Filler
Filler turns up in 2 fields: paper optics, forestry. Here is what it means to the people in each one.
Filler in paper optics
Fine mineral powder added to papermaking stock to raise opacity, brightness and smoothness and to cut cost.
Common fillers are precipitated and ground calcium carbonate, kaolin clay, talc and titanium dioxide. Their small particles create many air interfaces, so they scatter light far better than fiber, raising opacity and brightness while replacing more expensive pulp. Filler weakens the sheet by getting between fibers and reducing bonding, so levels are set against strength and dusting limits.
"Filler is up to 24 percent ash and the sheet is starting to dust on press, so we're at the limit."
Filler in forestry
A standard tree left standing above a coppiced understory to grow to full timber size.
In a coppice-with-standards silvicultural system, a filler is a tree selected to remain uncut while the surrounding brush or smaller trees are harvested. These trees fill the upper canopy and are grown on a longer rotation to produce large-diameter sawlogs, while the understory provides regular yields of firewood or pulp.
"Mark the straightest oaks as fillers and clear out the rest of the hazel understory."