Bleaching linen once took six months and involved sour milk
In 17th-century Western Europe, whitening cloth was a slow industry. Fabric was soaked in alternating baths of lye and acid, the acid often lactic acid from soured milk, and the whole cycle could last half a year. Chlorine bleach, invented in the late 18th century, cut that to a matter of hours.
Before any chemistry, people simply laid cloth out in bleachfields and let sunlight and water do the work. The breakthrough came quickly once chlorine was known. Carl Wilhelm Scheele discovered the element in 1774; in 1785 Claude Berthollet realised it could whiten fabric, and he also found sodium hypochlorite, sold as the first commercial bleach under the name Eau de Javel after the district near Paris where it was made. In 1799 the Scottish chemist Charles Tennant patented a solid alternative, bleaching powder, made of calcium hypochlorite.
Around 1820 Antoine Germain Labarraque showed that hypochlorites also disinfect and deodorise, and his campaigning transformed hygiene in hospitals, slaughterhouses and any trade handling animal products. Hydrogen peroxide, first made by Louis Jacques Thenard in 1818, was used for bleaching from 1882 but only became commercially important after 1930. Sodium perborate has whitened European laundry since the early 20th century and caught on in North America in the 1980s.
Bleaches work by attacking coloured organic molecules, such as natural pigments, and turning them colourless. Most are oxidisers, stripping electrons away; chlorine is the common active agent, delivered through hypochlorite because the pure gas is toxic and corrosive. Peroxide-based products are sold as oxygen or colour-safe bleach. A few bleaches work the other way, as reducing agents that donate electrons; sulfur dioxide, for instance, is used on wool.
Uses stretch well beyond laundry. Liquid bleach can destroy DNA completely, which is why labs rely on it to clean equipment, and bleaches keep swimming pools free of bacteria, viruses and algae and whiten wood pulp for paper. They also remove mildew, kill weeds and help cut flowers last longer. Because they react with far more than the target pigment, they can weaken fibres, leather and dyes such as the indigo in denim, and they can harm the body if swallowed, inhaled or splashed on skin or eyes.
Source: Bleach
