Dyes and dyeing

by Charles Ernest Pellew

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.

Chapter XII
SILK—II

BLACK DYEING OF SILK. WEIGHTING AND ADULTERATION
OF SILK. DYEING SILK WITH COLORS
FAST TO WASHING

The dyeing process described in the last chapter, while well suited for dyeing silk bright and lustrous colors, is not so well adapted to dyeing it black. To be sure, there are several good fast acid blacks, such as Silk Patent Black, 2R, Kalle, or Neutral Wool Black, B, Cassella, or Cashmere Black, 3BN, Elberfeld, or Amido Black, 4024, Metz, which, dyed in full shades in a broken bath of soap or boiled-off liquor, will give fairly good results. But the best of these are not always quite satisfactory, the resulting color generally showing a tendency to be a deep full grey rather than a perfectly true lustrous black.

Salt Colors.—Silk may also be dyed black with some of the good Salt colors—but unless the dyer takes the trouble to after-treat the goods by the troublesome process of diazotizing and developing, the results are no better, if indeed as good as those resulting from the Acid blacks mentioned above.

Sulphur Colors.—These have very often been tried on silk without much success, because for dark colors like blacks, it is necessary to boil the goods in the dye-liquor for some time and to have the latter very concentrated. Unfortunately the sodium sulphide, necessary for dissolving the sulphur dyes, is a powerful alkali, and hence readily attacks an animal fibre, like silk. It is possible, however, by the abundant use of glucose (Karo syrup, etc.) to greatly protect the silk from this tendering action. It is also possible for a dyer fairly well trained in chemistry, to very carefully neutralize the dye-bath by the cautious addition of acid sodium sulphite, until the dye-liquor is no longer alkaline and yet the dyestuff is not precipitated. This process, however, is hardly fitted for an amateur, and has not proved very successful even among the professionals.

Logwood Blacks.—Nearly all professional dyers continue to use the old vegetable dyestuff, logwood, about which some information was given in the first chapter.

To dye with this it is customary to use one of the many good logwood extracts on the market. Great care must be taken in the proper mordanting of the silk before it goes into the bath. For this purpose the silk is impregnated first with iron salts, and later with tannin, and in some processes, with salts of chromium or of tin, before entering the logwood bath. In all cases, therefore, silk dyed black with logwood contains a certain amount, say 15% to 20% of its weight, or 2-3 ounces to the pound, of foreign ingredients. When carefully done this does not injure the material at all, and the “pure dyed” logwood blacks are perfectly satisfactory both for shade, lustre, and durability.