Would the placement of the bubble wand affect the etch that much
that it etches more where it directly is hitting?
This will certainly affect it. The idea of air bubbles is to
agitate the whole liquid … if the bubbles are directed or impinging
just on one part then that area will tend to etch faster. A possible
disadvantage of using air burst agitation is that you will possibly
be creating minute droplets of ferric nitrate solution in the air
above your etch bath, so keep a lid on if possible.
Another thing you could do is thoroughly anneal the piece before
etching, so that it all has the same hardness.
I’m puzzled as to why you’re having to brush the plate to see the
depth of etch, as the etch action will be to form silver and copper
nitrate, both of which are soluble. One possible explanation would
be if you have any chloride in the bath, as silver chloride is
insoluble in water (though I don’t know how it would behave in ferric
nitrate solution). Using chlorinated tap water may be enough to
Kevin (NW England, UK)