Well, I wouldn’t put myself forward as an expert on iron founding, but to answer some of the questions above…The spokes would not fuse to the iron, it would chill when it hit them.
It is possible to make cast iron by melting down miscellaneous scrap and then adding carbon to bring the total carbon content up to what is desired. I have seen an article on this somewhere, I think it may have been ME but I can’t remember the details. They could also be doing cast steel I suppose, the same applies in that you can add alloying agents to the melt to get the desired properties.
As for who is going to do this when these guys have gone….well, you have seen the video, now you know how to do it. I would suggest purchasing a leather apron, leather trousers, gloves and so on myself if you plan to emulate them. I’m told that iron founders used to wear boots with the tongue outside, covering all the lace holes because molten iron is so fluid that it will run in through the holes and down inside the boot.
regards
John