How efficient is iron smelting ?
Fallout really isn't a problem and never really was. Freeman Dyson didn't like it, but on the numbers he calculated I really can't see why. There's just not enough radiaiton to kill anyone who wouldn't have died anyway. Someone calculated it's about the same dose you get from sleeping next to someone. (I'll try and check that at some point)
Since it's iron oxide that makes Mars red, there really has to be a lot of it in the soil. Ah... found a site :
""If you look at the soil composition of Mars, the one thing that really strikes you is that it's 5 to 14 percent iron oxide," said Dr. Peter Curreri, a materials scientist at NASA's Marshall Space Flight Center. "It's almost ore-grade material."
So my 20% figure's a bit high, but not worryingly so.
Could probably make a high melting-temperature ceramic from the materials on Mars for the furnace walls, will look into that. I doubt the power requirments to evacuate the furnace will be significant in an atmosphere 1% of Earth's density.