Blue (Woad) embroidery thread/weaving yarn

Woad is a dyeing plant known since ancient times. In Medieval it was favoured especially by Baltic tribes and Vikings, who were even called (from their favourite colours) "blue-green people" in Irish chronicles. Dyeing with woad required great skills and knowledge, as its leaves needed to be fermented in proper conditions (and with mysterious and smelly ingredients, like human urine!).