The process Richard is referring to we always used to call
’bombing’. We’d heat up a solution of sodium cyanide (maybe a
quarter of an egg at the most) in a glass or corning container
until it steamed, and then squirt in a couple tablespoons of
hydrogen peroxide (30%). The solution would boil and foam for a
few seconds, maybe quintupling its volume, and then collapse
turning a vivid blue color from the dissolved copper. The surface
of the article (usually gold) when then have an even bright 24k
finish ready for plating or whatever. Very handy when working on
chains which couldn’t be polished, and not quite as dangerous as
it sounds. Of course, we always worked under a fume hood.
Bright dip on the other hand is (was) a commercially available
solution (Max Friedheim used to sell it) composed of a blend of
nitric, hydrochloric, and sulphuric acids (kind of a dark brown
color). You’d heat it and then just dip the sterling or gold
into it. I used it once only, I hated it.