The metal feels oddly warm and smooth under his poking, for the record. (It LOOKS like silver, but that can't possibly be what it actually IS.)

Once Shephard has firmly bit down on his belt, the Physician lowers his arm and its metal hand into the bubbling cauldron.

It might be most accurate to say that the binding process feels exactly like having the arm cut off, only backward. The damaged bones and tissues of the flesh portion of that arm are being activated all at once and they are very unhappy about it.