which, while very
beautiful, are extremely fugitive. These two dyes, which give shades
of pink and red with yellow and blue fluorescence, respectively, are
considerably more fast than the rest of their group, and will rank in
the third class, if not at the foot of the second class, as regards
light-fastness.
With regard to fastness to washing, it must be remembered that
these Acid dyes are not fast at all, when dyed on silk in a broken
soap bath. They may stand very light washing in a cold soap bath,
but in boiling soapsuds will strip completely. This is important
for the amateur, and indeed, for the professional dyer, for
whom a dyed silk, either skein or in the piece, has come out
unsatisfactorily—uneven or spotted, or too dark in shade—for it is
possible, if the silk is of good quality, to clean off the color
completely by boiling soapsuds, without injuring the goods.
If the trouble is unevenness, while the shade is satisfactory, the
color can be dissolved off in the boiling soap bath and then, on
breaking the bath with a little acid, the same dye can be laid right
on again, it is to be hoped this time in a satisfactory manner. The
question of dyeing silk fast to washing, and also of dyeing silk
black, will be dealt with in the next chapter.