Interesting that the ancient term for nitric acid is used interchangably for a ferric nitrate solution.

Ferric nitrate should be much easier to obtain than nitric acid (altho it's not innocuous -- similar in that way to ferric chloride, that dissolves Cu off printed circuit boards).

I too would opt for more modern dyes, unless total authenticity were a major goal. One reason is the potential corrosive action of residual "dye" (I'm thinking of Brownings and salt wood, for example).

Fred