DYEING. 
mis the ingredients. Great care should be 
taken not to expose the vat to the air, ex- 
cept when stirring it. As soon as that ope- 
ration is over, the vat is covered witli a 
wooden lid, on w'hich are spread thick 
cloths, to retain the heat as much as pos- 
sible. Notwithstanding this care, the heat 
is so much diminished at the end of eight or 
ten days, that the liquor must be re-heated, 
by pouring the greater part of it into a cop- 
per over a large fire : when it is hot enough 
it is returned into tlie vat, and covered as 
before, 
Tliis vat is liable to two Inconveniences ; 
first, it runs sometimes into the putrefactive 
fermentation which is known by the fetid 
odour it exhales, and by the reddish colour 
it assumes. This accident ,.is remedied by 
adding more lime. The vat is then raked, 
after two hours lime is put in, the raking 
performed again, and these operations are 
repeated till the vat is recovered ; secondly, 
if too much lime; is added, the necessary 
fermentation is retarded ; tliis is remedied 
by putting in more bran or madder, or a 
basket or two of . fresh woad. 
When cloth is to be dyed, the vat is raked 
two hours before the operation, and to pre- 
vent it from coming in contact with the 
sediment, which would cause inequalities 
in the colour, a kind of lattice of large 
cords, called a cross, is introduced ; when 
wool is to be dyed, a net with small meshes 
is placed over this. The w ool or cloth be- 
ing thoroughly wetted with clear w'ater a 
little warm, is pressed out, and dipped into 
the vat, where it is moved about a longer 
or a shorter time, according as the colour 
is intended to be more or less deep, taking 
it out occasionally to expose it to the air ; 
the action of which is necessary to change 
the green colour, given the stuif by the 
bath, to a blue. Wool and cloth dyed in 
this manner, ought to be carefully washed, 
to can-y off the loose colouring matter ; 
and those which are of a deep blue ought 
to be even fulled with soap, which cleanses 
them effectually without injuring the co- 
lour. 
A vat W'hich contains no woad is called 
an indigo vat ; the vessel used for this pre- 
paration is of copper, into which is poured, 
according to its capacity, water ; in forty 
pails of which, six pounds of potash, tweh e 
ounces of madder, and six pounds of bran 
have been boiled ; six pounds of indigo, 
ground in water, are then put in, and after 
raking it carefully, the vat is covered ; a 
slow fire is to be kept up round it. Twelve 
hours after, it is filled, it is to be raked a 
second time, which is to be repeated at simi- 
lar intervals of time, till it comes to a blue, 
which will generally happen in forty- eight 
hours. If the bath be well managed it will 
be of a fine green, covered with coppery 
scales and a fine blue scum. In this vat 
the indigo is rendered soluble in the water 
by the alkali instead of lime ; the operation 
of dyeing with it is the same as the pre- 
ceding. 
Hellot describes two vats, in which the 
indigo is dissolved by means of urine. Mad- 
der is added to them, and in the one vine^ 
gar is put, in the other alum and tartar, of 
each an equal weight to the indigo. The 
quantity of urine ought to be very consi- 
derable. It is probable the indigo is dis- 
solved in them by the ammonia formed in 
the urine. These vats are not so good as 
those before described ; less work can be 
performed with them, so that they are 
adapted only to small dye-houses. 
The colour dyed by a solution of indigo 
in sulphuric acid, is called Saxon blue, from 
having been discovered at Grossenhayn, in 
Saxony, by Counsellor Barth. M. Poerner, 
wlio has paid great attention to this prepa- 
ration, directs four parts of sulphuric acid 
to be poured on one of indigo, in fine pow- 
der ; the mixture is to be stirred for some 
time. After having stood twenty-four hours, 
one part of good dry potash in fine powder 
is added ; the whole is to be again well 
stirred, and having stood twenty-four hours 
longer, more or less water is added gra- 
dually. 
To dye Saxon blue, the cloth is prepared 
with alum and tartar ; a greater or less 
proportion of indigo is put into the bath, 
according as tlie sliade required is deep or 
light ; for deep shades tlie stuff must be 
passed several times tlirough the batli ; 
light shades may be dyed after the deep 
ones, but they have more lustre when dyed 
in a fresh batli. 
Of dyeing Silk blue. 
Silk is dyed blue by the indigo vat be- 
fore described. In general a larger propor- 
tion of indigo is put in than is there di- 
rected, but nearly the same quantities of 
bran and madder. Macquer says, that half 
a pound of madder for every pound of pot- 
ash, makes the vat greener, and the colour 
more fixed. When the vat is come to, 
about two pounds of potasli and three or 
four ounces of madder should be added ; 
it slionld then be raked, and in four hours 
