[17] For woad, and the labour-intensive process of preparing the indigo vat, see Kirby, Van Bommel and Verhecken, Natural Colorants, 65-66; Edmonds, History of Woad, 13-47; Hartl, Proaño Gaibor, van Bommel, Hofmann-de Keijzer, “Searching For Blue”, 9-39.