- Ancient Egyptian mummies have been found with linen wraps, dyed a deep blue. This colour has survived for thousands of years when all other dyes have faded away.
- The dye comes from the plant Isatis tinctoria, commonly known as woad. Originally native to Asia, it has been cultivated in Europe since the Roman period at least.
- The dye molecule is indigo, which is normally insoluble in water and doesn’t stick well to fabric. But it can be chemically reduced to the soluble and colourless white indigo.
- White indigo has been used in ink for inkjet printers. This soluble form doesn’t clog the printer nozzle but it converts back to blue indigo on contact with the oxygen in the air.
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