D Y I N G. 
mcnt poured into linen bags. When the water has 
drained from it fufticiently, ir is formed into fmall lumps, 
and dried in the lhade. In this (late it is fold to the dyer 
under the name of indigo. 
Dr. Roxburgh, who firft: drew the attention of manti- 
fafturers to the neriufn tmff.orium, a bay-tree very common 
in Hindooitan, from the leaves of which indigo may be 
extrafted, has given a very (hurt method of obtaining 
that pigment. The leaves are kept in a copper full of 
water, fupported at the temperature of i6o°, till they 
affume a yellowifh hue, and the liquid acquire a deep green 
colour. The liquid is then to be drawn off, agitated in 
the ufual manner, till the blue flocculce appear; and 
then the indigo is to be precipitated with lime water. 
This procefs, which fucceeds equally well with the in- 
digofera, indicate that the plants, from which indigo may 
be extratfed, contain a peculiar green pollen, fallible in 
water. The intention, both of the fermentation of the 
common method, and of the Raiding, according to Dr. 
Roxburgh’s method, is merely to extraft this pollen. 
Mr. Hauffman firft lhewed, that this green balls of indigo 
has a ftrong affinity for oxygen ; and the fubfcqtient ex¬ 
periments of Drs. Roxburgh and Bancroft have confirmed 
hisobfervations, and put them beyond the reach of doubt. 
It gradually attrafts oxygen from the air ; in .conlequence 
of which, it acquires a blue colour, and becomes infoluble 
in water. The agitation is intended to facilitate this ab- 
forption, by expofing a greater fiirface to the aftion of 
the air. The lime water, by abforbing a quantity of 
carbonic acid, with which the green pollen feems to be 
combined, greatly facilitates the feparation of the indigo. 
The method of preparing indigo, and of applying it to 
the purpofes of dying, feems to have been very early 
known in India. But in Europe, though it had beenoc- 
cafionally ufed as a paint, its importance as a dye-fiuff 
was not underffood before the middle of the fixteenth 
century. The Dutch were the people who firft imported 
ir from India, and made its importance known in Europe. 
It was afterwards cultivated in Mexico and the Weft In¬ 
dies with fuch fuccefs, that the indigo from thofe coun¬ 
tries was preferred to every other. In confequence of 
this preference, they fupplied almoft the whole of the 
European market. But within thefe few years, the Eaft 
Indian indigo, owing entirely to the enlightened exertions 
of fome of our countrymen, has recovered its character, 
and is now imported, in very confiderable quantities, into 
Great Britain. For the chemical properties of indigo, 
fee the article Chemistry, vol. iv. p, 344. 
Indigo has a very ftrong affinity for wool, filk, cotton, 
and linen. Every kind of cloth, therefore, may be dyed 
with it, without the affiftance of any mordant whatever. 
The colour thus induced is very permanent; becaufe the 
indigo is already faturated with oxygen, and becaufe it is 
not liable to be decompofed by thofe fubftances, to the 
aftion of which the cloth is expofed. But it can only 
be applied to cloth in a date of folution; and the only 
folvent known being fulphuric acid, it would feem at firft 
fight that the fulphuric acid folution is the only ftate in 
which indigo can be employed as a dye. The fulphat of 
indigo is indeed often ufed to dye v’ool and filk blue ; 
but it can fcarcely be applied to cotton and linen, becaufe 
the affinity of thefe fubftances for indigo is not great 
enough to enable them readily to decompofe the fulphat. 
The colour given by fulphat of indigo is exceedingly 
beautiful: it is know-n by the name of Saxon blue ; be- 
eaufe the procefs, which was difeovered by counfellor 
Barth in 1740, was firft carried on at Grofftnhayn in 
Saxony. The method of the original inventor was very 
complicated, from the great number of ufelefs ingredients 
which were mixed with the fulphat. But thefe ingre¬ 
dients were gradually laid afide, and the compofition fim- 
plified by others, after the nature of it, which was for 
lome time kept fiecret, became known to the public. 
The beft procefs is that of Mr. Poerner : viz. One part 
of indigo is to be diffolved in four parts of concentrated 
Vol, VI. No. 338. 
149 
fulphuric acid ; to the folution one part of dry carbonat: 
ol potafli is to be added, and then it is to be diluted with, 
eight times its weight of water. The cloth muft be 
boiled for an hour in a folution, containing five parts of 
alum and three, of tartar for every Thirty-two parts of 
cloth. It is then to be thrown into a water'bath, con¬ 
taining a greater or linaller proportion of the diluted ful¬ 
phat of indigo, according to the fhade which the cloth is 
intended to receive. In this bath it muft be boiled tiil 
it has acquired the wifhed-for colour. The alum and 
tartar are not intended to aft as mordants, but to facili¬ 
tate the decompolition of the fulphat of indigo. Berg¬ 
man afcertained that alum poffeffes this property. The 
alkali added to the fulphat anfwers the fame purpofe. 
Thefe fubftances, alfo, by faturaling part of the fulphu¬ 
ric acid, ferve, in fome meafure, to prevent the texture 
of the cloth from being injured by the aftion of the acid, 
which is very apt to happen in this procefs. 
But fulphat of indigo is by no means th‘e only folution 
of that pigment employed in dying. By far the rnoft com¬ 
mon method, and indeed the only method known before 
1740, is to deprive indigo of the oxygen to which it owes 
its blue colour, and thus to reduce it to the ftate of green 
pollen; and then to diflblve it in water by means of al¬ 
kalies, or alkaline earths, which in that ftate aft upon it 
very readily. Indigo is precifely in the ftate of green pollen 
when it is firft extrafted from the plant in the Raiding 
procefs delcribed by Dr. Roxburgh. If, therefore, there 
were any method of flopping fliort here, and of feparat- 
ing the pigment while it retains its green colour, it would 
be precifely in the ftate beft adapted for dying. Nothing 
more would be neceftary but to diffolve it in water by 
means of an alkali, and to dip the cloth into the folu¬ 
tion. But as indigo is not brought home to us in that 
ftate, the dyer is under the necellity of undoing the lad 
part of the indigo-makers procefs, by l'eparating again 
the oxygen, and reftoring it to its original green colour. 
Two different methods are employed for this purpofe. 
The firft of thefe methods is to mix with indigo a folu¬ 
tion of fome fubftance which has a ftronger affinity for 
oxygen than the green bafis of indigo. Green oxyd of 
iron, for inftance, and different metallic fulphurets. If, 
therefore, indigo, lime, and green fulphat of iron, be 
mixed together in water, the indigo gradually lof'es its 
blue colour, becomes green, and is dilfolved, while the 
green oxyd of iron is converted into the red oxyd. The 
manner in whkh thefe changes take place is obvious. 
Part of the lime decompofes the fulphat of iron ; the 
green oxyd, the inftant that it is fet at liberty, attrafts 
oxygen from the indigo, decompofes it, and reduces it 
to the ftate of green pollen. This green pollen is imme¬ 
diately difiblved by the aftion of the reft of the lime. In 
like manner indigo is dilfolved, when mixed in water, 
with pure antimony and potafh, or with fulphuret of 
arfenic and potafh. The fecond method is to mix the 
indigo in water with certain vegetable fubftances which 
readily undergo fermentation. During this fermentation, 
the indigo is deprived of its oxygen, and diffolved by 
means of quicklime or alkali, which is added to the fo¬ 
lution. The firft of thefe methods is ufually followed 
in dying cotton and linen ; the fecond, in dying wool and 
filk. 
In the dying of wool, woad and bran are commonly 
employed as vegetable ferments, and lime as the folvent 
of the green bale of the indigo. Woad contains a colour¬ 
ing matter precifely fimilar to indigo ; by following the 
common procefs, indigo may be extrafted from it. In 
the ufual ftate of woad, when purchafed by the dyer, 
the indigo which it contains is probably not far from the 
ftate of green pollen. Its quantity in woad is but fmall, 
and it is mixed with a great proportion of other vegetable 
matter. Before the introduftion of indigo into Europe, 
woad alone was employed as a blue dye ; and even as 
late as the feventeenth century, the ufe of indigo was re- 
ftrifted in different countries, and dyers obliged to em- 
Q q " ploy 
