Bluing
Bluing turns up in 2 fields: gears, paper optics. Here is what it means to the people in each one.
Bluing in gears
Shop word for marking compound brushed on gear teeth to read the contact pattern after running.
A thin coat of colored paste or fluid, either Prussian blue or a yellow or white gear marking compound, applied to the teeth of one gear before the pair is rolled together. Where teeth touch, the compound is wiped off or transferred, leaving the contact pattern visible. It must go on thin and even, or the film itself hides the true pattern.
"Put a thin coat of bluing on the gear, roll it both ways under a little drag and let's see where it's touching."
Bluing in paper optics
Mill talk for adding blue or violet dye to make a white sheet look whiter by masking its yellow cast.
The word comes from laundry bluing, the blue added to a rinse to whiten cloth, and papermakers use it the same way: a trace of blue or violet dye pushes b* negative and hides yellowness from lignin or recycled fiber. It raises perceived whiteness but always lowers brightness, so it cannot rescue a brightness spec. Heavy bluing makes a sheet look grayish in daylight.
"Pulp came in yellow again, so we're leaning on bluing to keep the sheet looking white."