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.