I have welded rail road rails with thermite and I can tell you it is nasty stuff. Unless the iron you are wanting to cut is at least an inch thick I believe I would seek out another alternative. If you are set on using thermite, one way to control the flow of molten metal is to place the thermite in a clay pot with a weep hole in the bottom. This will allow the molten metal to flow down and through the piece below it. Please use great care.
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