Thanks a lot for your answer Corribus.
I bought the cup as part of a mourning/grief process and I'd really like to be able to use it. The producer said it did not contain any lead, but I'm unsure how reliable his words are.
With extraction in vinegar, I guess it wouldn't be as simple as boiling the cup in vinegar?
What's your opinion on the test kits I found? I found out they use sodium rhodizonate.