1. Newspaper Turns Yellow Even When Kept Next to High-Quality Paper
One resource describes a case where newspaper and department store wrapping paper (high-quality paper) were kept in the same place. Only the newspaper turned yellow [2]. If the location is the same, the difference must lie in the paper itself. The resources look for clues in the type of pulp used to make the paper [2][1].
Safety Note: When comparing paper, do not try to change the color yourself. Just observe what happens naturally.
2. Lignin, the Substance That Holds Wood Fibers Together, Remains in Newspaper
Lignin is the substance that holds fibers together inside wood. When exposed to light, oxygen in the air, or heat, this lignin changes, causing the paper to yellow [1]. There are two main ways to make pulp from wood. Mechanical pulp is made by grinding wood directly, so much lignin remains. Chemical pulp uses chemicals to remove almost all lignin. Therefore, paper made from chemical pulp, like high-quality paper, is said to resist fading [2].
Newspaper paper has traditionally used a lot of mechanical pulp [3]. However, another resource describes newspaper as a mix of chemical and mechanical pulp [1]. The exact ratio is not mentioned in the resources reviewed here.
3. How Light and Oxygen Turn Lignin into Colored Substances
A review paper organizes the yellowing process into three rough stages. First, parts that can become colored absorb energy from light. Next, an unstable intermediate substance called a "phenoxy radical" is formed. From there, substances that absorb visible light—meaning they have color—are created [4].
The main players are a group of substances called quinones. Phenolic parts within lignin are oxidized and mainly turn into quinones, which is considered the primary reason mechanical pulp yellows. For paper indoors, yellowing is expected to be greatest with ultraviolet light at wavelengths of about 330 to 385 nanometers [4]. However, this mechanism is complex, with multiple pathways working at once. Quinones alone may not explain all the yellow color [4].
4. Ultraviolet Light Causes Yellowing, but Visible Light Can Whiten Paper
The same review states that UV light tends to cause yellowing, while the visible light region can sometimes bleach paper, making it lighter [4]. However, the boundary and results change depending on the type of paper. This means light that yellows paper and light that whitens it coexist in sunlight.
In testing within the field of preservation, paper exposed to natural light showed a decrease in brightness overall, with yellowing confirmed, except for some parts [3]. The paper used in this test contained lignin but was not specifically newspaper. Bleaching is not something to try at home; read it as a research topic.
Safety Note: Home bleaching has not been proven safe or reliable. This article does not cover it.
5. Paper Can Keep Yellowing Even After It Is Moved to the Dark
Preservation guides state that chemical changes caused by light continue even after the light is removed or the paper is moved to a dark place [5]. Moving paper from sunlight to a drawer may not stop the process immediately.
The review paper states there is no cost-effective treatment that permanently prevents light-induced discoloration in paper containing lignin [4]. Therefore, papers made with mechanical pulp are considered suitable for uses with short lifespans, like newspapers.
6. Brittle Paper Is Damaged by Acid, Not by Lignin Yellowing
In acidic paper, acids break the connections between cellulose molecules, making the paper brittle [5]. The yellowing discussed in this article is caused by lignin changing under light. Acid damage to cellulose is a separate phenomenon. Therefore, explaining newspaper yellowing solely through acid damage would be inaccurate. Whether current Japanese newspaper paper is acidic or neutral was not determined in these resources.
7. How to Test Newspaper Yellowing Yourself with Cardboard and Sunlight
Choose one old newspaper you can use. Cover half of it with thick cardboard and clip it in a sunny spot. After several weeks, remove the cardboard and compare the covered part with the uncovered part. Sunlight exposure varies by home, so the time needed for changes is not fixed. If nothing changes, that is your result.
Another idea: Look at the inside and outside of the fold of an old newspaper. If the colors are different, try to predict in your own words if it is related to how the light hits it.