INDIGO-VAT.

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process at once, the vat is lost, and the contents have to be run away. Themode of working these vats is as follows :—They are stirred up first thing inthe morning, when the scum is also removed from the top ; then, in order thatthe goods to be dyed may not touch the sediment at the bottom, there isplaced in the liquid a net, made of thin strong twine, fastened to a woodenhoop : the goods are dyed, immersed in the vat, inside this net; the time ofimmersion varies, but is generally from twenty to twenty-five minutes. Whenwithdrawn from the vat the goods appear greenish-yellow, but very soonbecome blue on exposure to the air. The immersion is repeated as often as isneeded to produce the desired shade. The goods are afterwards washed, firstin acidulated water, next in a plentiful supply of clear water.

Silk is always dyed in the Indian vat, but, since the blue is never very deep,Prussian blue or the aniline blues are preferred, though indigo has the advan-tage in fastness.

M. Leuchs* has suggested to do away with all the vats above mentioned,and to utilise instead the property of pe&in and pe&ic acid of reducing indigoin presence of soda. Produ&s containing these pedtic compounds are verycommon in thevegetable kingdom,and the turniptribeyields them inabundance.The suggestion is valuable, but has never been tried in practice, except on avery limited scale. A large manufacturer in Belgium reports, in a privatecommunication, that when you have intelligent workmen this process is excel-lent, but the prejudice of foremen stands in the way of its general adoption,the fa< 5 t being that they think no good can be done with cheap materials,—rubbish as they term it,—as are turnips, mangolds, beets, and the like. Spe-cimens of woollens and silks vatted in this manner are not only not inferior,but, according to competent judges, superior to those done in the commonvats. In some exceptional cases the kind of vat above described as Germanis used for cotton piece-dyeing : this is principally for thick and heavy goods,into which the cold lime and copperas liquor-vat would not penetrate suffi-ciently well.

An entirely new vat has been proposed by Messrs. Schutzenberger andLalande. Bisulphite of soda, standing at 30° to 35 0 Baume, is brought incontadt in a covered vessel with twisted sheet zinc or granulated zinc, fillingup to the top of the vessel without occupying more than one-fourth of its realvolume. After the lapse of about one hour the liquid is poured into an excessof milk of lime, which precipitates the salts of zinc. It is stirred, and theclear liquid drawn off either by filtration and pressure, or by decantation afterthe addition of water. Air should be excluded as much as possible. Bymixing the hydrosulphite thus obtained with ground indigo and the amountsof lime or soda needful to dissolve the reduced indigo, we obtain at once ayellowish liquid containing no insoluble matter except the earthy impuritiesof the indigo. One kilogramme of indigo may be thus reduced so as to forma solution not exceeding 10 to 15 litres in volume. For dyeing, a certainquantity of the reduced indigo solution is run into a vat filled with water.This vat being clear in its entire depth, the operation of dyeing involves noloss of time. The excess of hydrosulphite constantly reduces the scum ofoxidised indigo forming on the surface of the dye-bath, the strength of whichcan be kept up during working by successive additions of the concentrated

* “ Polytechnisches Notizblatt,” 1865, p. 277.

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