In damascus bbl finishing, it is commonly used as the 'etch' soln betw the rusting cycles.
There it is used in a very weak soln,,I use it at about 3 to 5% soln in plain water at room temp.
The damascus/laminated/twist steel bbls are browned (or rustblued if you want a black % white pattern).
Then the bbl(s) are dunked very quickly into and back out of the etch soln.
Then placed under running cold water to flush the etch soln from the surface. You immedietly begin to card the surface with
fine steel wool with the water still running over the surfaces.
This carding removes the color layer that the etch soln loosened from the steel component of the bbl matr'l, but the color hangs tougher on the softer iron part of the bbl.
With repeated rusting, etching and carding cycles, the brown&white or black&white pattern is brought up on the surface.
It takes many more cycles than plain rust brown or rust blue finishing as you are constantly removing part of the color layer from the softer iron each time,,though leaving enough behind to build color and contrast with the completely color free 'in the white' steel of the pattern.
More than you probably wanted to know..
That etching soln will also place a plain matted steel finish of a light grey color on a piece of steel and is a good basis for a rust blue finish or rust brown. It gives the rusting soln a good 'foot' as they sometimes say to get started.
It doesn't take much (very weak solns...) and use them room temp,,no heat.,,and fast application. In and out. They work fast.
