THEORY OF THE INDIGO-VATS.

473

chemical and mechanical resists. In practice, however, the various substancesmentioned are suitably mixed, and so blended together as to make the resultsatisfactory.

The reader will find below instances given of the mixtures in actual use.The thickeners play in themselves a great part in this matter, and it is of thehighest importance to understand the mode of action of every thickener andthe proportion which should be mixed with the resist compound : it is clear,therefore, that the making of a good resist depends on understanding thevarious quantities of the constituents of the resist, calculated as well inreference to the strength of the vat as to the depth of shade to be reserved.It is a main condition that a resist should become hard immediately on thegoods being immersed in the vat; if the resist becomes soft, the consequenceis that the colour runs. The acidity of the resist mixture should therefore be inproportion to the alkalinity of the vat: a strongly acid resist, which would hardenreadily in a strongly alkaline vat, would soften and run in a feebly alkalinevat; on the other hand, again, a nearly neutral resist, which might do for aweak vat, would be useless in a strong one. It is clear, therefore, that thechoice of this or that salt or base is far from indifferent in this matter: sul-phates and nitrates are stronger resists than acetates, which latter, in theirturn, suit very well for weak vats. The most generally used white resist ismade up of pipe-clay, gum, verdigris, and sulphate of copper: the goods tobe dyed blue with a white pattern (white resist) are commonly not stronglybleached, but simply boiled in soda-lye, because in that state they are betterand more rapidly dyed. Such goods are, moreover, submitted to anotheroperation previous to being immersed in the vat; they are passed through aweak, very fluid, starch paste, to which some sulphate of copper and nitrateof ammonia are added, with the view first to render the pieces uniformlymoist, and next to promote oxidation as well as the penetration of the indigo.After this operation the goods are lightly dried and calendered; next, theresist is printed on; the fabric again dried, and after that vatted. The fabricis next cleared in a weak solution of tepid sulphuric acid, sp. gr. i'oi3.

When the resist, instead of being applied to a white ground, is printed onto a piece already blue dyed, the result will be the production of a light bluedesign upon a deep blue ground.

When a piece of goods has been arranged so as to leave, after vatting, whiteportions, it is clear that on these portions there may be printed steam-colours,or even a mordant, and that the piece so prepared may be treated as if thewhole of it were white, the result, after dyeing, being the production of apiece upon which several colours may be present on a blue ground. Thiskind of work requires a very nice adjustment of the engraved cylinders. If,instead of an ordinary white reserve, there is employed a composition whichcontains acetate or nitrate of lead and a salt of copper, the result will be thatby vatting a portion of the oxide of lead becomes fixed; when, after thevatting, the fabric is first immersed in dilute sulphuric acid, and next in milkof lime, and afterwards dyed in a bath containing a saturated solution ofbichromate, with or without lime, the final result will be the production of ayellow or orange pattern. When this resist is printed on previous to blue-vatting, and the piece afterwards dyed in bichromate, there will be produced agreen-coloured pattern surrounded by deep blue. Again, a reserve may be