Why do pages on Old Books turn yellow?

Paper is made out of wood pulp which contains cellulose. Another organic substance binding the cells, fibers and vessels that constitute wood, is LIGNIN. Apparently, it is the most abundant renewable carbon source after cellulose. Cellulose, a colourless substance, is remarkably good at reflecting light, which means we see it as being white.

To make a fine white paper (Bond paper), the wood pulp is put through a chemical process which separates and eliminates the lignin. This is why paper is usually white. However, when paper is manufactured, some lignin also ends up in it. Paper used as newsprint needs to be economical, and so ends up containing more lignin than other papers. Brown paper and cardboard used for packing purposes and as paper bags are stiff and strong because they also contain more lignin in them.


Paper is made from these components that yellow over time — at least when they’re exposed to oxygen. But when lignin is exposed to light and the surrounding air, its molecular structure changes. Lignin is a polymer, meaning it’s built from batches of the same molecular unit bonded together. In the case of lignin, those repeating units are alcohols consisting of oxygen and hydrogen with a smattering of carbon atoms thrown in.

Lignin, and in part cellulose, is susceptible to oxidation meaning it readily picks up extra oxygen molecules, and those molecules alter the polymer’s structure. The added oxygen molecules break the bonds that hold those alcohol subunits together, creating molecular regions called chromophores which reflect certain wavelengths of light that our eyes perceive as colour. In the case of lignin oxidation, that colour is yellow or brown. So after oxidation we see paper as yellow or brown sheets.

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