I think that silver soldering is used for refrigeration line fittings. I know that things have to be really clean for it to stick and it requires lots of heat to get it to flow into the joint. I don't think it is suited very well for cast iron, not sure how a person would get the cast clean enough and I would be nervous about the amount of heat needed. I know that brazing takes less heat and the flux used is more forgiving with the dirty properties on cast iron.
