Paper turns yellow over time mainly due to the oxidation of lignin and the action of ultraviolet light, which trigger chemical reactions that change its color.
What role does lignin play?
Lignin is a complex polymer present in the cell walls of plants and, although it is partially removed during paper manufacturing, traces remain in many types of paper, especially lower-quality ones such as newsprint. When lignin is exposed to oxygen in the air, it undergoes oxidation and forms compounds that absorb more light in the visible spectrum, which is perceived as a yellowish tone.
Influence of ultraviolet light
UV radiation, present in both sunlight and some fluorescent lights, accelerates the degradation of lignin and can also affect cellulose, although the latter is more resistant. The energy of UV photons breaks chemical bonds, generating free radicals that promote yellowing reactions.
Other contributing factors
- Paper acidity: Papers manufactured through processes that leave acidic residues (for example, some low-quality papers) experience accelerated hydrolysis of cellulose, which also promotes the change in color.
- Humidity and pollutants: Humidity facilitates the diffusion of oxygen and polluting gases such as sulfur dioxide, which can react with the components of the paper.
- Temperature: A warm environment increases the rate of oxidation chemical reactions.
Differences between types of paper
Archival or high-quality papers are usually bleached and have a low lignin content, and are also treated with alkaline agents that neutralize acidity. This is why they resist yellowing better and can be preserved for decades without changing color significantly. In contrast, newsprint paper, which contains more lignin and is usually acidic, turns noticeably yellow within a few months if exposed to light and air.
How to slow down yellowing
To preserve documents and prevent premature yellowing, it is recommended to:
- Store them in a dark place or under dim, UV-filtered light.
- Maintain stable relative humidity (between 40% and 50%).
- Use acid-free storage materials (archival folders, boxes and envelopes).
- Avoid high temperatures and abrupt climate changes.
These measures slow down the rate of oxidation reactions and help keep paper in its original color for a longer time.
