The fermentation is more or less prolonged according to the temperature. Its duration varies from nine to fourteen hours. The workmen judge as to the procedure of the operation by withdrawing a little of the liquid in the lower vat. If it is of a clear pale yellow when withdrawn, it will furnish a product less abundant but more pure than if of a deep gold color. At the moment of its issue from the fermenting vat the liquid is of a yellow color, more or less deep. The liquid is allowed to remain undisturbed for a brief period, when twelve naked men, armed with long bamboos, enter the vat to beat the water while it is still warm. During this time the upper vat is emptied and cleaned out for the succeeding operation. One vat requires seventeen workpeople (twelve men and five women). They thrash the water for two or three hours. The liquid passes by little and little to a pale green, and the indigo is found on suspension in the form of small floccules. The liquor is suffered to remain undisturbed for half an hour; it is then gradually decanted by opening, one after the other, the discharging holes placed at different heights. The water returns to the river, and the precipitate, under the form of a thin _bouille_, is turned into a reservoir. This _bouille_ is pumped up into a vessel, and made to boil for a moment to prevent a second fermentation, which would injure the quality of the product, by turning it black. It is suffered to rest about twenty hours, and the next morning it is again subjected to boiling, the ebullition being kept up three or four hours. The boiling deposit is then turned off upon a large filter, through which the water drips. This filter is composed of a vat constructed of masonry, covered with stucco, about eighteen feet long by six feet wide and three feet deep. This is covered with bamboos, upon which is a grating of smaller reeds, and above by a stout strained cloth. There remains upon the cloth a thick paste, of a deep blue and nearly black color. The water which is run into the vat deposits some indigo which has pressed through the filter. This is decanted after being allowed to rest, and the turbid liquid is boiled the next day with the fresh indigo. The paste of the filter is introduced into some small boxes of wood, pierced with holes, and provided above and below with a strong cotton cloth. The whole is again covered with a piece of stuff, and then with a covering of wood, pierced with small holes, and it is placed under a press, the force being gradually applied, so as to cause the water to run out as much as possible. There is withdrawn from the box a cake of the size of a cake of Marseilles soap. The water squeezed out flows back into the filtering vat, to be boiled again with the fresh indigo. The drying of the cakes ought to be done very slowly.