Short answers, pulled from the story.
Textile bleaching is a step in the textile manufacturing process that removes the natural color from raw fabric, known as greige material. It is done to prepare cloth for dyeing or printing, or to achieve a full white finish. Bleaching also removes natural impurities and manufacturing add-ons such as pesticides, lubricants, and sizing agents.
An Egyptian list found in the tomb of Rekh-mi-re at Thebes references bleached and unbleached linen, placing the practice at least as far back as 1000 BC. Some researchers, such as Mulrooney, argue it dates to around 5000 BC. Wood ash was used in bleaching from at least 1 AD.
Grassing involved boiling linen in lye made from wood ash, rinsing it, then laying it out on grass in the open for more than seven days to whiten in the sunlight. The fields used for this process, called bleachfields, were typically sited near watercourses and were common around British mill towns during the Industrial Revolution.
The French chemist Claude Louis Berthollet first demonstrated chlorine's bleaching properties and developed liquid bleaches around 1789. James Watt brought the technique to Britain, and Charles Tennant later patented a bleaching powder that made chlorine-based bleaching commercially practical.
Around sixty percent of the world's hydrogen peroxide output is used in chemical bleaching of textiles and wood pulp. Hydrogen peroxide works by releasing a highly reactive oxygen species when its single oxygen-to-oxygen bond breaks, which destroys the chromophores that give fabric its yellowish color.
Optical brightening agents are chemical compounds applied to textiles after scouring and bleaching to make fabric appear more brilliantly white. They absorb ultraviolet and violet light in the 340-370 nanometre range and re-emit it as blue light in the 420-470 nanometre range through fluorescence. They are available in blue, violet, and red tints.