DYE

DYE ( 374 )

number of vegetable matters, with lime or anyof the alkalies.

Silk is dyed in a fimilar indigo vat withthe foregoing, and without woad. For thevery deep colours however it is found necelfaryto prepare the filk with a high purple from archil.

Metallic fub - oxyds and fulphurets werementioned to be equally powerful in deoxy-genating indigo, with vegetable matters, and tobe employed accordingly. This chiefly takesplace in preparing the indigo vats for cottonand linen. A very Ample and efficacious modeof compofing this kind of blue vat is to mix to-gether one part of indigo, 2 parts of fulphatof iron, and 2 of lime with fufficient water ina vat, to ftir them together very well for a con-fiderable time, and then fuffer them to remaintwo days at reft. In this cafe part of the limefirft decompofes the fulphat of iron, feparatingthe fub-oxyd of iron, which then a&s on theindigo and brings it to the ftate of the green-ellow or deoxygenated indigo, at which time itecomes foluble in the remainder of the lime.This vat therefore confifts of a folution of de-oxygenated indigo by lime, mixed with fomefulphat of lime (arifing from the decompofitionabove mentioned) and at the bottom is a fedi-ment of the oxyd of iron, with any undifiolvedlime or regenerated blue indigo that may becontained. The cotton comes out of the vatgreen as in the former cafe, and turns blue byexpofure to air. It is the conftant practice afterthe cotton is dyed to pafs it through a citterncontaining cold water with a fmall proportionof fulphuric acid. This is found to heightenthe colour, and is of further ufe in diflolvingout any adhering lime or fulphat of lime whichwould give a harfhnefs to the ftu'ff and impairthe luftve.

Alkalies are alfo frequently ufed inftead oflime in the above vat. Their effe£t is precifelythe fame as that of the lime, only being them-felves much more foluble in water than lime,they will bear a much larger proportion ofindigo, and of courfe will form a much deeperand richer dye.

The indigo rapidly regenerates or recovers itsoxygen (and with it its blue colour) at the fur-face of all thefe folutions, and in fo doing itfeparates from the alkali or lime-water whichheld it diffolved, and partly finks to the bottom,partly remains entangled in the fcum. Hencethe ufe of the occafional ftirrings to mix theregenerated indigo with the other materials, andagain dilfolve it. Frequent additions are alfo

ufed of the deoxygenating an d diflolving fub-ftances to refrelh the vat when long expofed toair,, and bring it back to the proper ttate fordyeing.

It is remarkable that the falts of copper arefound by conftant experience to have a totallyoppofite effect on indigo from the falts of iron,the former being obferved not only to have noeffect in rendering indigo foluble, but even toprevent the operation of the fub-oxyds of iron,and to haften the regeneration of diffolvedindigo. This is fuppofed to be the reafon whycottons, firft foaked in fulphat of copper, andthen paffed through a very weak indigo vat, ex-tract at once all the colour, the copper deter-mining the immediate feparation of the indigo.

Orpiment or fulphuret of arfenic is chieflyufed in preparing the indigo folution for topicalapplication in calico-printing. Mr. Hauffmanmixes for this purpofe 25 gallons of water with10 pounds of indigo (more or lefs according toits quality), to which is added 30 lb. of goodpearlafh, 12 lb. of lime, and 12 lb. of orpiment.The whole is boiled for a fufficient time withftirring, and forms a very ftrong yellow li-quid folution, which turns blue by expofure toair. For calico-printing it is thickened eitherwith gum fenegal or with ftarch to the conflu-ence of a jelly, and when ftrongly impreffedon cotton it forms durable blue figures. Theextreme inconvenience of this and all otherfolutions of indigo in lime or alkalies, is theimpoffibility of preventing the perpetual rege-neration of the indigo before the whole is ufed;and as all the colour prematurely regenerated isreadily waffled away in the after proceffes, itbecomes extremely difficult to maintain an uni-form fhade of blue through the pattern of awhole piece of cloth. The gum or pafte areof further ufe in retarding this prematurechange of the indigo.

According to Profeffor Pallas k , cotton andfilk are dyed blue at Aftracan, by a bath veryfimilar to the fimpleft indigo vat for wool, thedeoxygenating fubftance being honey and thefolvent of the indigo being foda.

The proportions are 2 lb. of indigo, 5 lb. ofcarbonat of foda, 2 lb. of lime, and 1 lb. of cla-rified honey. Thefe are put with fufficientwater into large earthen jars fet in brick workover a fire, which will bear a boiling heat, andare heated with frequent ftirring, till the indigois diffolved.

Saxon Blue is the name given to a totally dif-ferent preparation of indigo from thofe hitherto

1 Ph. Mag. vol. I.