When I was satisfied with both drawings I placed the plates in a vat of nitric acid. The acid eats away the metal only where my lines have exposed plate. The longer the plate is in the acid, the deeper the grooves beneath my lines become.
Looking at my proofs at the areas I wanted to further develop, I once again drew into the Ground to expose the plate.
The shine of the metal shows where I am enriching the line work.
The Whistlers Needle is a twisted length of metal that comes to a point on either end.
I touch up the edges of the plates to protect them from the acid. And they get another half hour bath in the acid.