An article’s color fades when subjected to a chemical reaction in which oxygen is released. This oxygen combines with the natural coloring matter or with the dye to produce a colorless compound. Thus the color of the article becomes paler and is finally taken out together, or bleached.
The most famous bleaching agent in the sun, and the process by which it makes color fade is called photo-chemistry. The old-fashioned method of bleaching textiles by laying them out flat in the sunlight is still used in places.
Chemical bleaching is much quicker, but needs to be controlled with great care. In about 1790 it was discovered that chlorine gas and its compounds were good bleaching agents and chloride of lime (calcium hypochlorite), made by the action of chlorine on slaked lime came into use. This “bleaching powder”, as it was called, was dissolved in water. It was removed when the bleaching was complete by washing the article or immersing it in neutralizing solutions. More modern bleaching agents are sodium hypochlorite, hydrogen peroxide and sulphur dioxide.