Has this been proved?
because I have read another theory that says that this is unlikely and that he used a machine designed to stregnthen the 4th finger and it was this machine that caused his disability
I don't actually know a great deal about Schumann to be honest! I found out that infomation from one of those general books on composers, I would like to get a good book on him one day though.
It could of been possible. Using mercury as a medicine back in the day was quite common

. In the book it said he took it as a treatment for syphilis. So it could explan his madness, as mercury makes you go mad - Just like that Chinese emperor who took it to make him live forever or something

.