Why Does Paper Turn Yellow With Time?
Why does the paper turn yellow from white, and how can we protect it?
Paper: The Making and the curious case of Lignin
Paper is made from woods. Wood mainly contains two polymers: substance cellulose and lignin. It is lignin which causes paper to turn yellow. Lignin has a naturally dark colour, it is lignin which makes the wood hard and stiff. Lignin undergoes oxidation when exposed to sunlight and air , and it breaks down into many phenolic acids which are yellow in colour.
How to Stop yellowing and make books last:
Nowadays papers are made acid free, for this purpose lignin is removed from the wood. This makes paper last more compared to the paper made with lignin. An additional chemical process is required for removing the lignin from wood and this process is more expensive compared to making paper with lignin in it.
This is the reason why newspapers turn yellow with time, as they are produced in bulk and more importance is placed on economical cost of printing rather than long lasting requirement.
As far as old history books and documents (which were made using lignin) are concerned, they are protected from further damage by keeping it in a stable acid free environment; that is dry, dark and free of insects.