Removing tarnish/oxide excrescence on silver or silverplate

[silversmith_]

I have a cup, not sure if silver or silver plate. There are two spots, on the inside, consisting on some oxide, tarnish relief/excrescence. There were more oxydized parts but I have polished them.However these two spots are -as I say- like an excrescence, under which it seems it is the silver or silverplate.
How could I remove it?

Thanks

[JLDoggett]

First I would test for silver content. The excrescence could be something like solder. But assuming otherwise I would follow this rubric:

What colour are they? White or blue/green could be from cleaner or salt in long term contact. Dark coloured could be a foreign substance that has adhered to the inside surface.

What kind of texture do they show? Smooth would usually be the foreign substance suck on the inside. A rough surface could be a sign of chemical reaction.

I would typically fill the item with hot water and see if that got rid of them. If not I would try hot vinegar. If neither of these eliminates the excrescence then I would consider more drastic measures.