The following is things I've learned over the years that have helped me in the preservation of older reclics and coins and are listed here as helpful SUGGESTIONS... If you think it is worth alot get PROFESSIONAL assistance.
Preservation
Initial cleaning I soak the object in hot water with a mild dish soap like Dawn for an hour or two. Then I brush with a soft tooth brush.
--EXTRA VIRGIN OLIVE OIL, For those brass and copper pieces soak it in olive oil for anywhere from a day for those lightly affected pieces up to two months for those items in bad shape. Again lightly brush with tooth brush and soapy water after the oil soak.
--Pewter items, Hot soapy water and soft toothbrush is the extent of it. The pewter becomes fragile after years in the ground and great care must be taken.
--Silver items, Hot soapy water initially. If it is in rough shape or not worth anything (monetary, not sentimental) then I'll run it on electrolysis in small spurts of 10 to 20 seconds, lightly brushing between each session.
-- A couple new methods have come around as of late. One is the use of Hydrogen Peroxide. I have not used it much, but I have seen it used on older copper coins and it did great in leaving the patina but yet showing good detail. The other thing is the use of Naval Jelly, it is great for bringing out the remaining gilt on buttons, HOWEVER what it will also do is turn any of the base metal (copper/brass) an orangish color. So, if you don't mind that, it will suprise you with the brightness of the remaining gold, but if you were to sell it, most collectors don't like for the patina to be removed.
Long Term Preservation
--Long term preservation this is what I was taught. It will seal your artifcat form the environment for a LONG TIME.
Obtain some Bees wax (can be bought at Wal-Mart in the plumbing area, the seal used at the bottom of toilet bowls is Bees wax). While there get some parafin used for canning. At the Dollar Store get a small tin pot suitable for placing on a stove and also get a small metal sifter that can fit inside the pot. Mix the parafin and wax at a mixture of 60% parafin and 40% Bees wax in the pot and heat on a low heat until it liquifies and mixes. Place the artifact in the sifter and lower into the hot wax mix (do not allow it to come in contact with the bottom of the pot, it has been known to melt some items). After letting the object absorb the wax for a minute or two, remove the sifter from the hot wax and then remove the item from the sifter with a pair of tweezers. Take a toothbrush and brush the hot wax off of the item. This will seal the item from the environment for several years. If you have any questions about this feel free to email me.