BENCH TIP FROM ALAN REVERE
Flux is necessary for soldering. It provides an impenetrable glassy shield, which prevents oxygen in the air from reaching and oxidizing your work. Flux also cleans the seam, trapping dirt and other foreign compounds do that they do not come into contact with the metal. After soldering, jewelers usually remove the hard, glassy flux by immersing the workpiece in "pickle," usually either a weak sulfuric acid solution or a solution of water with dry powder acid. The "pickling" action cleans off the flux and oxidation.
But what if a piece is hollow, and you don't want to deal with the difficulties of removing trapped acid from the interior? In that case, try boiling the piece.
That's right; just place that hollow ring or other item in a water-filled beaker, and heat. When the water reaches a boil, let the piece bounce around for a few minutes as the flux dissolves into the water.
Boiling removes all of the flux, leaving only surface oxidation, which can be removed mechanically by sanding and buffing.