THEORY OF THE INDIGO-VATS.

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yellow the redudion is complete, and the vat ready for use. According toSchwartzenberg the liquid contains in solution reduced indigotin, some indigo-red, and all the gluten, with a trace of the brown matter. The sedimentcontains the whole of the indigo-brown combined with lime, the greater partof the indigo-red, some gluten, a very small quantity of indigotin, sulphate oflime, and hydrated peroxide of iron. The same author states that indigo-reddoes not sensibly affed the dyeing process, but that this assertion is an erroris known to all pradical blue dyers who have used qualities rich in redcolouring matters.

The vats in large dye-works are oblong cisterns, made of flag-stones largeenough to form the four sides and the bottom, so that there are joints only atthe corners. They are g feet long, 3£ wide, and 9 deep.

In many dye-works small tanks (tubs) only are used, the size being such asonly to suit a piece of goods of some 50 metres in length : these tanks areconstrudted of wood, the shape being cylindrical. The piece or pieces ofgoods to be dyed are stretched on a peculiarly construdted apparatus, con-sisting of a wooden pole, bearing at each of its ends another piece of wood,shaped as a star, with six or eight arms; one of these is made movable bymeans of a screw: the arms are provided with hooks, on to which the goodsare fastened. When small tanks or tubs are used this arrangement admits ofreadily transferring the wooden framework containing the goods from one tankto another.

Some manufadturers of colours precipitate the indigotin from old vats,and employ it again in making up new vats. This operation is condudtedas follows:—The clear liquid as well as the sediments of the vats are, bysuitable means (pumps or other contrivances), poured over into very largewooden tanks, and therein diluted with a large quantity of water; sulphate ofiron and lime are next added, in such quantities as to cause the redudtion, aswell as complete solution, of all the indigo present, a laboratory assay beingmade of the old vat residues, so as to have some guide to the quantity presentand to the ingredients required to be added. The contents of these tanks arethoroughly stirred, and next left to settle; the clear liquid is made to run inthin layers over shallow wooden troughs, so as to expose as much surface ofliquid as possible to the adion of the air; hydrochloric acid is added, and theprecipitated blue indigo is colleded on filters, consisting of woollen blanketsstretched in suitably construdted wooden framework. The sediment left inthe tanks is repeatedly treated with water, and some lime and sulphate of ironare also added, and thus the indigo is as much as possible recovered.

Calico, linen yarn, linen thread, and hemp yarn or thread, are alwaysdyed, if with indigo, by means of the cold green vitriol vat: a simple im-mersion in the vat is sufficient to produce, after proper exposure to air, auniform blue colour; and by varying the strength of the vat, and the length oftime of each immersion, it is possible to obtain, as may be desired, a series ofshades from the very lightest sky-blue to a tint so deep as to appear nearlyblack. The length of time allowed for the immersion of the goods has a greatinfluence on the quantity of indigotin which becomes fixed. The tissueattrads by its pores indigotin, and this attradion is the cause that the vatsbecome exhausted of colouring matter long before all the liquid which servesthe purpose of keeping the dye-stuff in solution is absorbed.