Jan,
A friend that conserved hundreds if not thousands of ancient coins told Suzi and I what we needed to do with very grungy metal. Bronze Copper and Silver.

Suzi and I have conserved more than a few old Byzantine and Roman coins with very good results. I have attached two images of an 850 year old Byzantine coin to show the results of this type of conservation.As you can see in the images the results can be amazing.

Make a saturated salt water solution, enough so you can hang your piece in it. Put the solution in a glass/plastic container. Wrap your piece in a few places with copper wire fine but not too fine, if you have three or four wires bring them together up top and twist them together a bit. Get an old piece of stainless steel and old butter knife will work.

Get yourself (thrift stores are great for this)an old wall plug transformer, most go from 120volt to either 9, 6, or 3 volt. The kind used for calculators and such. Get a three volt (the best)I would get a few if they have them.

Cut the plug that goes into the electronic item 3 volt output. Split the wires and put an alligator clip on each wire. Put one clip on the copper wire twist and the other on the stainless steel piece that are in your solution keep the stainless steel from touching the piece being worked on. Plug the transformer in and look in your solution if bubbles are coming off of the piece to be cleaned this is right if off the stainless switch the clips.

Depending on how much grung is on the piece will determine how long this process will take. We usually slowly spin the coin back and forth so the electricity is not a constant in one spot. Every once in awhile remove the piece and see if things are lifting, if so pick at them with a tooth pick to help in removal.

Might be best if you want to give me a call so I can make this a bit clearer. I assure you that this process works perfect and seeing as though it works at such a microscopic level on the metal you have little chance of causing damage to it.

All my best .......... Danny

contact me if you want my phone number.