Ok if its anything of any value it "should" have a hall mark of sorts.
However its filthy, Leave in in some brown sauce for a few hours then clean it, if its still not clean enough either repeat it with fresh brown sauce or leave it to soak for a couple of hours in cheap and nasty full fat coke.
That should clean it up
Harpic toilet cleaner should also do it,might dissolve it though if left on for to long