Ferric nitrate mordant for silver

Gail Nelson’s article on “The Edinburg Etch”, a method for improving
ferric chlodide as an etchant for copper and brass in the most recent
“Tips from the Jeweller’s Bench”

makes me wonder if anyone could speculate on how to achieve similar
improvements with ferric nitrate when etching silver? I have been
using it for years, often with much frustration as there is a lot of
cleaning up to do on background areas which develop a horrible crust
and has to be stoned off. The crust is slightly spongy when wet and
if you press a blunt point on the surface it seems to give a little
and you can see the solution being squeezed away actually under the
surface. These are silversmithing scale pieces, so I mean a LOT of
stoning. I etch in a number of layers, painting each one successively
with a homemade pitch resist and the deeper the etch, the bigger the
build-up of crust on the uncovered areas. I now stone it between each
layer instead of waiting til the end of the job. It would be
fantastic if someone with chemistry know-how could suggest a possible
way to dissolve this surface crystalline sediment and give some of
the improvements afforded by using “The Edinburg Etch” on other

Some of the things I’ve done in trying to isolate where the problem
lies are: to mix the ferric nitrate crystals with purified or
distilled water; keep the solution out of the sun; and at someone’s
suggestion tried soaking the finished piece in an ammonia solution
which was supposed to dissolve the adhering crust but none of these
made any difference. You are probably wondering how I could persist
in such ignorance but I just keep hoping it might be okay next time!
Any brainstorms about where the problem may lie?

Many thanks, Kay