I tend to go low tech for prep and use hot soapy water and a clean kitchen scrubber.
The closest I've come to achieving that patina affect was to use platinol, then I popped then into a tumbler with some ceramic pns for 15 mins which also left them matte. So they then went into the other tumbler with steel shot to bring the shine back up. Not quite the huge contrast you are looking for but as I recall these were lighter and I was after a darker look so I redid the process leaving them in the ceramic pins for a shorter time.
These items were patina'd with platinol and then just thrown in the tumbler so all that happens is the the silver is dark and shiny at the end of that process with just a little bit of highlighting. In fact I might have gone over these with a brass brush in my rotary tool (I really should take notes!)
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