The bullino style engraving creates it's image by shadow.
That shadow is created by the tiny burr pushed up by the graver point.
The graver is sharpened to a very small V point with a narrow included angle and a very low face angle. This so the burr is raised with little effort. Just barely touching the surface will
produce a burr.
The Bullino graver is fragile compared to a standard line cutting graver because of it's low face angle. But with it you get the necessary sharp burr needed.
By changing the pressure and the angle that the graver makes with the surface of the work, those burrs can be manipulated in depth and size.
This produces differing shadow effects.
Add to that then how closely those burrs,,or 'dots' as they are often called,,are placed together and you can get anything from Black all the way to White and anything inbetw
on the Gray scale for 'color'.
Again placing them closer together or farther apart is the (simple!) key to forming the images themselves. The same idea as newsprint only in a much more detailed form of course.
Bullino scenes can loose some of their detail and photographic look when viewed at different angles. It's simply those shadows not showing up as well or not at all as the light and the standing burr that casts it are moved.
Wear to the surface removes the everything the engraver took countless hours to place there to create the image. So with heavy wear the image can disappear down to only a mear outline of character lines of the subject and some heavier background cuts.
If done on Gold inlay work, the wear can be gone very quickly from the soft alloys.
Bank Note Engraving is different in that it is done with lines cut with a graver.
The lines are incised (cut) and not done by raising small burrs to create an image.
BankNote does use dots and tiny dash markings but they are cut into the surface, not raised as burrs.
In BankNote the dots are used the same way as in Bullino in that closely grouped dots will show as a dark 'color' and loosely spaced as a lighter color.
Images can be made the same way by placing the dots and dashes together.
Cross hatch lines are also used extensively to shade engraving in banknote style
But they are all surface level cut lines on BankNote engraving.
BankNote was/is a form of print engraving, the shallow, thin lines holding the ink on the plate cut with the image.
When done on guns, the same style of cutting was used.
Nimschke used BankNote style on most all of his figures and scenes.