 In Europe, use of "Isatis tinctoria", or woad, can be traced back to the 8th century BC, although it was eventually replaced by "Indigofera tinctoria" as the superior dye product.However, most denim today is dyed with synthetic indigo dye.In all cases, the yarn undergoes a repeated sequence of dipping and oxidation — the more dips, the stronger the color of the indigo.