"Bleach
· Sunlight: is a natural bleaching agent. To bleach white cotton, wash it and spread it out to dry in the sun. Leave it outside for at least a day or two to three days if you can."

The Scots used to whiten their yarns and their cottons etc in trays filled with urine and left in the sun, as the sunlight turns the urine into bleach!
(Useless fact for the day!)