466
DYEING AND CALICO PRINTING.
over this again fine calico. This triple filter is held in its place by ribs ofwood pegged down to the frame.
Another mixture is—io lbs. indigo, stirred into 80 lbs. sulphuric acid, asbefore. Care is taken that the heat does not rise too high during mixing.Allow the whole to stand for twenty-four hours, at the temperature of 8o° F.;then add 5 gallons of water, and mix well. Next dissolve 80 lbs. commonsalt in the smallest possible quantity of water, and add it to the indigosolution. The whole is then thrown upon the filter and allowed to drain.
The following receipt is given for a “ sweet ” extra# of the first quality:—Take 4 lbs. of the best indigo, dry, well ground and sifted through a lawn orsilk sieve. Add to it by degrees 37 lbs. of the best sulphuric acid, stirringwell, and not allowing the heat to become too strong. Let the mixture standat about 6o° to 70° F. for eight days. Next dissolve 40 lbs. of clean commonsalt in water, and pour the solution into the acid. Then dissolve 60 lbs. ofsoda crystals in water, and add it gradually, lest the mixture should frothover. 2 lbs. of precipitated chalk are next added: the whole thoroughlystirred up, thrown upon the filter, and allowed to drain. The liquid thatdrains through the filter will be green: if its shade continues very deep, it iswell to wash the paste on the filter with a weak solution of salt till thedrainings become nearly colourless. The yield from the proportions givenshould be 120 lbs.
It will be observed that in this formula the indigo is not added to the acid,but the acid to the indigo, which, it is maintained, produces a richer and purercolour.
For a second quality, 8 lbs. of indigo are treated with 74 lbs. sulphuric acid,144 lbs. of salt, 112 lbs. of soda crystals, and 4 lbs. of chalk, working in thesame manner.
For a red extra# of indigo, consisting chiefly of sulpho-purpuric acid, andresembling Boiley’s purple-blue, take 1 lb. indigo, 3i lbs. sulphuric acid, andleave them in conta#, after incorporation for twenty minutes only. Thenthrow the solution into a very large excess of water, and wash with watercontaining a little hydrochloric acid.
Extra# in which the acid is entirely neutralised is called “ free extra#,”“ soluble indigo,” and frequently, though improperly, “indigo-carmine.” Theyare in the form of a dry powder. Some good authorities maintain that car-bonate of magnesia is preferable to soda for neutralising extra#s.
All these acid preparations of indigo should be perfe#ly free from undis-solved particles, which, if present, are very liable to render the goods spottyin the dyeing process.
The paste extra#s should form a rather consistent mass, of a purple colour,without any green or brown refle#ion.
Extra#s of indigo are sometimes adulterated with farina and other inertmatters. This fraud may be dete#ed by dyeing comparative swatches withthe suspe#ed sample against an extra# of known purity.
The more acid varieties of extra#s are applied on wool and worsted, whilstthe more neutral are reserved for silk and cotton goods.
The material commercially known as refined indigo maybe obtained by twodifferent processes :—The one consists in treating common indigo, in finepowder, several times with strong hydrochloric acid not containing free