I don't know anything about iron casting (Sadly- my grandfather was a grey-iron foundryman for 50 years. I wish I'd learned from him when I had the chance.), but that kind of porosity/roughness in a jewelery casting would indicate sprues (gates?) that are too small. The cooling, contracting liquid metal needs a reservoir to pull from, or it will pull from the piece.
But lost-wax gold may not work the same as iron...
It looked as though you leave the foam cores in the molds, and just let the iron burn them out? Weird. Jewelers burn the wax out in an oven before pouring metal.
We had part of a Slinky. But I straightened it.