You can melt silver using a reducing flame to soot the iron crucible (burning a layer of oil into the iron mold or melting pot will work similarly to put a layer of carbon soot on the fixture), silver and iron will not alloy in the melt. Basically, you can melt silver in a greased iron skillet. Although a ceramic melting dish, crucible, or cupel would be better, they are cheap and are much more suited for the tasks.
Forget about melting gold with iron or in a cast-iron crucible, one metal will become a solvent for the other metal and the two metals will make an alloy together, ceramic melting dishes or crucibles are cheap and readily available...