How to get a French Grey finish?

[William]

Anyone have a recipe for achieving a French Grey finish on a gun barrel using vinegar?

[beowulf]

found a couple of methods , but none using vinegar , at least not yet ! ;Brownells 44/40 cold blue and then soak with chlorine bleach wiped on with cotton balls, then card it back with green scotchbrite or steel wool to the color you want. ;ench Grey formula is to cold blue, then naval jelly, then blue again, then card down to what I want ; and here`s one that uses vinegar; I like the acid etch better than the bleach because I think I can get better control of the process.  Bleach seems to react more with the grain structure of the steel giving a slightly pitted effect.  You don't use strong acid, if I'm starting with straight HCl, I dilute it to 10%, the same with the Acetic acid ( and vinegar is 5% acetic acid so we're not talking strong here).  As Jerry said, phosphoric will also work - you can get that easily at the auto parts store.  In the replies above two different things have been discussed - a total french grey and selective greying.  When I've done single shot rifle actions that I wanted to totally grey after the engraving, I hot blued them and then submerged them in the acid.  I used a strong light and when the blue was gone and the surface minutely frosted, I pulled them out, rinsed immediately with lots of water, dunked them in some baking soda to completely neutralize, and then rinsed lots more.  Because the surface that that point is super clean it will rust almost immediately if the humidity is high so you need to get some oil on it quickly.   Needless to say on all of this the surface of the piece needs to be nearly perfectly polished as any scratches, etc. will be easy to see after the surface treatment.

What this type of greying does is to frost the surface so that light isn't bouncing back  which is what hides the engraving.  It doesn't leave a dark background in the engraving which is what Jerry has done with his selective removal from the surface.  You can take a totally greyed piece when it comes out of the rinse, dry it and wipe some india ink into the engraving and then wipe the surface clean.  The ink will stick quite well.  Once the ink is dry, oil.  This will highlight the engraving.
hope these are helpful !

[Hanshi]

While I don't know the method he uses for that finish, Matt Avance at TVM built me a rifle - this was quite a few years ago - with that nice, grey finish.  I think it can be beautiful.
Here's a photo that shows the nice, satin color.