Once again Primitive Tech amazes me. If you haven't heard of this YouTube channel, you should check it out (you should read his blog posts too). So far, he's mostly built huts (one even had a fireplace) and primitive tools, but with the latest video, he's taken on building a furnace hot enough to smelt metal. It's super simple in concept, and looks pretty effective.
It's amazing what he's able to build with clay and water.