I found a Darlot Paris Petzval at my local antique shop (score!) but the thing is in pretty poor condition. The glass actually looks very good except for separation on the front element which is an easy fix if I could get the thing out. It looks like the lens was either damp or in water for some time and there's white crusty stuff inside the barrel and I'd imagine it's on the threads too. The rear element threaded out fine but the front is still stuck.
I've tried using PB blaster, a penetrating oil for several hours and that's not getting me anywhere. Any suggestions on what would break these things apart would be greatly appreciated.