The natural colour of wood pulp is influenced predominantly by the amount of lignin it contains.
Lignin is a natural adhesive that binds cellulose fibres together to form wood and imbues the pulp with a natural tint, which varies from light to fairly dark brown.
Quite obviously, while paper made from unbleached pulp serves well as packaging material, it is unacceptable for most printing and writing applications.
Bleaching is the term used for a series of process steps where the pulp is exposed to various chemicals that either remove or alter the colour substances it contains, so that it appears whiter. The chemistry involved in bleaching depends on the type of pulp being processed.
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