First off, GREAT find!
I'd try lye first but I'm probably the only person on here who would suggest that process. I've cleaned thousands of ancient coins and relics over the last decade or so and lye is often my choice.
Most people on here will probably suggest either olive oil or electrolysis.
I doubt olive oil will do it and electrolysis may work fine if the core metal is solid. If the core metal has been penetrated by the corrosion, electrolysis could kill the piece (been there and done it).
Lye eats away encrustation much faster than olive oil or distilled water soaks and doesn't harm base metal.
You may still end up using some electrolysis in the end.
To use lye, buy Red Devil drain cleaner from any Ace Hardware store. Mix as much lye into cold water as the water will hold. Wear gloves and eye protection. Use a glass jar and expect it to get very warm as you mix the lye into the water. Soak the artifact for about 12 to 24 hours and check it.
If the lye is going to work you'll see a tremendous improvement within 24 hours.
Best to you.