This chemical vat used copperas (iron(II) sulfate) and lime (calcium hydroxide) to reduce the indigo pigment. The indigo needed to be finely ground.
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The copperas vat (1)
The indigo is usually ground in a metal basin somewhat inclined, by means of three or four twelve pound cannon balls, which boys roll in the basin till the indigo is ground into a paste, that can be scraped off the basin without leaving any sticking to it. A boy of twelve years old can thus grind ten pounds of indigo in a day. If a brandy cask holding about 120 or 130 gallons, is used as a vat, the ingredients should be five pounds of indigo, one pound of soda, five and a half pounds of lime, and five pounds of copperas or green vitriol. Dissolve a pound of soda and half a pound of lime, in which solution you may grind your indigo; when ground, put it in the vat, which should be filled with water within a foot of the top. Then dissolve in hot water five pounds of English green copperas, which must also be turned into the vat; then slack your lime with a small quantity of water, and turn it in powder into the vat; then throw in the grounds of the soda, lime, and indigo; stir the whole together, and let it rest covered all night. Early in the morning stir it, and about nine o'clock it will be ready to use, if it has been left to settle four hours. If the vat should not be in order for some hours afterward, dissolve half a pound of copperas in four or five quarts of water. The vat should be of a yellowish green; if too yellow, a small quantity of lime will correct it.
Thomas Cooper, (1815), A practical treatise on dyeing, and callicoe printing: exhibiting the processes in the French, German, English, and American practice of fixing colours on woollen, cotton, silk, and linen, Philadelphia. pp. 71, 72
The copperas vat (2)
Indigo blue or Vat Blue
6¼ lbs of Indigo
18½ lbs of Lime
18½ lbs Green Copperas or Sulphate of Iron.
To prepare an indigo vat you grind 6¼ pounds of indigo with water.
When it is perfectly pulverized, you throw it in a vat; then you slack 18½ lbs. of lime and pour it in the vat; stir well for some time, then you dissolve in hot water the 18½ lbs. of sulphate of iron, and pour it in the vat. Stir anew for some time, fill the vat with cold water and let it rest about twelve hours before working it.
Louis Ulrich, Hippolyte Dussauce, (1863), A complete treatise on the art of dyeing cotton and wool, as practised in Paris, Rouen, Mulhausen, and Germany, Philadelphia, H.C. Baird, p. 78.
The copperas vat (3)
Copperas or Ferrous Sulphate Vat. - This vat, which forms one of the oldest methods of indigo dyeing, is made up with copperas, lime, and indigo. The lime decomposes the ferrous sulphate, forming calcium sulphate and ferrous hydrate, and the latter, having a great tendency to absorb oxygen and pass into ferric hydrate, decomposes water liberating hydrogen. The hydrogen is not given off as gas, but immediately combines with the indigotin to form indigowhite, which dissolves in the excess of lime present. The amounts of indigo, lime, and copperas vary according to the work to be done, and the order in which the ingredients are added varies with different dyers. The following figures give a general idea of the proportions of indigo, copperas, and lime required for setting a vat :
Water, 200 gallons.
Indigo (60 per cent.), 10 lbs.
Copperas, 30 lbs.
Quicklime, 35 lbs.
Some dyers add these substances direct to the dye vat, but it is much better to mix them previously in a more concentrated form, and to accelerate the reaction by the aid of heat, A cask furnished with a lid answers the purpose well. The 10 lbs. of indigo, ground thoroughly well to a smooth paste, is mixed with 8 gallons of water at 65° C Milk of lime (containing 35 lbs. quicklime) of the same temperature is added during constant stirring, and this is followed by the 30 lbs. of ferrous sulphate dissolved in about 10 gallons of hot water. More water, at a temperature of 60° to 65° C, is added until the whole measures about 50 gallons. The lid is put on the cask and the mixture allowed to stand (stirring occasionally) for three to four hours, by which time the liquor will have assumed a fine amber yellow colour. The mixture may then be poured into the dye vessel which has been already three parts filled with water, stirred up, and allowed to settle. Before entering the goods, the flurry is removed by means of an iron scoup called a skimmer.
The "copperas" vat is used chiefly for hank and "resist" dyeing. It is not suitable as a continuous vat.
Edmund Knecht, Christopher Rawson, Richard Loewenthal, Manual of dyeing : for the use of practical dyers, manufacturers, students, and all interested in the art of dyeing., 6th ed. (1920), London: Charles Griffin and company, limited; Vol. 1, p. 316
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