MANUFACTURING INDIGO. 
307 
fermentation in the steeper is the most difficult 
point in the whole process of making indigo; for 
should not the fermentation be carried far enough, 
a considerable loss of coloring matter will be the 
result. It is necessary, therefore, to carry it on to 
a certain point, and to draw it off the instant it 
arrives at that point; and this can only be known 
by a skilful observer who has obtained his knowl¬ 
edge by practice. 
There is no chemical operation so difficult to 
describe as that of fermentation, and I almost de¬ 
spair of making myself clearly understood by prac¬ 
tical workmen in the following description of the 
steeping process. 
Fermentation has been divided by chemists into 
four kinds, the panary, vinous, acetic, and putre¬ 
factive. The kind of fermentation given in the 
indigo-steeper is evidently of that kind called pana¬ 
ry, or the first stage of fermentation. It is known 
to be the panary by the large quantity of carbonic 
gas given out, which, rising to the surface, floats 
on the top, covered with a thin pellicle of the li¬ 
quid. The difficult point for the operator to dis¬ 
tinguish is when it arrives at that degree of fer¬ 
mentation, and begins to assume the acetic. The 
same difficulty occurs with the woollen blue dyer; 
and the losses so frequently complained of by the 
vats being out of order, and often being irrevocably 
lost, arise from the fermentation being permitted 
to proceed too far. 
The following directions are given as a guide 
for those who may be engaged in the making of 
indigo. While the plant is in steep, draw off a 
little of the water, and with a pen dipped in it 
make a few strokes on white paper. The first 
will probably be high-colored, in which case the 
indigo is not sufficiently fermented. This opera¬ 
tion is to be repeated every quarter of an hour, 
until it loses its color, when it will have arrived 
at its true point of fermentation. 
Let a small hole be made in tire stopper, six or 
eight inches from the bottom, exclusive of the 
opening'or aperture, for drawing off the impregna¬ 
ted water. Let this hole be stopped with a plug, 
yet not-so firmly but that a small stream may be 
permitted to ooze through it. After the plants 
have been steeped some hours, the fluid oozing 
out will appear beautifully green, and at the low¬ 
er edge of the cistern, whence it drops into the 
battery, it will turn of a copperish color. This 
copperish hue, as the fermentation continues, will 
gradually ascend upward to the plug, and when 
this circumstance is perceived, it is proper to stop 
the fermentation. 
During the progress of this part of the business, 
particular attention should be paid to the smell of 
the liquor which weeps from the aperture, for 
should it discover any sourness, it will be neces¬ 
sary to let the fermenting liquor run immediately 
into the battery, and lime-water of sufficient 
strength must be added, until it has lost its sourness. 
As it is running off it will appear green, mixed 
with a bright yellow or straw color, but in the 
battery it will be of a beautiful green. 
Another maker has given the following descrip¬ 
tion of the fermenting process. 
When the plant is gathered, a large quantity is 
put into a vat, and some wood laid above to pre¬ 
vent its rising above the water. The mass begins 
to ferment sooner or later, according to the warmth 
of the weather, and the maturity of the plant— 
sometimes in six or eight hours, and sometimes 
in not less than twenty. The greener the plant, 
the more rapid the fermentation. The liquor 
grows hot, throws up a plentiful froth, thickens 
by degrees, and acquires a blue color inclining to 
a violet—-at this time, without touching the herb, 
the liquor impregnated with the tincture is let 
out, by cocks in the bottom, into another vat pla¬ 
ced for that purpose, so as to be commanded by 
the first. 
The boiling process, for extracting the color 
from the dry plant, was obtained from Mr. Dal- 
rymple, who had for many years been an extensive 
indigo maker in Bengal. He says: “ Take an 
iron, brass, or copper boiler, fill it within three 
inches of the top with the plants, press down with 
stones, and cover the plants with water. The 
liquor must be heated, not until it boils, but until 
it begins to blubber, or simmer. The water, by 
this time, will look greenish, then draw it off into 
a shallow vessel or vat, and beat for one or two 
hours to incorporate oxygen with it. On taking 
some of the liquor in a white saucer, little parti¬ 
cles will appear in it as big as a pin’s head and 
smaller, then stop beating, and throw in a little 
lime-water, upon which the indigo will precipitate 
to the bottom, and the supernatant water will look 
like brandy. The water has now to be drained 
off to a level with the top of the sediment, lay the 
sediment on a cloth to drain, and when stiff enough 
put it into moulds to dry. 
I have been recently informed, that many first- 
rate makers of indigo in Bengal condemn the prac¬ 
tice of obtaining it from the dried leaves, on the 
plea that the article obtained is no better, and is 
much less in quantity. If any of our planters 
should be disposed to try the process, it will be 
necessary to inform them, that should the leaves, 
between gathering and drying, be subject to fer¬ 
mentation, only a small portion of coloring matter 
will be obtained, and that the loss sustained will 
be as the degree of fermentation. 
During the precipitation of the coloring feculse 
the coarsest particles, possessing the greatest 
specific gravity, subside first, constituting the lower 
strata of the pulp, and the lighter and finer parti¬ 
cles subsiding the last, form the upper part. It is 
necessary that indigo-makers should take advan¬ 
tage of this circumstance, by first taking off the 
upper layer, and moulding it by itself, and the 
lower by itself. By this means they may obtain 
several qualities of indigo from one mass of pulp. 
It appears from analysis made by Bergman, 
Quatremere, and other chemists, that indigo of 
good quality does not contain more than from 46 
to- 47 per cent, of coloring matter, and that the 
very best samples do not contain more than 48 per 
cent. 
The following table will exhibit an analysis of 
indigo of good quality, and of the menstruua in 
which the impurities are soluble. 
Mucilaginous parts separable by water - - 12 
Resinous parts soluble in alcohol - - - 6 
