Woad called also Glastum, arises from a seed sown annually, in the spring, which produces a plant called Isatis Tinctoria. It has usually three, four, or five crops of leaves every year, the first of which is best, and the rest in their order. When the leaves are ripe they are gathered, let lie sometime, and then put under a wheel to bruise or grind them; after which they are laid eight or ten days in piles or heaps, and at last reduced into a kind of balls, which are exposed in the shade, on hurdles, to dry. This done, they are broken or ground to powder; and when ground, spread on a floor and watered. Here the woad is allowed to smoke and heat, till by terrifying it every day it becomes quite dry, which is called silvering. A week, or more, after which, it is in a condition to be used for dyeing. The ancient Britons used to dye their bodies with it, and some hold that it was from this plant, called glastum, that glass took its denomination ; though others derive both glass and glastum from the British glass, which signifies a blue colour. A woad blue is a very deep blue, almost black; and is the base of so many sorts of colours, that the dyers have a scale, whereby they compose the different casts, or degrees of woad, from the brightest to the deepest.