Hi Stew
A couple of years ago I needed to silver solder some cast iron cylinders and the timing of the job coincided with my annual visit to the Midland show. While there I spoke to Keith from Cup Alloys about how to go about it and he advised heating the cast iron to red heat to burn off any contamination and allowing the parts to cool before the actual soldering process which could then go ahead as normal. (He also sold me some silver solder foil to sandwich between the parts which worked well.)
Jim