At this point, I'd guess the issue is with the composition of your test barrel. I regularly use C.10 and C.19 for blacking, both yield good results. In fact, I usually begin diluting the solution after the third application or so, because the black is so deep it's harder to etch. (C.10 used on Damascus and Twist). Having problems with C.19 is also surprising, as IMHO it produces an exceptionally deep black on fluid steel barrels.
The only other thing I do differently than you is allow longer rusting times, with four to six hours of moist heat, followed with dry heat throughout the overnight. I use shorter periods much farther down the process, after the color has been established and I'm trying to maintain contrast. (Damascus and Twist).
I suspect, though I'm not certain, that an Aqua Regia mix (HCL and Nitric Acids, along with Iron) is reactive enough to color some rifle barrels. It's certainly unnecessary for Vintage shotgun tubes.
Regards
Ken
Last edited by Ken61; 03/06/17 12:11 AM.
