WEIGHTING OF SILK
This moderate increase of weight, however, which is hardly enough to replace the weight of the gum lost in the stripping process, was far from satisfying the demands of the manufacturer for a cheaper raw material. And accordingly both dyer and dyeing chemist have exhausted all their energies and skill in trying to increase this percentage of cheap foreign matter in the finished silk, to the utmost limit of what the market will stand.
The first efforts in this direction were based upon the saving of some or, indeed, nearly all, of the gum which is wasted in the stripping or degumming process previously described. This gum, which amounts to from 20 to 35 per cent. of the raw silk, makes the silk stiff in texture and dull in color and more difficult to dye. Accordingly, in former years, it was invariably washed out of the silk with the greatest care before any attempt was made to dye it. But by modifying the dyeing, and especially the finishing process, it was found possible to produce the so-called “souples”—i.e., silks with little or no lustre, but with the characteristic “scroop” or “feel”—capable of replacing bright silk as a filling in many fabrics and yet containing almost all the natural gum left in the fibre.
The black silks were then attacked and an elaborate system of mordanting was introduced before the dyeing proper began. For instance, the silk can be steeped alternately in one solution after another, first of iron salts and then of ferrocyanide of potash, thus forming Prussian blue in the fibre. Then the excess of iron can be converted by immersion in tannin solutions, such as Gambier or Cutch, into black tannate of iron, or ink, and finally, after perhaps a light bath in chromium salts, the real black color is brought out by boiling in logwood extract. The silk is then brightened by boiling with good neutral Castile soap, is shaded, if necessary, by dyeing with either an Acid or Basic dye in a weak bath, and, after drying and finishing, the finished product may easily weigh two or even three times as much as the original raw silk, and still retain its strength, lustre, and elasticity.
Tin Weighting.—The weighting of colored and bright silks did not proceed so rapidly, and it was not much more than ten years ago that, by accident, some French dyers discovered that by immersion in a strong bath of tin chloride (stannic chloride acidified with some hydrochloric acid) the silk fibre would absorb a large percentage of tin salts without necessarily losing lustre, dyeing capacity, or even strength. This at first was kept a secret, but its use gradually spread, until now it is a very poor silk dyer who cannot weight his silk 100 or 150 per cent. without spoiling its immediate commercial value.
Without going into unnecessary details, the process is somewhat as follows: The silk, after being degummed and thoroughly washed free of soap, is plunged into a bath of tin chloride and kept there for some hours. It is then taken out and the loose tin salts are washed off in a tank of water (technically called a box), or in a washing machine. To further “set” the tin, the silk is then placed for a short time in a solution of phosphate of soda and again washed thoroughly. It has now gained from 15 to 25 per cent of its original weight (2½ to 4 ounces to the pound of raw silk).
If further weighting is desired, this treatment, first in tin chloride and then in phosphate of soda, can be repeated three or four up to five or even six times, increasing the weight with each immersion. Then a bath is usually given of silicate of soda, which adds a little weight, ½ to ¾ of an ounce, and, it is claimed, benefits the lustre and strength of the goods. Then, after a final washing, the silk is ready for the dye-bath.
The weighted goods are dyed, dried, and finished about the same as with the “pure dye” process, and the proud dyer can rejoice at returning to the honest manufacturer from 150 to 250 pounds of finished silk for every 100 pounds of raw silk (containing, by the way, 25 to 30 pounds of gum) which was sent in to the dyehouse! This “tin-weighing” process is also applied to black dyeing, and enables the black dyer to build up his weight with tin salts instead of limiting him to iron, chromium, ferrocyanide of potash, tannin, and logwood.
Properties of Weighted Silk.—It is scarcely necessary to point out that silk, weighted to the extreme limit, is hardly to be considered as the most durable and trustworthy of fabrics, even when dyed by the most expert workmen. And when carelessly prepared heavily weighted silk is an abomination, liable to crack and wear away with the least provocation.
It may be worth reminding some of my fair readers that the old test of a silk taffeta, “so thick and stiff that it will stand of itself,” is nowadays anything but a proof of good quality. One or two manufacturers in this country a few years ago tried to revive the almost forgotten art of making and selling pure-dyed goods, and one trouble they experienced in disposing of their products, outside the high price, was the criticism that their silk felt so light and thin.
Prevalence of Weighted Silk.—At present it is almost impossible, at least in New York, to buy pure-dyed heavy silks. The writer, at any rate, has tried diligently, during the last year or two, to find for some special experiments a piece of white taffeta which was not markedly weighted. After visiting department stores and the very best dry-goods stores in the city, at all of which he was informed that no such material now existed, the best that could be obtained was one make of silk where the organzine or warp was fairly pure, the tram being well weighted. Light-weight Japanese and Chinese silks, however, undyed or dyed in the piece, can still be procured with little or no weighting.
Tests for Weighted Silk.—This silk may be identified by a very simple test. Pure-dyed silk, when dry, is easily inflammable. When touched with a lighted match it catches fire at once, “carries the flame” well, especially if in the form of thread; and, if followed up with a flame, it will before long burn away completely, leaving little or no ash or residue.
On the other hand, weighted silk, especially when the added mineral matter amounts to 25% or over, is quite hard to burn. If it catches fire at all, it just flashes up for a moment and then the flame dies right out. And when persistently heated, until the organic matter is all burnt away, it still leaves a very considerable residue of ash.
When this test is to be made on unwoven or skein silk, it is enough to take two or three threads, five or six inches long, and to light them in the flame of a match. For piece goods it is best to pick out the threads carefully, with a pin or fine knife blade, separating the tram from the organzine, and then, with a match, to test each of these in turn. A very little practice will enable the most inexperienced student to make this test satisfactorily.
Of course, for an accurate determination of the percentage of weighting contained in a given sample of silk, it is necessary to resort to delicate chemical analyses. But for all ordinary purposes this simple flame test is quite sufficient.