Woad (Isatis tinctoria) was the Anglo-Saxon source of indigo blue. Although woad is not a true native plant (i.e. it was not present here before the formation of the English Channel), it is thought that it was introduced in the neolithic age when farming began. Some of the earliest textile fragments show evidence of having been dyed with woad and it was probably one of the first dyes to be used. As extracting blue from the indigo-bearing plants is somewhat more complicated than the method of extracting colour from most other plants, it may seem strange that blue was among the first dye colours. However, the indigo-bearing plants, including woad, were generally considered to have healing properties and it may be that their use as dyes developed from their use medicinally. For example, if woad leaves were applied to damaged skin as a poultice, perhaps together with urine, which was regarded as an antiseptic, the conditions necessary for extracting blue from the leaves might have developed. These conditions would be heat (from the skin) an alkaline medium (from the urine as it became stale) and bacteria from the urine. So one can imagine that, if the poultice was removed to reveal blue skin beneath it, people would have been able to work out how to use the leaves to dye textile fibres. Another possible scenario might occur if woollen fleece was being cleaned in a tub of urine and someone dropped woad leaves into the tub by mistake. The woad leaves might remain in the tub long enough for the urine to act on them and could, in effect, create a woad vat in the tub. When removed, the fleece would become blue on contact with the air. Once people noticed the presence of the woad leaves in the tub, they would probably have been able to work out why the fleece had become blue. All this is purely speculation, of course. Woad vats would have been organic in the Anglo-Saxon period and might have been made using stale urine, which provides both the source of alkali and the bacteria needed to make the vat active. Woad leaves may have been harvested and used fresh or they may have been allowed to ferment and processed into woad balls and stored for later use. Another method of dyeing with woad may have been the fermentation vat, made using wood-ash water as the source of alkali and madder and bran to induce fermentation and remove the oxygen from the vat. The recipes for these vats can be found in "Colours from Nature". The first photo below shows pale blue shades from woad. In the second photo the first three skeins show a range of shades from a woad fermentation vat. (The other skeins show lichen purple and black achieved by dyeing.