Cleaning and Preservation of Coins - A Complete Guide, page 21
HOW TO BRIGHTEN SILVER, COPPER and GOLD COINS: Effective Methods
HOW TO BRIGHTEN SILVER COINS
1) Complete brightening is done in simple fashion by immersing the coins in a silver immersion bath such as Ripolin, Land's Precious Metal bath, etc.
These immersion baths very quickly dissolve silver sulfide without harming the coins.
After immersion, the coins are washed and the still moist coins are rubbed briefly with table salt which removes the dissolved silver sulfide from all the crevices.
2) When Silver Polishes are used, the luster often is unpleasant in contrast with a silver immersion bath, so that such coins, when cold, have the value-reducing term of "polished" applied to them.
3) An old method uses the water of recently Boiled Potatoes poured hot over silver which then remains in it. After about 10 minutes, the silver is rubbed briskly with a woolen cloth to brighten it.
4) Electrochemical Reduction Method: Another old recipe uses Aluminum Foil placed on the bottom of an enamel pot in which the silver is boiled for a few minutes. The silver sulfide transfers to the aluminum foil, forming aluminum sulfide on it. Electrochemical Reduction Method - Galvanic Cleaning of Coins is described on page 15 and is fully covered in my detailed and illustrated Tutorial - "Galvanic Cleaning of Silver Coins".
5) A simpler version of the Galvanic Cleaning method is to boil dark silver in an Aluminum Vessel. The coins must touch the bottom and thus the aluminum.
6) The dark silver coins may be covered with moist Table Salt. After drying, the salt is removed and the coins are brightened. It is simpler to rub the coins with moist salt, giving the same effect.
7) "Never Dull" - cotton pads or wads saturated with chemical in a can, works wonders on silver, brass and steel, and does not leave a film. It might slightly harm the coins, care must be exercised.
8) Dipping coins into Taco Bell Sauce is an effective method.
9) Dipping coins into Coca Cola Classic for an hour can remove the tarnish. This popular drink is usually used for removing rust off the steel nails.
10) If only parts of the coins are to be brightened, because they are spotted or too uniformly dark, thus lacking the natural partial brightening caused by circulation, then they can be brightened readily with a Soft Eraser, and old coins can be brightened with an Ink Eraser Pencil.
HOW TO BRIGHTEN COPPER COINS
1) Copper, like bronze and brass is brightened with Copper Soap. A moist cloth is passed over the copper soap and then the cloth is rubbed over the coin.
2) "Never Dull" - cotton pads or wads saturated with chemical in a can. It effectively brightens the copper coins, and does not leave a film. It might slightly harm the coins, care must be exercised.
HOW TO BRIGHTEN GOLD COINS
Gold is rarely discolored. If this should occur, the gold coin is immersed very briefly into diluted Aqua Regia, which is prepared by mixing three parts of concentrated Hydrochloric acid (HCl) with one part concentrated Nitric acid (HNO3) followed by dilution to about 1:10.
NOTE: Aqua Regia is the only acid that will dissolve gold and platinum.
After immersion the gold coin is rinsed and then soaked for 30 minutes with frequent changes of water.
