108 EXPLOITATION OF PLANTS 
By June the plants are three to five feet high, with a 
stem } inch in diameter. The maximum colour is 
developed by the end of August, and the young leaves 
are the richest. Generally two crops are taken from the 
plants, one in the middle of June, the other two to three 
months later. 
To extract the dye the plants are cut early in the 
morning and closely stacked together upright in a large 
vat. Bamboo rods are laid across the vat and fixed 
down by heavy timber. Water is then run into the vat’ 
till it nearly reaches the bamboos. The whole is left 
nine to twelve hours at a temperature of 30°-35° C., 
while an active fermentation takes place. The whole 
mass expands, and high pressures are developed in the 
vat, so that the beams are sometimes broken. Mean- 
while several gases are evolved, carbonic acid, marsh 
gas, and hydrogen; the latter often catch fire and form 
a blue flame playing about over the surface of the vat. 
The course of this fermentation must be closely watched, 
as it is important to stop it at exactly the right point. 
After a time the liquid subsides ; then a valve is opened 
at the bottom of the vat and the liquid is run out. It 
should now be orange yellow or olive in colour, and 
contains reduced indigo in solution. It is run into a 
beating vat, where a rotating wheel mixes the liquid 
with air. Atmospheric oxygen quickly converts the 
reduced indigo in solution into insoluble indigo blue, 
which comes down as a blue precipitate, until the liquid 
is clear. It is collected, dried, and exported in powder 
form. The refuse plant or “seet” is valuable as a 
manure ; but, curiously enough, it cannot be used direct 
for indigo lands, or the resulting crop is poor in the dye. 
