In ancient times, woad was an important source of blue dye and was cultivated throughout Europe for that purpose. It was eventually replaced by the more colourfast Indigo (Indigofera tinctoria) until the early 20th century when synthetic blue dyes were developed that replaced both woad and Indigofera tinctoria. Woad leaves react to rubbing and crushing by producing a bitter-tasting liquid that serves as a natural defence. This liquid fermented in stale urine for several weeks turns into blue woad pigment that floats on the service of the urine vat. Textile soaked in this substance turns blue once it is exposed to oxygen and sunlight.