Reversion happens when residual lignin, degraded hemicellulose or extractives form new light-absorbing groups. Mechanical pulps are prone to photoyellowing in daylight, which is why newsprint browns on a windowsill, while bleached kraft reverts more slowly and mainly from heat and humidity. Labs measure it with accelerated aging in an oven or humid chamber and report the points lost or a PC number.
Kraft left the bleach plant at 88 and sat at 86 a week later, that's too much brightness reversion, so look at the final stage pH.
Heat and moisture speed reversion, so pulp bales stacked while still warm lose more brightness than bales left to cool.
Also heard as
- reversion
- color reversion
- yellowing
Related terms
PC number
Post color number: a Kubelka-Munk measure of how much a pulp yellows during an aging test.
Yellowness index
Single number that rises as a near-white sample shifts toward yellow, used to track aging and lignin effects.
Brightness
The blue-light reflectance of pulp or paper, read at an effective wavelength of 457 nm and reported as a percentage.
Points
Mill shorthand for units of brightness, opacity or whiteness on their percentage or index scales.