Why Do Old Books Turn Yellow?

When you go into a big library, you’ll see many old books that have become yellow and brittle. Why did that happen? You may once wondered that, let’s find out the reason.

Make your own ancient treasure map
Draw a treasure map on a sheet of paper. Meanwhile, ask mom or dad to make you a cup of
black tea (without milk or sugar). Pour the tea onto a plate, place your map in it and let it soak overnight. In the morning, take out the map carefully and let it dry in the sun. Does it now have an ancient, yellow effect? Show it to your friends and tell them that an ancient pirate gave it to you!

Lignin and paper
As we know, paper is made from wood. Wood is in turn made of carbohydrates like cellulose
and lignin. Lignin is a very complicated molecule that adds hardness to wood. More the lignin, hardier is the wood. However, in paper it is a problem. Over time, lignin breaks down to form many phenolic acids, which are yellow in colour. These acids then react with cellulose. This causes the paper to become very brittle.

That’s what happened when you put the map in tea. There was tannic acid in the tea, which reacted with the acid in the paper.

How to make books last
William Barrow was a librarian in the 1930s, who was very interested in knowing how to
preserve old books (perhaps some of them had old treasure maps!) He was the one who discovered that it was the acid from lignin that caused it.

Since then, paper manufacturers remove lignin from the wood pulp before it is made into paper. These require additional chemical reactions. In addition, the paper is made alkaline by adding calcium bicarbonate. If any lignin is left in the paper, when it forms acid, the calcium bicarbonate will immediately react with it and ‘neutralise’ it. This kind of paper is called acid-free paper.

All this makes the paper expensive. Things like newspapers, tickets, notebooks etc are therefore not printed on it. But all books nowadays are printed on acid-free paper.