Indigo dyeing from Isatis tinctoria L.: From medieval to modern use

Since ancient times, indigo has been one of the most widely used natural pigments for textile dyeing. In Europe, the only source of indigo dye was from woad (Isatis tinctoria). Woad leaves were processed to obtain an insoluble indigo pigment, which had to be reduced to leuco-indigo to dye textiles. Today, most indigo comes from the chemical industry, the production of which raises public health and ecological problems. For the past few years, renewed interest in natural pigments has led to the revival of I. tinctoria cultivation for indigo pigment production. However, the woad blue is still obtained with uncontrolled and inconsistent yields. The aim of the following paper is to provide an overview of what is known about the production of the woad blue pigment, from the leaves of I. tinctoria to its use as a dye, from medieval times to the present day. Despite numerous studies, the behaviour of the woad indigogenic precursors and the mechanisms leading to indigo formation remain unclear.