This author adds: “Woollen cloth, before being dyed, is boiled in water for one hour, then passed immediately under cold water. If it be suffered to lie in heaps after being boiled it undergoes some change, which renders it afterwards incapable of taking up color in the vat. In dyeing, the cloth is placed on a net-work of rope attached to an iron ring, which is suspended by four iron chains to a depth of about three feet beneath the surface of the liquor. The cloth is stirred about in the liquor by means of hooks for about twenty or thirty minutes. It is then taken out and well wrung. It now appears green, but, on being unfolded and exposed to the air, rapidly becomes blue. When the vat has an excess of lime the cloth has a dark green color when taken out. It is then passed through hot water, and dipped again if a darker shade is required.”
_The Indian Vat_.—This presents much analogy to the woad vat, as the fermentation of vegetable matters effects the transformation of the indigo-blue. According to Dr. Calvert, the Indian vat, probably so called from its origin in the East, is taking the place in England of the old woad vat for dyeing wool and woollens. He describes its preparation as follows: 8 lbs. of powdered indigo is added to a bath containing 3½ lbs. of bran, 3½ lbs. of madder, and 12 lbs. of potash, which is maintained for several hours at a temperature of 200° F. It is then allowed to cool to 100° F., when fermentation ensues. After about forty-eight hours the indigo is rendered soluble, being reduced by the decomposition of the sugar and other products contained in the bran and the madder root during the process of fermentation. The distinguishing feature of this vat is the use of _potash_. The Indian or potash vats are spoken of by the best authorities as more easy to manage than the woad vat. They are less subject to accidents, and yield their coloring material more readily to the fibre, while three times as much wool can be dyed in the same time. On the other hand, they do not last so long, and require to be renewed at the end of twenty-five or thirty days. Besides, the fibres dyed in the potash vat have a darker shade than those dyed in the woad vat, owing to the large quantity of the coloring matter of the madder dissolved by the potash, which becomes fixed on the stuff with the indigo-blue.