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The Challenge: Make Paper and Ink

Chemical Pulping

The chemical pulping process was discovered in the nineteenth century, and allowed wood to be pulped for strong, white, long-lasting paper. The goal of chemical pulping is to remove all the parts of wood which are not cellulose. Chief among these is lignin, a carbohydrate that cements adjacent cells together in wood. This material can be made more soluble by cooking the pulp either in a strong acid or a strong base.

Originally, wood pulp was cooked in lye alone but this produced a rather weak paper. The addition of sodium sulfide to the pulp produced a much stronger paper. This process is called the kraft process, from the German word for strong. About 80 per cent of this kraft pulp is wood and the remaining 20 per cent is lye and sodium sulfide. The pulp is cooked, or digested, at 170°C (338°F) for 3 hours until most of the lignin is broken down. The liquid is drained off and the pulp is washed to remove the chemicals.

The resulting pulp is dark brown in color. If it is made directly into paper, the resulting paper is strong but brown. This is the kind of paper out of which grocery bags and corrugated cardboard are made. For white paper, the pulp is bleached.

Brown paper bag