DYE

DYE

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always of a deep green, but by expofure to airit foon changes to a fine deep blue. Thischange of colour of the folution of indigo fromgreen (or, if much alkali has been ufed, yel-lowiffi green) to blue, forms a very entertainingexperiment in the fmall way. The fame changeis conftantly going on upon the furfaces of indigovats, which, being in contact with air, are al-ways covered with a fine variegated green andcopper-coloured fcum that is perpetually pallingto blue, unlefs ftirred in and mixed with themafs below.

This conftant change in the nature of theindigo demands an equally conftant attentionto the ftate of the vat on the part of the work-man, to keep the liquor at the proper point ofoxygenation; for when the blue colour is re-generated, the indigo again partly feparatesfrom the lime or alkaline folvent, or remainsonly fufpended in it as a fine impalpable pow-der which will not adhere to cloth with anypermanence. A frefii addition of the deoxy-dating materials alfo becomes from time to timeneceflary; but again, too great a quantity of thisand of the lime or alkali is equally detrimental,and fo far alters the nature of the indigo thatit will no longer change to blue by air, but re-mains of a rufty yellowifh green.

Woad, which is a fecula or dried pulpmade of the fermented leaves and ftem of theIfatis TinBoria, and in nature not unlike indigo,is commonly ufed along with indigo in the dye-ing of woollen. Woad is never employed alone,for the blue colour which it gives, though full-bodied and very permanent, is not fufficientlybright and gloffy. In its nature and affinitiesfor oxygen it appears confiderably to referableindigo.

The bath in which wool is dyed blue is nota copper boiler as is the cafe with other dyes,but is a large wooden vat generally funk in theground fitted with a cover to protect it fromthe draught of external air, and to preferve thetemperature more uniform. The colour isfometimes procured from indigo alone, fome-times from a mixture of indigo and woad. Thelatter is prepared in the following way. b

Two hundred parts of woad broken fmall arefi.rft thrown into the vat, to which is thenadded a hot deco£lion of 15 parts of weld, asmuch madder, and a balket of bran all boiled ina feparate veffel with water enough to fill thewooden vat. This is allowed to remain at reftfor fix hours, when the whole is well ftirred fo-

mentation is thus produced in the vat, attendedwith very ftrong acrid vapours, owing to thebeginning decompofition of the woad. Aftera fufficient time (during which the ftirring isoften repeated) 4 parts of lime are added whichturns the vat of a black colour, and makes thefumes ftill more acrid. Immediately afterwardsfrom 5 to 15 parts of indigo (more or lefs ac-cording to the depth of colour to be dyed) mixedup with water into a cream-like fluid are pouredin, and the whole ftirred and covered. Thedifoxygenation of the indigo is effe&ed by theaction of all the vegetable matter already in thevat, and when thus altered it is difiolved bythe lime. The figns of the folution of the in-digo are the rifing of a blue copper-colouredand variegated fcum, and the liquor beneathbecoming of a bright green.

Before the cloth is put into the vat, it iswetted with clear hot water and wrung out.The time of remaining in the vat depends onthe depth of colour wanted and on other cir-cumftances.

It comes out green, as already mentioned,but the colour changes to blue by expofure toair. It is then thoroughly wafhed to difehargeall the colouring matter that is not fixed, anddried in the field.

Dr. Bancroft thinks (and with feme reafon)that the weld and madder added to this vathave little other effect than to fupply vegetablematter for the fermentation which is to difoxy-genate the indigo, and hence that inftead ofthefe expenfive ingredients, any herbs, or agreater quantity of bran, or coarfe fugar, ormany other cheaper materials might be fub-ftituted.

A richer blue bath, that gives a finer andfofter colour (but is more expenfive on accountof indigo alone fupplying the blue colour) ismade by boiling in a copper with fufficientwater, 9 parts of pearlaffi, with as much branand x part of madder, after which 9 parts ofindigo ground up with a little water are added,and a gentle heat kept up for about forty-eighthours, ftirring the whole well together threeor four times during the folution. In this cafe-the alkali is the folvent of the indigo when de-oxygenated by the bran and the madder.

In other indigo baths the ammonia containedin ftale urine is the folvent of the colouringmatter, and the deoxygenating matters are mad-der and tartar. The variety is endlefs of com-binations thus afforded by the ufe of a raft

-ether and again let to reft. A kind of fer-

k Bcrfliollct.