DYE

DYE

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ounce of bruifed galls and a quart of prettyllrong glue, and when it has boiled for iominutes add one ounce of vitriol of iron, andallow the whole to cool with frequent ftirring.Then foak the thread for an hour in the liquor,after which wring it out and dry it in the {hade.The dye is then compleat. Though the aboveprocefs includes many operations it does notappear needlefsly complex, but on the contraryis very judicious, and agreeable to the long efta-bliftied practice of the art. Cloth of cotton orlinen may be dyed in this way, according tothe author, as well as thread.

Of Grey.

Many of the varieties of grey, iron-grey, flatecolour, &c. are given by procefies in generalfimilar to thofe for black, but with fmaller quan-tities of the ingredients, and efpecially a fhortertime of immerfion. They are often finifhedwith a weak bath of weld, cochineal, Brazilwood, and other livelier colours to give fomeparticular tints.

Of Blue.

The principal material for this molt im-portant colour is indigo, a Angular vegetablepreparation produced by the fermentation ofthe leaves of a fmall plant, the Indigofera Tinc-toria, and one or two varieties of the famegenus, cultivated abundantly in many tropicalcountries, particularly in Mexico and other partsof Spanifh America, and of late years verylargely in the Britifli Eaft Indies.

Many important chemical faCts are connectedwith the preparation of this colouring matter,which, with the analyfis and fuller account ofthe methods of folution for the purpofe of dye-ing will be referred to the article Indigo. It willbe fufficient to obferve in this place that indigois brought over in the form of cubical cakes orlumps, which (when the article is of the beltkind) are of a very deep blue colour almoftblack in mafs, very light, breaking fhort andeafily reduced to powder; when rubbed withthe nail {hewing a polilhed copper-colouredftreak, and of a peculiar fmell, which particu-larly {hews itfelf when in folution.

Indigo is foluble of itfelf in fcarcely anyknown menftruum except the ftronger acids;water, alkalies, fpirit of wine, &c. added finglyhaving no action on it whatever. The reafonof this great infolubility appears from multi-plied experiments to confift in the high degreeof oxygenation in which blue indigo in its com-mon hate exifts. That the blue colour dependson the action of the air is obvious from the

circuni(lance, that in the preparation of thisdye the colour of the fermented plant is at firftgreen, but by expofure to air it takes the deepand permanent blue, for which it is much va-lued. Other experiments which will be men-tioned under this article make it appear that thechange from green to blue is owing to thiscaufe.

In order to render indigo foluble for the pur-pofes of dyeing a faft colour, a complicatedprocefs is requifite. Firft, fome fubftance muftbe added which is capable of partially deoxy-genating indigo, or of reducing it to a ftatefomewhat fimilar to the recent green pulp ofthe indigo while under manufacture; and next,a liquid muft be added capable of diflolving thedeoxygenated indigo, that it may be applied tothe fibres of the ftuff immerfed in it. Theformer objeCt is fulfilled by a great varietyof fubftances, fo different in nature, that it ishardly to be conceived that they can have anyother property in common than that to whichtheir utility is attributed, namely, that of de-priving indigo of its oxygen.

The additions actually employed for this pur-pofe, or found by experiment to be capable ofbeing fo ufed, are other dyeing matters, fuchas madder or weld, vegetable mucilaginous fub-ftances, fuch as fugar or gum, g and alfo manymetallic fub-oxyds, or metals only partially com-bined with oxygen and capable of abforbingfrom the indigo that additional quantity of thisprinciple which is requifite to complete fatu-ration. The fub-oxyd of iron is chiefly ufedfor this purpofe, and is made extemporaneouflyby mixing lime and fulphat of iron; and as aproof of the neceflity of ufing fub-oxyd, and nota perfeCt oxyd, it may be added that the redperfeCt oxyd of the fame metal has no effeCtwhatever in bringing indigo to a foluble ftate.Dr. Bancroft alfo has found the fub-oxyd of tinequally beneficial. The fulphuret of arfenicor orpiment is alfo commonly ufed for the famepurpofe.

The indigo thus deoxygenated is now folublein the alkalies or in lime-water, which are thefolvents actually employed in the compofitionof the indigo vat.

Indigo is a fubfantive colour, or one that re-quires no mordant to be previoufly united withthe ftuff to be dyed. It is alfo one of the fafteftcolours known, but to render it permanent, itmuft be prefented to the fibres of the doth inits green deoxygenated ftate. Hence it is thatcloth when it comes out of the indigo vat is

* Bancroft.