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Metalworking is one of the skills that separated the eras of human history — the Stone Age, Bronze Age, and Iron Age were defined by our ability (or lack thereof) to produce tools and weapons from durable metal. These days, we may feel as though we've progressed far beyond the need for such simple tools, but the ability to forge and repair metal items remains extremely valuable. It's not hard to imagine a scenario in which our fancy CNC mills and power tools become unusable, but a hot fire, hammer, and anvil will always get the job done.
Whether you're forging a knife, repairing a tool, repurposing some scrap, or simply learning metalworking as a hobby, you'll need a forge. One of the simplest ways to achieve this is by forcing air up through a layer of burning charcoal or coke. This was originally done through manually-operated bellows, but can now be done with a small shop vac or leaf blower. Even if the power grid is down, this could easily be run off a battery or a small generator, and it saves a tremendous amount of work that would be necessary to keep bellows pumping.
Swedish YouTuber and blacksmith Torbjörn Åhman recently posted a video that shows how he made an extremely simple forge in his backyard without any special tools. The forge consists of a wood box atop two sawhorses. A hole is cut in one side of the box, and a metal pipe (called a tuyere) is inserted with the protruding end connected to a shop vac hose. The space in the box is then filled with dirt and heat-resistant stones — concrete or other types of stone could explode, so these must be avoided. Wood kindling and coke fuel are added and ignited atop the pipe opening, forming a hearth.
After the first attempt at forging, the stones fracture, so Åhman replaces them with more durable hard-burned bricks.
Unless you're going to go 100% primitive and rely on manual bellows, this is about as simple as it gets. Check out the video below if you're interested in trying your hand at blacksmithing or blade-forging: