KarloinPerth's Profile

Museums and respectable authorities have a history across time of wrecking pieces because 'knowledgeable' people have little underlying grasp of physics and chemistry. Galvanic action only occurs when there is a solution permitting the two metals to act such as a salt solution in water, it cannot continue without a solvent!

Now metal plating bonds two dissimilar metals to one another - one of these processes is for galvanic protection, such as zinc plating (galvanizing) where the anode - the more reactive metal is attacked first before the least reactive metal, usually the iron it is protecting. The higher the reaction potential of the metal, the more that metal will corrode. The other use for plating metals is decorative, like the silver we're discussing. In this case the silver with a very low reaction potential is plated over brass which contains zinc with a higher reaction potential. Similarly, chrome plate covers steel on car bumpers - it's aesthetic only and does not protect the steel except by covering the steel completely AND having a very low reactive potential. Once the chrome is breached the iron will corrode quicker than if there had been no plating at all - in this case the iron is sacrificing it's self to protect the chrome!

Another example of galvanic action is the nails in the lemon battery where you put an iron and a copper nail in a lemon (acid solution) and draw off electricity. The reverse action is where you do the same thing but pump electricity into the circuit and you get metal being plating instead.

In the case of silver plate, we have decorative plating but silver is actually a 'noble' metal and is not actually that reactive.. however it does react to oxygen and sulfur very easily and this causes the tarnishing we see. By reversing the corrosion process as described in other posts, we are using a very reactive metal in a salt-bridge with the silver to re-plate the silver back onto the flatware.

In plating metals you don't want a solution that would actually attack either metal, so bicarb soda is perfect for the silverware.. but you do need it to allow the electrons to flow, once they begin flowing the aluminum corrodes and it takes the ions from the silver tarnish - and the silver which is released from the silver sulfide or silver oxide corrosion then sticks it's self back onto the silverware. This process is not simply harmless, it's actually putting the silver back where it came from.

The only other methods to clean silverware aside from galvanic re-plating involve abrasion or 'polishing' or worse, acid stripping. No matter how gentle the polish may appear, removing any tarnish involves abrading or rubbing off a portion of silver. Usually Canuba or another hard wax is involved to create a fine finish, but it still removes silver. Again, any method of polishing removes silver and that is not ideal. Acid stripping etches and strips some metal - that is extremely bad, while galvanic cleaning replaces silver back to where it came from and you can't really get better than that.

Wikipedia has a fine article on conservation and restoration of silver objects which emphasizes this better.