Short answer
Mostly because of what the paper is made from. Cheap wood-pulp papers such as newsprint keep much of the wood's lignin, which oxidises in light to yellow and brown compounds, and many papers made from the later nineteenth century onwards were also acidic, which breaks down cellulose in the dark. Rag and modern alkaline, lignin-free papers yellow far more slowly. Light, heat, pollutants and acidic enclosures all speed the process, and fading optical brighteners can make it look worse.
Wood is roughly 30% lignin, the aromatic polymer that binds cellulose fibres together. Chemical pulping — the kraft and sulfite processes — removes most of it, though kraft pulp keeps enough to be brownish unless bleached. Mechanical pulping, used for newsprint and other groundwood papers, grinds wood into fibre and keeps most of the lignin in. Lignin absorbs ultraviolet strongly and photo-oxidises: laboratory studies of lignin isolated from chemimechanical pulp show UV breaking it down into aldehydes, acids and increasingly condensed structures. The products absorb violet and blue light, so the paper turns yellow, then brown, fastest where light reaches it. The Canadian Conservation Institute notes the high UV sensitivity of low-quality papers such as newsprint.
Yellowing is often accompanied by embrittlement, and that is largely acid's work. Acidic wood-pulp paper became commonplace in the late nineteenth century, sized with alum–rosin systems that generate acid. Acid catalyses the breakdown of cellulose chains, so the paper loses strength, and the degradation products add to the discolouration; librarians called the process slow fire. It continues in the dark and speeds up with heat and humidity. From the 1950s alkaline sizing made acid-free paper practical, and permanent paper now contains an alkaline reserve, usually calcium carbonate, to neutralise acids from the paper or the air. ISO 9706, published in 1994, is the international standard for permanent paper.
The Image Permanence Institute describes paper yellowing in prints as a natural ageing process whose rate falls as storage temperature falls, which is why cool or cold storage is recommended for chemically unstable, lignin-rich papers. Heat-driven yellowing of most papers is slow, but light, pollutants and poor-quality storage and framing materials can accelerate it dramatically. Contact matters: acidic card mounts and boxes transfer acid into the paper next to them, leaving a darker band or burn line. And brighteners complicate the picture. Optical brightening agents make new paper look whiter by fluorescing blue; as they break down under light and pollutants the paper loses that boost and appears to yellow, even if its fibres are unchanged.
The practical steps follow from the chemistry. Keep valuable paper out of light, especially daylight, because lignin-rich paper reacts to ultraviolet; frame with UV-filtering glazing and acid-free, lignin-free mounts; store in cool, stable, moderately dry conditions; and separate newspaper cuttings from other documents, since their acidity migrates. For important papers the conservation options — deacidification, washing, lining — are specialist treatments. For documents whose content matters more than the object, making a good copy is often the best protection. Recycled paper adds its own considerations: residual inks and mixed fibre shift its starting colour, which the recycled-paper and de-inking pages in the materials and colour-technology pillars describe.
| Paper | Lignin | Acidity | Yellowing tendency |
|---|---|---|---|
| Newsprint and groundwood papers | High | Often acidic | Fast, especially in light |
| Unbleached kraft (brown paper) | Some | Varies | Darkens further with age |
| 19th–mid-20th-century book paper | Varies | Often acidic (alum–rosin sizing) | Yellows and embrittles in the dark |
| Modern alkaline, lignin-free paper (e.g. ISO 9706) | Very low | Alkaline reserve | Slow |
| Cotton rag paper | None | Usually neutral | Slow |
| Brightened office paper | Low | Usually alkaline | Loses brightener glow; looks yellower |
Why: An acidic mount board has transferred acid into the paper along the bevel.
Fix: Have it reframed with acid-free, lignin-free mount board; a paper conservator can advise on the stain.
Why: Acidic, lignin-rich newsprint migrates acid and discolouration into adjacent paper.
Fix: Photocopy cuttings onto permanent paper and store originals separately in acid-free folders.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactModerate evidence
Paper made from wood-based pulp that has not had its lignin removed turns yellow, becomes brittle and deteriorates, faster when exposed to light or heat; acidic wood-pulp paper became commonplace in the late 19th century.
Source: Acid-free paper
FactStrong evidence
Paper yellowing is slowed by lower storage temperature, is faster in lignin-rich and groundwood papers, and is accelerated by light, pollutants and poor-quality storage and framing materials; breakdown of optical brighteners also makes paper look yellower.
FactModerate evidence
UV irradiation of lignin from bleached chemimechanical pulp produced benzaldehyde- and benzoic-acid-type compounds and increasingly condensed structures.
Caveat: Isolated lignin on filter paper, not whole newsprint.
Source: Photodegradation of acidolysis lignin from BCMP (Molecules, 2008)
FactStrong evidence
Low-quality papers such as newsprint are highly sensitive to UV and yellow on exposure.
Source: Agent of deterioration: light, ultraviolet and infrared
FactModerate evidence
Wood is roughly 30% lignin; kraft pulping removes most of it but kraft paper retains some lignin and is brownish.
Source: Lignin
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.