That's an interesting picture of the failed cap. I'd guess the molten blob on the can end is just some electrical solder that ended up there by coincidence sometime in the past, or maybe from inside the part. Try hitting it with a soldering iron - it should melt. If it doesn't, then I guess it could be Ta.
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Metallic tantalum does not normally look like that, all silvery and shiny, because oxides form on the surface with a range of iridescent colors. It's more of a refractory metal that would not exist molten at a temperature that wouldn't have turned the rest of the part to ash. Besides that, the "solid" Ta caps are mostly Ta oxides and very little metal, sintered into solid slugs, and I can't picture there being that much actual Ta metal present (in proportion) inside a cap of that size. There could have been that much electrical solder in there though. It's used to make internal contacts, and especially to solder the electrical leads and glass/metal hermetic sealing disc into the can.
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Anyway, I think it's just solder on there, but if it is actual Ta metal, then it was quite a feat to nearly instantaneously melt down, collect a decent glob of liquid Ta inside the core, then burst the can end and eject the Ta gently into a nice external glob. All without a hint of discoloration or deformation of the can. I'm very curious about it. If you still have the part, it would be interesting to see what that actually is. Please check it out if possible.
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Ed
