The Dyer's Guide / Being a compendium of the art of dyeing linen, cotton, silk, wool, muslin, dresses, furniture, &c. &c.; with the method of scouring wool, bleaching cotton, &c., and directions for ungumming silk, and for whitening and sulphuring silk and wool; and also an introductory epitome of the leading facts in chemistry, as connected with the art of dyeing

by Thomas Packer

Rules to judge of the state of the woad vat.

The vat is ready for working, and to dye blue, when the sediment at the bottom, on being taken out of the vat changes to a fine brown-green. When the froth which rises in great bubbles on the surface is of a fine Prussian-blue, and when the pattern which has been steeped an hour, comes out of a dark grass-green, and changes in the air to a blue; when the liquor is clear and reddish, and the drops which stick to the rake are brown; when the sediment changes colour on being taken out of the liquor, and becomes brown on exposure to the open air: when the liquor is neither harsh nor greasy to the feel, and neither smells of lime nor of ley, the vat is known to be in a proper state for working.