Why does old paper turn yellow? Old paper turns yellow because lignin and cellulose break down.
Most things in our world fade with time. The most common culprit is ultraviolet light. If you have a photo on your desk or a nice painting on your wall, over time, it will lose its vibrant color. And if you move house and take that painting off the wall, you will still be able to see where it was hanging because the color of the rest of the wallpaper will have faded. This happens because sunlight can bleach materials. Ultraviolet light can break down the chemical bonds in the pigments that give color to materials. This is called photodegradation, and it increases with the amount of UV light. The longer something is in sunlight, the more it will fade. Colors can also be affected by oxygen and heat. That is why old masters are kept out of sunlight in temperature, UV, and climate-controlled rooms. Interestingly, some pigments fade more quickly than others. Organic pigments fade more quickly than synthetic pigments, and the binder and pigment can drastically affect the length of time it takes to fade. They have double bonds that absorb UV light very easily and break apart.
So, why does paper go yellow rather than just fade? If you look at any old manuscripts, they are often yellow. Unless they have been kept in an airtight location or are made of vellum or another material, paper will turn yellow with time. It is all to do with something called lignin in the paper. Generally, the cheaper the paper is, the more lignin it has, and the higher quality it is, the less lignin it has.
Lignin is a complex, natural polymer that gives rigidity and support to plants. It is found in all plants that have a vascular system and is vital for the plant’s height and strength. It is part of what gives the crunch to vegetables and is in the fiber we need in our diet. Without it, plants wouldn’t be able to stand up against the wind. Without lignin, trees would not be able to grow taller than about 2 meters. Lignin is the second most common organic polymer on Earth, coming second after cellulose. Lignin is like a glue in plants that binds to the cellulose and holds them together. It is incredibly strong and is often the last part of a plant to decompose. There are only some organisms that are able to break it down. White-rot fungi are one of them, and they need special enzymes to be able to do it. It takes them a long time.
Paper is made from pulped wood, which, of course, contains lignin. When higher-grade paper is made, it is chemically treated and bleached, which removes a lot of the lignin. This doesn’t happen with cheaper paper, such as that used for newspapers, and it didn’t happen with old paper because the bleaching process was only invented at the end of the 18th century. If it isn’t bleached, lignin turns a brown color in processing, which you can see in brown grocery bags. They have more lignin because they are designed to be strong. When the paper is bleached, the lignin loses its color. Cellulose, which stays in the paper no matter what, is naturally white.
When lignin is exposed to oxygen, it starts to oxidize. The oxygen molecules react with the lignin polymers and degrade their structure. Lignin contains chromophores, which are molecules responsible for color. They reflect certain wavelengths of light. The chromophores in lignin are called aldehydic chromophores. After paper is bleached, it reflects nearly all the light that hits it, making it white. When the chromophores bind with oxygen, they form new compounds and start to absorb some of the light, reflecting the brown and yellow parts of the spectrum. The more oxygen the paper is exposed to, the more it changes and the more yellow light it starts to reflect. Bleached and chemically treated paper will stay white for a lot longer, but even that may eventually begin to yellow. The only way to reverse this yellowing is to bleach the paper again, but that will ruin whatever is written on it.
Newspapers may go yellow in a matter of days because they have such a high quantity of lignin. The quality of paper is improved if the lignin is removed, but that makes it more expensive, and newspapers are printed in such high quantities that costs need to be kept down. To make newspaper paper, the wood is ground up and pulped. Keeping the lignin in increases the amount of pulp that can be made because about 95% of the wood can be used, meaning less wood is needed overall. Older paper made from wood was made in the exact same way. The quality of the paper is low, and it degrades quickly, but nobody expects to keep a newspaper for longer than a couple of days. And this is what I learned today.
Sources
https://en.wikipedia.org/wiki/Lignin
https://www.science.org/content/article/scienceshot-why-old-paper-turns-yellow
https://science.howstuffworks.com/question463.htm
https://www.sciencefocus.com/science/why-does-paper-go-yellow-over-time
https://www.loc.gov/everyday-mysteries/physics/item/why-does-ultraviolet-light-cause-color-to-fade
https://cordis.europa.eu/article/id/448740-why-red-pigments-fade-in-historical-art-works
https://en.wikipedia.org/wiki/Chromophore
Photo by Jess Bailey Designs: https://www.pexels.com/photo/open-textbook-762687/