When a sheet enters a hard nip with excess moisture or under high load, fibers collapse and bond tightly, closing the air spaces that scatter light. The crushed areas lose opacity and brightness and look gray or glassy, often in streaks matching wet bands in the moisture profile. Lowering moisture or nip load, or evening out the moisture profile, stops it.
Calender blackening on the back side of the reel, and the moisture profile shows a wet streak right there.
Also heard as
- blackening
- calender darkening
Related terms
Calender
Stack of hard rolls that presses a paper web to make it smoother, glossier and more even in thickness.
Supercalender
Off-machine stack of alternating steel and resilient filled rolls that gives paper high gloss and smoothness.
Light-scattering coefficient
Per-grammage measure of how strongly a paper redirects light, written s and given in square meters per kilogram.
Opacity
A sheet's ability to hide what lies behind it, expressed as a percentage from readings over black and white backings.