Calculated as caliper divided by grammage, bulk tells how thick a sheet is for its weight. High-bulk sheets hold more internal air and free fiber surface, so they scatter more light and give better opacity per gram. Refining, wet pressing and calendering all lower bulk, which is why mills fight to keep it while chasing strength and smoothness.
Bulk is 1.35 on this lot, a good tenth above the last trial, and the opacity followed it up.
Also heard as
- specific volume
Related terms
Caliper
Thickness of a single sheet of paper, measured under a set pressure and given in micrometers or thousandths of an inch.
Grammage
Mass of paper per unit area in grams per square meter, the base for most optical calculations.
Light-scattering coefficient
Per-grammage measure of how strongly a paper redirects light, written s and given in square meters per kilogram.
Calender
Stack of hard rolls that presses a paper web to make it smoother, glossier and more even in thickness.