why do old newspapers turn yellow?
Old newspapers turn yellow due to a process called degradation, which is influenced by a combination of factors including light, heat, humidity, and the paper’s composition. Here’s a more detailed look at the processes involved:
- Papermaking Process: Modern papermaking involves using wood pulp that contains lignin, a natural polymer that helps hold the cellulose fibers together. Lignin is a complex organic compound that can be broken down by chemicals during the papermaking process, leaving behind a whiter paper. However, in older papers, especially those made before the 19th century, the papermaking process was less refined, and lignin was not completely removed. This can result in a browner or yellowish color.
- Light Degradation: Exposure to UV light can break down the cellulose fibers and the lignin in paper, causing it to yellow over time. This is why newspapers stored in direct sunlight or near a window for extended periods may turn yellow more quickly.
- Hydrolysis: The breakdown of wood pulp by water can also contribute to the yellowing of paper. As paper ages, it can absorb moisture from the air, and the cellulose fibers can be broken down by hydrolysis, which makes the paper more susceptible to browning.
- Thermochemical Degradation: Heat can accelerate the chemical breakdown of lignin and cellulose, leading to yellowing. Additionally, the combination of heat and humidity can cause the paper to swell and then shrink as it dries, leading to a crinkled or pitted appearance.
- Ink Chemistry: The type of ink used in older newspapers can also play a role in yellowing. Some historical inks contained metals like iron, which can react with other compounds in the paper over time, contributing to a yellow or brown color change.
- Bacterial and Fungal Degradation: Moisture can also allow bacteria and fungi to grow on the paper, which can break it down and contribute to discoloration.
The combination of these factors can lead to the characteristic yellowing seen in old newspapers. To preserve old papers and slow down the yellowing process, it is recommended to store them in a cool, dry, and dark environment. Archivists often use deacidification processes to treat old papers to remove acidity and slow down further degradation.