Woad, or Isatis, L. a genus of plants comprehending 6 species, of which only the tinctoria, or Wild Woad, is a native of Britain. It is biennial; grows in corn-fields, principally at New Barns, in the Isle of Ely; where it flowers in the mouths of June and July. The stalk attains a height of from 3 to 4 feet; and the leaves are eaten by cows, but refused by horses, goats, and sheep : these leaves are highly valued by dyers, for the beautiful blue colour which they impart to wool; hence, this vegetable is cultivated to a considerable extent.
Woad prospers in a deep fat loam, though a moderately rich and mixed soil is the most proper : as this plant remarkably exhausts the ground, more than two crops should never be taken in succession. The land ought to be ploughed four times ; first, shortly before the winter ; a second time in the spring, when it will be advisable to form the ridges ; a third time in June ; and, lastly, towards the end of July, or early in August: in the intervals between each ploughing, it will be necessary to harrow the soil, so that all weeds may be destroyed.
Woad, in this country, is sown early in August, and generally broad-cast, though the drill-husbandry is the most advantageous. At the end of two or three weeks, the plants must be hoed, at the distance of at least six inches: after which they will require no farther attention, except a careful weeding in October, and particularly in the month of March.
The proper time for gathering the leaves, is determined by their full growth, and the first change of colour at their points : they are cut with an edged tool, and collected into baskets by women and children. If the land be good, three or four successive crops may be taken; but the two first are the finest, and produce from 25l. to 30l. per ton; whereas, the third or fourth do not sell for more than 7 or 8l. per ton. After the leaves are gathered, they are submitted to the action of mills, similar to those employed for grinding oak-bark; and in which they are reduced to a kind of pulp. The woad is then laid in small heaps, which are closely and smoothly pressed down. As often as the crust, formed on the outside, cracks or separates, it is again closed, to preserve the strength of the colouring matter. In this state, it remains for a fortnight ; at the expiration of which, the heaps are broken up ; the external part is worked into a mass; and the whole is formed into oval balls, either by the hand, or by means of moulds. The balls are now exposed to the sun, under shelter : when perfectly dry, they are ready for sale ; or are prepared for the vat, in the manner stated vol. ii. p. 202. - Such is the process which woad undergoes, before it becomes fit for dyeing blue colours ; but M. Astruc is of opinion, that, if this vegetable were cured in the same manner as indigo, it would produce a colour of equal lustre to that obtained from such expensive foreign drug. Thus, considerable sums of money, that are annually exported for indigo, might probably be saved ; a conjecture which is now corroborated by the test of experience.- Dam-bourney directs to boil the fresh leaves of woad with diluted bullocks' blood, or more effectually with caustic soap-boilers' ley : ill this simple manner, a dark-green decoction of a blueish shade will be obtained ; and, after clarifying the liquor, it will form a blue precipitate; which, dissolved in oil of vitriol, and properly diluted, imparts a beautiful colour to woollen cloth. Farther, even the leaves, in a state of fermentation with pure water, on adding a small portion of a caustic alkaline ley, afford a fine blue sediment, resembling the true indigo.