Scientific American 16, 8.1.1848
For the Scientific American.
Good indigo is as high in price as cochineal, therefore this color is a valuable one not only on account of its permanency, but because the stuff that makes it is so expensive. It has, therefore, received great attention in the economising of the indigo, so that no particle of its coloring matter may be lost. The art of indigo blue dyeing, therefore is distinct in itself and it lakes a long time to be master of it, and unless the operator has a good eye for color, he will never be profitable, either to himself or employer, as the feeding of the vats and the striking of the sidderent shades of color, all depend upon this faculty. This color is so well arranged and systematised by the dyer, that the shades of half a cent in price per pound, in the ratio of prices are made and these must be done, so that there will just be a certain quantity of indigo taken up on the goods and none lost, for such a valuable dye drug cannot be lost with impurity in the smallest quantities.
To Sett A Blue Vat
Take 10 lbs. of good Bengal or American indigo and grind it in water so that it will be as fine as flour, so line that no grit will be felt in the fingers when rubbed between them.— Indigo grinds easier if steeped in warm water for four hours before being put into the grinding mill. Ten pounds of indigo thus ground is put into a clean cask, such as a wine or oil puncheon, filled nearly full with clear water. To this is added fourteen ?rounds of the sulphate of iron (copperas) and sixteen pounds of the flour new slacked pure lime. This mixture is to be well stirred every few hours for two or three days, when the liquor in the vat will have a fine deep green color, a sure sign that it is in good order for dyeing. There is one thing to be observed, however, which is, that the rake for stirring the vat must be of an oval shape, or like a disc on the end of a long stick, the disc to be steel, very thin, as the lime, copperas and indigo unite together and stick in lumps at the bottom of the vat, and the vat must be raked from the bottom until all the stuffs are mixed in a wet powder.
Before a blue vat is used for dyeing, it must not have been disturbed alter being raked for twelve hours, and then whatever is wanted to be dyed must be dipped and handled, so as not to disturb the sediment at the bottom, and then where a vat is used, it must be stirred up again and not touched until it is to be used for dyeing. A thin crust gathers on the surface of a blue vat after it is stirred up, which keeps out the action of the atmosphere, and if this thin crust is broken the indigo sinks and will not give out its color until stirred up and left to settle again. The cause of this is the effect of a law which is too abstract to be of any use to explain here, and it would take up too much space to do so. We merely state the fact. When cotton is dipped in a blue vat it is not blue but green, and holds this green color until it is exposed for a short lime to the atmosphere, when oxygen is absorbed and a deep blue the result. Blue dyeing is, therefore, an unhealthy occupation, as in a close room the atmosphere is deprived of much of its vital principle. The deepness of blue shades are made by frequent dippings and airings. Two vats, the one set weaker than the other and the cotton squeezed out of the one (the weakest) aired and finished out of the strongest, will enable a small factory to do considerable work. We might describe the system of blue dyeing upon a large scale, but this of itself would require almost a volume of matter; our object is to describe so as to be a benefit to a number who would manufacture a little for themselves. Blue vats must be fed as it is termed, from time to lime, that is when they look blackish, they must get a little copperas and lime added, and be well stirred up, as the indigo does not give out its coloring in the blue vat but very slowly, but more economically than by any other plan. By frequent dippings and feedings as we have directed, a cotton blue vat will dye a great quantity of goods and work up all its indigo until the liquor is quite whitish. When the vat must be renewed, by saving all the clear liquor and throwing away all the grounds or sediment, and adding as before directed, filling up with clear water what is wanting in the old liquor to till the vat.
Five vats are called a sett, and to work indigo very advantageously, five setts are necessary. A water tight wooden box may be used for vat instead of a tin hogshead, and some vats are made of iron. The proportions we have given for setting a blue vat will enable any person to set a larger or smaller one, by varying the quantities of the dye drugs. The liquor wrung or dripped out of goods must not be thrown away, but always returned to vat again.