Scientific American 21, 3.2.1855
The oldest method of dyeing blue on cotton is with indigo. It is believed that the Greeks and Romans were unacquainted with the use of indigo, but it has been used from time immemorial in the East. The first indigo employed for dyeing in Europe, wwas brought by the Dutch from the East Indies. It was also used by the Mexicans upon the arrival of the Spaniards, as mentioned by Clavigero. The beet indigo is now raised in Bengal, but as good can be cultivated in the United States. It makes the richest blue color on cotton, but is expensive. The entering of indigo-blue is a branch of dyeing peculiar in itself, and requires much experience. There is so much dependent on the skill of the eye, that no amount of word instruction can enable a pereon to conduct the business, still the way to dye the color can be taught, and a number of useful hints given to all. A work recently published in London by David Smith, named the Dyer's Instructor, is worse than useless to any person who desires information on indigo dyeing, more especially on the blue vat.
The bath for dyeing indigo blue on cotton is called "the blue vat." The most common vessels used are large wine casks, five of which are called a set, each capable of handling ten pounds, that is for yarn. Many vats are made of cast iron, well bolted, and rendered water tight at the seams. These are made of a rectangular form, and capable of handling about from twenty to twenty-five pounds of yarn at once. They are made very deep, so as to allow the sediment to is undisturbed on the bottom, when the yarn is being handled. Pieces are dyed in these cast iron vats by using a frame with rollers, and making the piece, which are sewed with their rude together, dip down and turn over a roller sunk is the vat to a certain depth. It is also a common thing to suspend a screen down in the vat, to prevent the disturbance of the sediment.
A blue vat may be set with more or less indigo, so as to make it strong or weak. The best proportions are for tell pounds of indigo good quality — ground in a mill, until so grit is felt when rubbed between the ringer and thumb — sixteen pounds of powdered quicklime, and fourteen pounds of the sulphate of iron (copperas,) that is for a ten pound vat. These are stirred up occasionally for two days, in the water in the vat — which is filled to within four inches of the top — with an iron rake, which is a disk of thin plate steel set on the lower end of a long shank, to reach the bottom. Care must be taken to rake well from the bottom, until no hard lump is felt sticking to it. When the liquor assumes a deep rich green color, with a violet froth floating on the top, it is a sign that the coloring matter of the indigo has been given out to the water, and the vat ready for working, after it has completely settled. A thin crust of the carbonate of lim gathers on the surface of a blue vat, and this prevents the admission of air. When this is broken, by handling the goods it, dyeing, the vat has always to be raked up, and allowed to settle before it can be worked again. This takes about ten hours. Only part of the indigo is given out to the liquor, at first, and as the vat is worked, it has to be mended with lime and copperas, from time to time. The wants of the vat are known only by its appearance. As the indigo is worked out, the color of the vat becomes a lighter green. It takes five ten-pound vats to work out the indigo economically, each for ten pounds. They are worked out and made up in rotation, which takes about four weeks, working every day. The yarn, to attain the deepest shade, gets tire dips, commencing with the weakest vat and finishing with the strongest, wringing and scotching the yarn after every dip. The cotton comes out of the strong vat a deep green color, and becomes blue as it is exposed to the air by absorbing oxygen. The business of indigo blue dyeing is on this account very unhealthy. A little pearl ash added to the vat makes it produce a clearer color.
— When fifty pounds of yarn are dyed at a batch regularly, it requires twenty-five ten-pound vats to work the indigo economically. It is scarcely possible to maintain all the vats at one particular strength; there is generally a difference of two or three shades in fits ten pound bundles. These are examined and compared with one another before the last dip, and are handled in the cat such a length of time as will bring all to the same shade when finished. After being dyed the goods arc run through a tub of diluted sulphuric a id, then washed, wrung, and are ready to be dried. The sulphuric acid blooms the color, makes it look richer. and the goods cleaner. In emptying indigo vats, when they are worn out, to be set again, the sediment only is thrown out, and the clean liquor retained, to be used in place of water. Large vats cannot be so economically worked as small ones.
The blue vats in calico print-works are thrown out long before the indigo is so completely worked up, as in the establishments for dyeing yarn in New York and Philadelphia.
Great care must be exercised in the selection of good copperas. The best has a dirty green appearance, not a red rusty look, which some mistake for the genuine. If bad copperas is used, the blue vat, as it got, old, will float — that is, the sediment or sludge will not sink — and in that state a vat is unfit for use.
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