If you find an old book or newspaper in a bookcase and compare it with a new one, you can see that old paper sheets have become yellow. Make sure that this process is happening everywhere, and not only in your bookcase, you can, having departed in the museum where old archival documents are stored. Of course, it can be assumed that people in the old days specifically printed the text not on white, but on the yellow pages, but in fact the purchase of white yellow paper is a natural process proceeding due to substances that are in the paper.
It is believed that the paper was invented in China in the 2nd century BC. For its manufacture, the fibers of hemp stems were used, mixed with chopped wood bark and an old cloth, rolled into a thin layer and then dried. The invention quickly spread throughout Asia, but was used solely to record state importance reports. The paper came to Europe much later - in the 11th century and was manufactured mainly based on flax fibers. The invention quickly gained popularity, but manual work was used for its production, respectively, the proposal could not satisfy the demand, and the cost of one leaf was quite expensive, so the paper could afford only representatives of the highest class. But the situation has changed when a patent for a paper machine was registered in 1799, which was created in several years later. Now paper sheets were made on an industrial scale and cost much cheaper than before. Thus, we have a large number of documents on 19 and 20th century. Initially, they were white, but now completely yellow. Why is this happening?
The fact is that the paper is made of wood, which mainly consists of two components: cellulose and lignin. Cellulose is a transparent material that reflects the light well, so it looks white for us. Under the influence of oxygen, the cellulose changes its structure and begins to reflect the light differently, so we begin to perceive its color as yellow. But the change in the structure of cellulose is not the only reason why paper becomes yellow. Much more susceptible to the oxidation of lignin.
Lignin is a substance, in large quantities contained in the wood. It fastens her plant fibers and gives the tree strength. Biologists calculated that without lignin, the tree could not grow more than one and a half meters and resist the streams of wind and other aggressive environmental impacts. Under the influence of oxygen, lignin changes its structure and rapidly yellowing (remember the coarse packaging paper or, for example, cardboard boxes), so it is trying to remove it from paper to avoid her yellowing.
Modern white paper passes several stages of purification from lignin, so yellowing is much slower compared to the one that the eyelid used. But, of course, its production costs more. To save money, most newspapers and passing publications are printed on cheap paper, in which most of the lignin remains, which contributes to its rapid yellowing. If you put a newspaper sheet and a white sheet for printing, then the first will get much earlier, and the second treated with special chemicals will remain unchanged for a long time.