As early as in the 18th century it was known that pulp fibres could be bleached with chlorine
(Cl2) and hypochlorite (OCl– i.e. alkaline chlorine). One stage hypochlorite batch bleaching was
the standard bleaching process in pulp mills until the 1930s when bleaching sequences with
chlorine-alkali-hypochlorite were developed. Such a simple sequence was sufficient for chemical
pulps that were easy to bleach like sulphite pulps but kraft pulps were still not possible to
bleach to high brightness with such a sequence without a severe loss of pulp strength.