this is from sharr choate book, creative gold and silversmithing.
i think what you are descibing is a ‘bright dip’. the metal is
heated to produce cuprous oxides of the surface. after it is
cooled, dip it in your solution for only 2-3 seconds, use
something that won’t react to the solution when yu dip your
piece in it (wood or plastic tweezers, string). the solution for
gold is one part sulphuric acid and one part distilled water.
the solution for silver is one part nitric and one part
distilled water. a small bit of abrasion will rub the finish off
though. it is described as a bright, frosty finish.
one could get a more matte finish by first bright dipping, then
dipping the piece into a second solution consisting of one part
of each: nitric, sulfuric, and water.sharr choate doesn’t
mention if this solution works for both silver and gold. of
course, one could thicken the surface treatment by repeated
heating and dipping. i hope you can tell that i’ve never tried
this, so don’t curse me if it doesn’t work. i’ve read and
enjoyed many of your posts, i hope this works for you.