Theory 158/Paper ageing/Threshold lens
Yellowing Needs a Lignin Threshold
The claim
Yellowing occurs above a threshold in lignin content, so papers below it age by a different route entirely.
01Mechanism
Lignin oxidises to chromophores; below the threshold, oxidation of cellulose dominates and colour change is slight.
02Prediction
Yellowing should track lignin content.
03Refutation
Yellowing tracking age would remove composition.