Large containers or casks in which textile materials are soaked in reducing baths, which utilize reducing agents like sodium hydrosulfite to convert water-insoluble vat dyes into a soluble form, as a means of altering the color of fibers for aesthetic purposes. When textile materials are exposed to dyes in these containers for prolonged periods of time, their individual fibers are impregnated with the dye color; the dyes, now infused in the fibers, are then oxidized back to their original insoluble forms for increased color brilliance and light fastness. First used in medieval Europe, these containers derived their names from the large vats once used in the reduction of indigo plants through fermentation.
