Feel free to add more tips here.
Copper
Remove green verdigris.
Powdered chalk + methylated spirits mix, rub hard.
Use fine steelo on stubborn areas.
Then apply metal polish.
A soft cloth buffing wheel.
Finally buff by hand with soft cloth.
Brass
Wash in detergent
Rub with 1 heaped teaspoon salt + 2 Tablespoons vinegar mixed into 600ml water.
Clean dirt & old polish from engravings with Ammonia on toothbrush, then detergent, and finally water.
Immerse badly corroded brass in warm strong solution of washing soda for 1 hour. Wipe or brush, if corrosion remains, repeat.
Brush stained areas with glass jewelers brush.
Rub off spots of corrosion with scouring powder on cloth, or if difficult use steelo.
Polish out scratches with paste of Whiting (finely ground chalk, obtainable from hardware store).
- Metal polish may rub off engravings *
Clear Lacquer (Hardware or Art Supplies) can preserve the polished appearance. Lacquer goes cloudy & picks up dust in cold weather, so should be applied in warm environment.
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Silver Oxide is a minor dull grey tarnish. The two main corrosions are Sulphide & Chloride.
Silver Chloride occurs near the sea.
Silver Sulphide occurs near geothermal areas or from certain materials which contain Sulphur; eg orange/red foods (Apricots, Plums, egg yolks), also avoid contact with rubber.
Resoldering & hammering dents is complex and best left to an expert.
A previous post here:
http://www.paydirt.co.nz/forums/yaf_postst6348_Non-destructive-Preservation-Techniques.aspx#post45047
Discussed cleaning techniques.
Here are some more tips for touching up some remaining staining.
Clean any remaining bad spots with a paste of French chalk & Methylated spirits or in water with a few drops of Ammonia.
You may want to skip this some tarnish can be appealing, even a fully tarnished look can be desirable.
Green corrosion can be removed with a bit of lemon (alternately 1 tsp citric acid in 1 cup water). Work on small areas one at a time, wash clean between each.
Remove any scratches by rubbing with Jewelers Polish or Tripoli powder.
Metal or Plate polish can be used.
A lacquer may also be applied, it will last for a year.
First wipe free of grease with Turpentine, then dry thoroughly.
Apply lacquer in warm well ventilated room. Remove with Acetone.
To store silver, wrap in tissue, then polythene ziplock plastic bags.
Gold can be shined with Jewelers Rough on a leather chamois (used for polishing cars Repco/Bunnings).
Small items can be burnished with a piece of steel, such as knitting needle - do not do this with engravings (unless you want to remove them).
Clean with warm soapy water & soft cloth or chamois.
(Including this because, often digging turns up non metal loot)
Bone, Ivory, Soapstone
Fixing cracks
Warm piece in hotwater cupboard.
Dip in melted wax for a few minutes, wipe off excess. Tints to match original can be done by adding a crayon shaving, becareful to add colour slowly & test drop onto parchment as drying colour will be different.
Alternately, use Epoxy Resin (eg Araldite), diluted in Acetone until liquid enough to run into surface cracks & holes. Can be coloured with oil or powdered paints.
Rejoining Broken Pieces
Clean breaks, Bleach old glue stains with Whiting & Hydrogen Peroxide paste, join with Epoxy Resin.