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Paper making. Engraving of wood chips being macerated in caustic soda in the 1870s. The raw materials used to make paper include wood pulp or cloth rags. In order to make paper this raw material must be cleaned and the fibres within it broken. As part of this process the raw material is cooked in caustic soda in a large rotary boiler for several hours. The caustic soda attacks the fibres chemically. It also reduces coloured dyes to colourless compounds and combines with grease and other impurities to form soaps which can later be washed away.
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