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.

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.

DYEING SILK WITH COLORS FAST TO WASHING

As a rule the method previously described of dyeing silk with Acid dyes in a broken bath of soap, or better, of boiled-off liquor, will be found perfectly satisfactory. The shades are easily obtained, the colors are brilliant, and, if the right dyes are used, exceedingly fast to light, and the material, if properly rinsed, suffers no deterioration.

On the other hand these colors are not, in the slightest degree, fast to washing.

The dyed goods can be cleaned with gasoline and the like, but when passed through a lukewarm bath of soap and water they bleed badly, and in boiling soapsuds the color can be completely stripped from them.

In most cases this is not a serious objection, for a person who will send a handsome hand-dyed silk scarf or piece of embroidery to the family washtub is entitled to scant sympathy if the results are disastrous. But occasionally it is important to have colors on silk which can be guaranteed against moderate or even against, severe, washing.

_Fast Colors on Silk._—There are two grades of fastness known to the dyers—“fast” and “embroidery fast.”

“Fast” means simply that the silk is to be dyed fast to ordinary, careful handling so that the colors will not bleed or run in a warm or even hot soap bath, but does not guarantee them against every possible maltreatment.

The best way of doing this is by the use of the Direct Cotton or Salt dyes, described in Chapter III, which, it will be remembered, only dye wool or silk at a high temperature, at or near the boiling point and, preferably, in an acid bath, but, when once on, are very hard to dislodge. The selected ones are very fast to light and present a great range of bright, attractive colors, which are nearly, if not quite, as brilliant as those produced by the Acid dyes.

They are applied in a boiling bath containing a little acetic acid, and a good deal of salt, especially for full shades. For lighter shades, the presence of salt is hardly necessary. The goods are to be finished just as with the Acid dyes, with a soap bath followed, if the scroop is desired, by a weak bath of acetic acid.

The results, when carefully done, are very good. They possess, however, one disadvantage for the amateur dyer. These colors are quite hard to strip, and so, the desired effect must be produced the first time, or not at all. It is not possible to strip an unsatisfactory shade in a hot soap bath, and dye it over and over again without injury, as in the case with Acid dyes. They are best stripped by soaking in a bath of sodium hydrosulphite, and then washing.

_Embroidery Fast Colors._—While the above process gives shades fast enough against all ordinary washing, it sometimes happens that silk must be dyed fast enough to withstand exactly the same treatment that coarse cotton or linen goods are subjected to, without bleeding or staining. The salt dyes are not quite fast enough for this, particularly because, not having been converted in the dyeing process into a special insoluble condition, if they should be detached from the fibre by strong or hot soaping, they would be liable to stain the neighboring tissues and not wash off quite clear.

One of the hardest tests that colored silk is called upon to stand is when, in small quantities, it is used with a large amount of white linen or cotton goods. Thus, for instance, when monograms are embroidered in red or blue silk upon white towels or napkins, and the latter are scrubbed, week after week, in the regular wash, the color must be fast, indeed, not to show some evidences of running. Hence the term “embroidery fastness” as applied to this class of dyes. Thanks, also, to the amiable practice of the modern laundress of lightening her labors by the addition of bleaching powder and other strong chemicals to the washtub, it is very important that a silk dyed “embroidery fast” should be able to withstand the action of these agents as well as of soap. Up to the last few years these colors were only obtained by the use of the Alizarine dyestuffs, the full rich scarlet so often used for this purpose being the modern form of the old, madder-dyed, Turkey red of our forefathers.

But, during the last few years, the troublesome and tedious mordanting processes necessary for the proper development of color by the Alizarine dyes, have been replaced, for craftsmen, and, indeed, by most professional dyers, by the much simpler and shorter processes of vat dyeing. As long as Indigo was the sole representative of the class, it was of very little use for silk dyeing. But since the introduction of the splendid series of new vat dyes, the Algol, Ciba, Helindone, Indanthrene, and Thio Indigo colors, which, dyed in a single bath, give a whole range of brilliant shades, wonderfully fast to light and to washing, the necessity for mordant colors has very largely disappeared.

DYEING SILK WITH VAT DYES

It must always be remembered when working with silk, wool, leather, or any other animal material, that such materials are extremely sensitive to the action of alkalies, especially when hot or caustic, while they are but slightly injured, if at all, by the action of dilute acids. For this reason it is always better, whenever possible, to dye silk with the Acid dyes or the Salt dyes, in an acid or neutral bath, rather than to use dyestuffs like the Vat dyes or the Sulphur colors, which need an alkaline dye-liquor. Furthermore, the silk is likely to have a more brilliant lustre when dyed with a color which fastens to it by chemical affinity, from a solution, rather than one where the color is fixed because the oxygen of the atmosphere changes it into an insoluble powder, while in the pores of the silk. It is, however, perfectly possible to dye silk full shades with the Vat dyes and even—though this is not often advisable—with the Sulphur dyes, by using some simple precautions.

The best Vat dyes for silk are Indigo itself, and its substitution products, like Brom-Indigo, _Elberfeld_, or the Thio Indigo dyes, _Kalle_, or else the rather closely related colors like the Helindones, _Metz_, and the Ciba colors, _Klipstein_. It is of importance to use only those which are shown in the table on page 102, as dyeing in a cold, or at most, a lukewarm bath.

The dye-bath should be made with a considerable amount of dyestuff, so as to avoid the necessity of keeping the goods in it long. And the amount of caustic alkali should be kept as low as possible, consistent of course with dissolving the reduced dyestuff. It has been found in practice that the presence of glue or gelatine in the bath, or even of glucose (molasses, corn syrup, Karo syrup, etc.), protects the silk, wool, and other animal fibres greatly from the action of alkalies. It should, therefore, be added in quantities of two or three large tablespoonfuls to the gallon of dye-liquor.

The wet goods should be immersed in the cold or lukewarm bath, and turned constantly for a few minutes only, before taking them out, wringing them, and hanging them up to oxidize. As soon as the color sets, which is shown generally, by the change of shade and which never takes more than, say, twenty minutes if the materials are well opened up, the goods should be brightened in a hot bath of good, neutral, olive oil soap, and then finished as previously described. It will be remembered that several, indeed most of the best Vat colors do not develop their final shade at all, until after the soaping process.

When carefully done, this process will give exceedingly fast and quite brilliant colors, without injury to the strength of the goods.

_Comparative Results of Vat Dyes and Sulphur Dyes on Silk._—It is hard to get full shades with Sulphur colors because it is generally necessary to heat the dye-bath, and this, owing to the powerful alkaline properties of the sodium sulphide, is very injurious to the silk. Besides this, the sulphur dyes are much less brilliant than the Vat dyes, and have no good red or orange shades in the whole class. They accordingly should not be used, excepting where no other are available, or, as will be described in a later chapter, when doing “resist stencilling” on silk.

On the other hand, such very unusual advantages do some of these new Vat dyes possess, for the dyeing of silk for special purposes, that large quantities of Helindones, Thio Indigoes, and other good specimens of this class are being sold, at comparatively very high prices, to manufacturers of fine shirtings where the patterns are made by weaving fine lines or figures of brightly dyed silk into the linen or cotton fabric. Until the introduction of these dyes in the last two or three years these shades could not have been produced fast enough for this purpose.

Sulphur dyes can also be used on silk without injuring the goods, by taking the precautions described earlier in this chapter. The shades, however, are quiet and dull, as compared to those produced by other classes of dyestuffs; and it is almost, if not quite, impossible to get a good full red and, especially, a good scarlet, by using these colors.

Silk properly dyed with Sulphur colors is extremely fast to washing. But these dyes, unlike the best Vat colors, are as a rule quite sensitive to bleaching agents, and therefore are not so well adapted for general use on “embroidery fast” silk.