Indigo is, as all dyers know, or should know, a natural dye-stuff, prepared from the leaves and twigs of the indigo plant by a species of fermentation which produces the indigo in a soluble form from the indigo substance in the plant, followed by oxidation which results in the separation of the indigo from this solution. It comes into this country in the form of lumps, which have a dark (p. 138) blue to bronze blue colour. The dye-stuff is insoluble in water, cold alcohol, alkalies or weak acids. When heated with strong and fuming sulphuric acid it dissolves, forming a blue liquor from which the colouring matter may be obtained on addition of soda in the form of a paste, which is used in wool and silk dyeing under the name of indigo extract. But dissolving in sulphuric acid materially affects the properties of indigo as a dye-stuff, as will be seen later on. By the action of reducing agents the insoluble blue indigo is converted into a soluble white indigo. This body is rather unstable, and on exposure to the air it rapidly becomes oxidised and converted back again into the blue indigo. Upon this principle is based the application of indigo in dyeing by means of the vat. Various methods may be adopted to cause the indigo to become dissolved. These may be divided into two groups: (1) Fermentation vats, in which the action of reducing agents is brought about through the influences of the fermentation of organic bodies, such as woad, bran, treacle, etc; (2) Chemical vats in which the reducing effect is brought about by the reaction of various agents on one another. Of such vats the copperas and lime and the hydrosulphite vats are examples. The fermentation vats, when in good order, work well and give good results, but they are most difficult to prepare or set. The chemical vats are the easiest to work, and (especially the hydrosulphite vats) are coming to the fore, and are gradually driving out the fermentation vats. The actual method of dyeing with the indigo vat is the same with all methods of preparation. The material to be dyed is well wetted or wrung out in water. It is then dipped into the vat, handled a few minutes to ensure its thorough impregnation, then lifted out, the surplus liquor wrung out, and the material exposed to the air, (p. 139) when the indigo white on it soon absorbs oxygen and turns into blue indigo. With these few preliminary remarks the methods of setting the various indigo vats will now be described in detail.