Woad (Fa., Guede, Vouede ; GEM, Waid).—The leaves of Isatis tinctoria, a variety of the indigo plant, yield a blue dye, which will ever be historically interesting, as the substance with which the ancient Britons stained their bodies, but which has been driven out of the English market by true indigo. In districts where indigo cannot be obtained, its culture may still be remunerative. The plant thrives only in deep friable loam, such as the fen lands of Lincoln and Huntingdon, and 3-4 years is the maximum period for which it can be continuously grown in one spot. The seed is drilled into the ground in March-May ; the young plants are thinned out as soon as they are 3-4 in. high. By the middle or end of July, the first sowing will be ready for cropping, which should be commenced as soon as the leaves of the plants (8-9 in. high) begin to change colour. The leaves are pulled off separately by hand, and conveyed to the manufactory ; in about 6 weeks, a fresh crop of leaves is produced ; and sometimes a third crop is taken in the same way. The produce, however, decreases each time. A portion of the crop is left to produce seed ; the flower stem is thrown up in spring, and the seed-pods ripen about July, when they are plucked, and threshed with flails. The dye is prepared from the leaves in the following way :—The leaves are crushed in edge-runner mills ; the pulp is removed, and laid up in small heaps to drain, till the mass is sufficiently dry to cohere ; it is then " balled," or pressed by hand into lumps, 4-6 in. thick.
The balls are taken into a drying shed, which is roofed and well ventilated, and are spread on hurdles. When thoroughly dry, the balls are stored in a dry and airy place till the whole crop is completed ; they are then submitted to the final "couching," or fermentation. This forms a winter occupation. The balls are ground to coarse powder in the edge-runners, and spread 2-3 ft. thick on the floor of the " couch." The powder is now watered and constantly turned over, to ensure the utmost possible equality in the fermentation. Considerable heat, and abundant offensive fumes, are generated ; if the fermentation be too slow, the product becomes "heavy," if too rapid, "foxey"; in either ease, its value is much affected, consequently the operation needs to be conducted with great skill. When completed, the mass is simply turned, till its temperature sinks low enough to admit of its being packed in casks for transport to market.
Shortt, ' Culture and Manufacture of Indigo' (Madras : 1862) ; P. L. Simmonds, Tropical Agriculture' (London : 1877).
(See Coal-tar Products ; Dyeing ; Pigments.)