I love this sort of stuff!
The screaming bell looks like a warm copper colour.
I would build up layers with:
Rhinox hide or dark brown
Bronze (I forget the current color name)
Vallejo bronze (again, but lighter)
Finally I'd add a little oxide into the lower recesses where corrosion might have accumulated, keeping in mind that high friction areas would be bright.
Awesome, thanks for the suggestions. I still have lots of experimentation before I'll settle on everything, but I'll try some of this. Not sure I have all the GW bronzes/brasses and such, but I'll make due.